# 8.6.26 Tuberculosis 1126 # 8.6.26 Tuberculosis 1126 section 8  Infectious diseases 1126 raw meat, and can be carried by flies. The distribution of enterotoxin-​ producing strains may be more restricted. However, surface con- tamination of meat with C. perfringens is common and subsequent rolling or grinding distributes these organisms throughout. Spores germinate and multiply to 106 to 107 cells/​g in the anaerobic envir- onment created when meat or meat gravy cools slowly or stands at ambient temperature. Reheating may not kill these cells and, when ingested, they multiply still further, sporulate, and release their toxin. Enterotoxin-​producing strains of C. perfringens may sometimes cause diarrhoea by means of overgrowth in the gut. Patients, usually elderly, can experience diarrhoea without known contact with con- taminated food. The diarrhoea may be short-​lived or persist inter- mittently for several months. Colony counts of 108 to 1010/​g of faeces are associated with the presence of high titres of free toxin. Previous antimicrobial treatment may encourage the overgrowth and the same strain has been found to cross-​infect patients. FURTHER READING Botulism Arnon SS, et al. (2006). Human botulism immune globulin for the treatment of infant botulism. N Engl J Med, 354, 462–​71. Cherington M (2004). Botulism: update and review. Semin Neurol, 24, 155–​63. Chertow DS, et al. (2006). Botulism in 4 adults following cosmetic in- jections with an unlicensed, highly concentrated botulinum prepar- ation. JAMA, 296, 2476–​79. Fox CK, Keet CA, Strober JB (2005). Recent advances in infant botu- lism. Pediatr Neurol, 32, 149–​54. Lalli G, et al. (2003). The journey of tetanus and botulinum neuro- toxins in neurons. Trends Microbiol, 11, 431–​7. Sobel J (2009). Diagnosis and treatment of botulism: a century later, clinical suspicion remains the cornerstone. Clin Infect Dis, 48, 1674–​5. Gas gangrene Aldape MJ, Bryant AE, Stevens DL (2006). Clostridium sordellii in- fection: epidemiology, clinical findings and current perspectives on diagnosis and treatment. Clin Infect Dis, 43, 1436–​46. Bryant AE, Stevens DL (1996). Phospholipase C and perfringolysin O from Clostridium perfringens upregulate ELAM-​1 and ICAM-​1 ex- pression, and induce IL-​8 synthesis in cultured human umbilical vein endothelial cells. Infect Immun, 64, 358–​62. Bryant AE, et al. (1993). Clostridium perfringens invasiveness is en- hanced by effects of theta toxin upon PMNL structure and function. FEMS Immunol Med Microbiol, 7, 321–​36. Bryant AE, et al. (2000). Clostridial gas gangrene I: cellular and molecular mechanisms of microvascular dysfunction. J Infect Dis, 182, 799–​807. Bryant AE, et al. (2000). Clostridial gas gangrene II: phospholipase C-​ induced activation of platelet gpIIbIIIa mediates vascular occlusion and myonecrosis in C. perfringens gas gangrene. J Infect Dis, 182, 808–​15. Bryant AE, et  al. (2006). Clostridium perfringens phospholipase C-​ induced platelet/​leukocyte interactions impede neutrophils diape- desis. J Med Microbiol, 55, 495–​504. Centers for Disease Control (2000). Update: Clostridium novyi and un- explained illness among injecting-​drug users. MMWR Morb Mortal Wkly Rep, 49, 543–​5. Cohen AL, et  al. (2007). Toxic shock associated with Clostridium sordellii and Clostridium perfringens after medical and spontaneous abortion. Obstet. Gynecol, 110, 1027–​33. Darke SG, King AM, Slack WK (1977). Gas gangrene and related in- fection: classification, clinical features and aetiology, management and mortality: a report of 88 cases. Br J Surg, 64, 104–​12. Maclennan JD (1962). The histotoxic clostridial infections of man. Bacteriol Rev, 26, 177–​276. Shouler PJ (1983). The management of missile injuries. J R Nav Med Serv, 69, 80–​4. Stevens DL, Bryant AE (2005). Clostridial gas gangrene: clinical cor- relations, microbial virulence factors, and molecular mechanisms of pathogenesis. In: Proft T (ed) Microbial toxins: molecular and cellular biology, pp. 313–​35. Horizon Bioscience, Norfolk, UK. Stevens DL, et al. (1993). Evaluation of therapy with hyperbaric oxygen for experimental infection with Clostridium perfringens. Clin Infect Dis, 17, 231–​7. Stevens DL, et al. (2004). Immunization with the C-​domain of alpha-​ toxin prevents lethal infection, localizes tissue injury, and promotes host response to challenge with Clostridium perfringens. J Infect Dis, 190, 767–​73. Stevens DL, Bryant AE (2017). Necrotising soft tissue infections. N Engl J Med, 377, 2253–65. Gastrointestinal infections Abrahao C, et al. (2001). Similar frequency of detection of Clostridium perfringens enterotoxin and Clostridium difficile toxins in patients with antibiotic-​associated diarrhea. Eur J Clin Microbiol Infect Dis, 20, 676–​7. Alfa MJ, et al. (2002). An outbreak of necrotizing enterocolitis asso- ciated with a novel clostridium species in a neonatal intensive care unit. Clin Infect Dis, 35, S101–​5. Bos J, et al. (2005). Fatal necrotizing colitis following a foodborne out- break of enterotoxigenic Clostridium perfringens type A infection. Clin Infect Dis, 40, e78–​83. Fisher DJ, et  al. (2005). Association of beta2 toxin production with Clostridium perfringens type A human gastrointestinal dis- ease isolates carrying a plasmid enterotoxin gene. Mol Microbiol, 56, 747–​62. Lawrence GW, et al. (1990). Impact of active immunisation against en- teritis necroticans in Papua New Guinea. Lancet, 336, 1165–​7. Li DY, et al. (2004). Enteritis necroticans with recurrent enterocutaneous fistulae caused by Clostridium perfringens in a child with cyclic neu- tropenia. J Pediatr Gastroenterol Nutr, 38, 213–​15. Obladen M (2009). Necrotizing enterocolitis—​150 years of fruitless search for the cause. Neonatology, 96, 203–​10. Sobel J, et  al. (2005). Necrotizing enterocolitis associated with Clostridium perfringens type A in previously healthy north American adults. J Am Coll Surg, 201, 48–​56. 8.6.26  Tuberculosis Richard E. Chaisson and Jean B. Nachega ESSENTIALS Tuberculosis is caused by organisms of the Mycobacterium tuber- culosis complex, including M.  tuberculosis (the most important), M. bovis, and M. africanum. It has been present since antiquity and 8.6.26  Tuberculosis 1127 is the leading infectious cause of death ahead of HIV infection. An estimated 2 billion people worldwide carry latent infection, when M. tuberculosis persists within cells and granulomas, with the poten- tial to reactivate to cause disease decades later. Tubercle bacilli are transmitted between people by aerosols gen- erated when an infectious person coughs. Proximity to an infectious person determines the risk of infection. Host immunity and factors affecting it—​most importantly HIV infection but also diabetes, cig- arette smoking, and alcohol and drug abuse—​determine the risk of active disease following infection. Clinical presentation of active tuberculosis is highly variable, depending on the site, extent of disease, and the immune status of the host. Disease is generally classified as pulmonary or extrapulmonary, with considerable clinical heterogeneity within each group. Clinical features—​pulmonary tuberculosis Following deposition of tubercle bacilli in the alveoli of the lungs, they are ingested by alveolar macrophages, multiply intracellularly, and eventually cause cell lysis with release of organisms. Over a period of weeks, infection spreads to regional lymph nodes, else- where in the lungs, and systemically. Infected people who success- fully contain viable bacilli in granulomas may retain a latent infection, with lifetime risk of reactivation of about 10%. Active pulmonary tuberculosis—​this is usually a subacute respiratory illness, the most frequent symptoms of which are cough, fever, night sweats, and malaise. The cough is initially non​productive, but often progresses to sputum production and occasionally haemoptysis. Loss of appetite and excessive weight loss are common. Clinical features—​extrapulmonary tuberculosis This can be generalized or confined to a single organ, and is found in 15–​20% of all cases of tuberculosis in otherwise immunocompe- tent adults, more than 25% of cases under 15 years of age, and in more than 50% of HIV-​related cases. Children under 2 years of age have high rates of miliary or disseminated tuberculosis and menin- geal disease. Infection spreads from the lungs by lymphatic and haema- togenous routes. The tissues and organs most likely to be affected are the pleura, lymph nodes, kidneys, and other genitourinary or- gans, bone, and central nervous system. Tuberculosis bacteraemia is unusual, but seen most often in patients with HIV infection and low CD4 lymphocyte counts. Pleural tuberculosis—​this is usually the result of relatively small numbers of tubercle bacilli invading the pleura from adjacent lung tissue, in which case the duration of symptoms is generally brief, with patients complaining of symptoms including fever, chest pain, and non​productive cough. Pleural tuberculosis involving larger numbers of bacilli produces frank empyema and is more common in older patients. Lymphatic tuberculosis—​classic scrofula of the cervical or supraclavicular lymph node chains is the most common presen- tation, but multiple lymph node groups can be involved in HIV-​ infected patients. Genitourinary tuberculosis—​the most common manifestation is renal tuberculosis, resulting from haematogenous seeding of the renal cortex during primary infection; this is frequently asymptom- atic, but might be evident as sterile pyuria. Bone and joint tuberculosis—​the most common form is vertebral tuberculosis (Pott’s disease), resulting from haematogenous seeding of the anterior portion of vertebral bodies during primary infection; presentation is typically with back pain; constitutional symptoms are not prominent in most cases. Tuberculous meningitis—​meningeal and leptomeningeal bacterial replication results in a robust inflammatory reaction that increases cerebrospinal fluid pressure and can cause cranial neuropathies. Common symptoms are headache, stiff neck, meningism, and an al- tered mental status, including irritability, clouded thinking, and mal- aise. The condition is not common, but usually fatal if untreated. Miliary/​disseminated tuberculosis—​these describe widespread in- fection with absent or minimal host immune responses, usually arising as a result of primary infection, and seen more frequently in children and immunocompromised adults. Typical presentation is with fever and other constitutional symptoms over a period of several weeks. Diagnosis Tuberculin skin testing—​intracutaneous injection of purified proteins of M. tuberculosis provokes a delayed hypersensitivity reaction which produces a zone of induration in those who are infected, but cannot distinguish disease from latent infection and may be falsely positive from Bacille Calmette–​Guérin vaccination or non​tuberculous myco- bacterial infections. Interferon-​γ release-​based assays—​these detect in vitro responses to M. tuberculosis antigens. These appear to be more specific than tuberculin skin testing because false-​positive reactions due to sen- sitization from Bacille Calmette–​Guérin vaccination are less likely to occur. They may also be more sensitive, and are appealing because they do not require patients to return for reading of induration. Detection of tubercle bacilli—​microscopical staining of acid-​fast bacilli in sputum or other tissue is the method most widely used to diagnose tuberculosis because it is inexpensive, rapid, and techno- logically undemanding. However, a relatively large number of bacilli are needed for a positive test, and up to 50% of patients with sputum cultures positive for M. tuberculosis have negative acid-​fast smears. Culture of M. tuberculosis is the gold standard for confirming the diag- nosis, but takes 10–​40 days, depending on the method used. Nucleic acid amplification assays and other rapid diagnostic methods allow faster detection of both the presence of mycobacteria and assess- ment of drug resistance: these have promise in resource-​limited set- tings, but further validation in endemic countries is needed. Nucleic acid amplification—​several new commercial assays that amplify M. tuberculosis DNA can result in rapid diagnosis of tuber- culosis (<1 day). Some tests also can detect drug-​resistant mutations, providing timely detection of multidrug-​resistant tuberculosis. Particular issues—​(1) Pulmonary tuberculosis—​this can involve any portion of the lungs, hence radiographic findings are usually only suggestive, not diagnostic. (2) Pleural tuberculosis—​diagnosis can be inferred from pulmonary findings when pulmonary parenchymal in- volvement is manifest, otherwise analysis of pleural fluid is essential. (3) Lymphatic tuberculosis—​swelling of involved nodes accompanied by a positive tuberculin skin test and typical biopsy findings are strongly suggestive of tuberculosis and warrant presumptive therapy. (4) Tuberculous meningitis—​diagnosis requires a high degree of sus- picion; presumptive therapy is frequently necessary. section 8  Infectious diseases 1128 Treatment Drug-​susceptible tuberculosis—​combination therapy with isoniazid and rifampin (and other antituberculosis drugs in the first 8 weeks) is highly effective. Treatment is usually once daily but can be given as infrequently as twice per week, with two major interventions to im- prove adherence and prevent bad outcomes being directly observed therapy and the use of fixed-​dose combination tablets. Modern ‘short course’ combination chemotherapy is curative in 6 months, except for bone and central nervous system tuberculosis, which re- quire 12 months. Second-​line agents are reserved for treatment of drug-​resistant tuberculosis and are generally less potent, more toxic, and less readily available. Drug-​resistant tuberculosis—​this significant challenge arises both through infection with drug-​resistant strains (primary or ‘new’ drug resistance) and by selection for drug-​resistant strains due to inef- fective therapy (secondary or ‘previously treated’ drug resistance). Multidrug-​resistant tuberculosis is defined as resistance to at least ri- fampicin and isoniazid. Extensively drug-​resistant disease, which has been reported in more than 70 countries, is defined as multidrug re- sistant plus resistance to fluoroquinolones and at least one injectable second-​line agent (capreomycin, amikacin, or kanamycin). Patients with drug-​resistant tuberculosis should be managed by a physician who is a tuberculosis expert because of the complexity of their regi- mens and their high risk of failure of death. Prevention Strategies to control tuberculosis include: (1) Identification and treat- ment of infectious tuberculosis cases, which rapidly eliminates in- fectiousness. (2) Treatment of latent tuberculosis infection—​the use of preventive therapy in high-​risk individuals known or strongly suspected to be latently infected with M.  tuberculosis can benefit not only the individual patient who does not fall ill with tubercu- losis, but also potential contacts of that patient, who might become secondarily infected were disease to develop. (3) Prevention of ex- posure to infectious particles in air, especially in hospitals and other institutions—​infected patients must be identified and managed in respiratory isolation. (4)  Vaccination—​the attenuated live vaccine, Bacille Calmette–​Guérin, is widely administered throughout the world, but remains controversial. Proponents argue that it provides about 50% protection against active tuberculosis disease and also diminishes haematogenous dissemination of primary tuberculosis infection, thereby reducing the incidence of miliary tuberculosis and tuberculous meningitis in children. Introduction Tuberculosis (TB) is one of the most important diseases in the his- tory of humanity, and remains an extraordinary burden on human health today. Archaeological evidence demonstrates that tubercu- losis was present in antiquity, and large epidemics of the disease emerged in Europe in the Middle Ages. While contemporary phys- icians consider tuberculosis to be one of the classic infectious dis- eases, recognition of the clinical manifestations of the disease has evolved over the past two millennia. The Greek term phthisis was used by Hippocrates to describe the wasting disease later known as tuberculosis. While the Greeks recognized various clinical manifestations of tuberculosis, understanding of the connection between the forms was limited. In the Middle Ages, the study of anatomy and the correlation of pathological findings with clinical syndromes led to a better understanding of the disease. The term ‘tuberculosis’ was used first only in the early 19th century, derived from the tubercles characterized in the study of pathological fea- tures of the disease. The impact of tuberculosis on the human population cannot be overstated, as the disease has killed hundreds of millions of people over the centuries and has had economic and social effects perhaps unparalleled in the history of medicine. Between 1700 and 1950, tuberculosis was a great killer in the developed world, earning the sobriquet ‘the captain of the men of death’ from John Bunyan, and ‘the White Plague’ from René and Jean Dubos. The inspiration that artists have drawn from tuberculosis, portrayed in literature, opera, and art, testifies not only to the importance of the disease within their contemporary societies, but also to the extent to which tuber- culosis affected artists themselves. The annals of art are filled with those who succumbed to tuberculosis including Keats, Chopin, the Brontë sisters, Stevenson, Poe, and many, many others. The conquest of tuberculosis through the development of vac- cines, drugs, and diagnostics was a principal goal of biomedical re- search in the 19th and 20th centuries. The first description of the tubercle bacillus as the cause of tuberculosis by Robert Koch in 1882 was a scientific landmark. The postulates established by Koch for determining the microbial aetiology of disease have continuing in- fluence today, and molecular correlates of those derived by Koch fur- ther strengthen the ingenuity of his thesis. Koch also developed the microscopic and culture methods for detecting tubercle bacilli, still widely used today. Calmette and Guérin developed an effective vac- cine for tuberculosis in the early 20th century, but use of the vaccine was not broad enough to control the disease and it may no longer be effective. The discovery of streptomycin by Schatz and Waksman in 1943 was a major triumph; both Koch and Waksman received the Nobel Prize for their work. The development of additional anti- microbial agents against tuberculosis in the 1950s, 1960s, and 1970s, and the evaluation of chemotherapy in elegant studies conducted by the British Medical Research Council, the United States Public Health Service, and the United States Veterans Administration led to a marked apathy about tuberculosis in the closing decades of the 20th century. Despite the availability of curative chemotherapy for more than half a century, however, tuberculosis continues to kill more than 1.5 million people/​year, and causes an enormous amount of suf- fering and disability. In 1994, the World Health Assembly declared that tuberculosis was a global health crisis, and the situation has only grown more serious since then. Epidemics of HIV-​related tu- berculosis and multidrug-​resistant disease have expanded in re- cent years, and global control of tuberculosis remains a formidable challenge. The unique biological properties of the causative organism, Mycobacterium tuberculosis complex, allow for a long incubation period between the time of infection and the development of symp- toms. Latent tuberculosis infection can persist for decades before causing disease, or can persist for the lifetime of an infected person without ever causing clinically evident illness. Because latent infec- tion creates a large reservoir of carriers of the infection, disease elim- ination is difficult to envisage. 8.6.26  Tuberculosis 1129 Aetiology Tuberculosis is a granulomatous disease caused by organisms of the M. tuberculosis complex, including M. tuberculosis, M. bovis, and M. africanum, of which M. tuberculosis is the most important. M. tu- berculosis and the other mycobacteria are small rod-​shaped or curved bacilli in the order Actinomycetales, family Mycobacteriaceae, with a unique thick cell wall composed of glycolipids and lipids. The lipid-​rich coat of the mycobacteria renders these organisms resistant to acid decolorization following carbol-​fuchsin staining, hence the term ‘acid-​fast bacilli’. Classification of the mycobac- teria was based for many years on the staining and growth prop- erties described by Runyon, but this unwieldy system has been largely replaced with modern techniques that identify mycobac- teria by specific DNA sequences and, to a lesser extent, biochem- ical assays. Mycobacteria are frequently considered according to the diseases they cause more than their behaviour in the labora- tory: M. tuberculosis complex causes tuberculosis; M. leprae causes leprosy; and the non​tuberculous mycobacteria, including rapid growers, are associated with a variety of manifestations, particularly in immunocompromised hosts. The organisms of the M. tuberculosis complex are remarkably slow growing, with a generation time between 20 and 24 h. The exceed- ingly slow intrinsic reproductive rate of M. tuberculosis contributes both to its behaviour as a pathogen and to difficulties in recovering the organism in cultures. Moreover, M. tuberculosis is able to persist in a latent form within cells and granulomas for many years, and can reactivate to cause disease decades after infection is acquired. Tubercle bacilli are not known to form spores, but both typical bacilli and non​staining forms of the bacteria persist in cells and tissues, as evidenced by detection of DNA, years after infection is acquired, and retain the capacity to replicate and produce clinical illness. These unique biological characteristics make the tubercle bacillus exceedingly difficult to combat and control. Epidemiology Global incidence Despite the widely held belief that tuberculosis was waning during the 1980s, global tuberculosis incidence has been steady or increasing for several decades. In Western Europe and North America, the incidence of tuberculosis peaked in the 1700s and 1800s, and then declined over a period of years before the devel- opment of chemotherapy. Improvements in hygiene and nutrition, along with reductions in household crowding, were credited with these trends. Following the introduction of curative treatment for tuberculosis in the era following the Second World War the inci- dence of disease fell even further, and tuberculosis deaths were greatly decreased. The success in controlling tuberculosis experi- enced in the western nations was not replicated in developing coun- tries, and increasing epidemics of the disease have been occurring in these areas. In addition, progress in tuberculosis control in the western nations ironically led to neglect of public health pro- grammes that were responsible for reductions in morbidity. As a consequence of inattention to control, the United States of America experienced a resurgence of tuberculosis between 1985 and 1992, with a 21% increase in the annual number of reported cases during that time. In the United Kingdom, tuberculosis incidence has lev- elled off in recent years, with an annual incidence of 11 cases per 100 000 people since 1991. Worldwide, tuberculosis continues to kill more than 1.5 million people per year, making it the leading infectious cause of death ahead of HIV infection. Tuberculosis is a leading cause of death in AIDS, and HIV-​related tuberculosis deaths are attributed to AIDS not tuberculosis. The World Health Organization (WHO) estimates that 1.5 bil- lion people, or one-quarter of the world’s population, are infected with M. tuberculosis. From this seedbed of latent infection, about 10.5  million people became ill with TB and 1.7  million died in 2016, and most of the cases were detected in developing coun- tries. The global distribution of tuberculosis case rates is shown in Fig. 8.6.26.1. Disease due to M. tuberculosis is most common in developing nations, both in absolute numbers and incidence of new cases. Twenty-​two countries account for 80% of all cases of tuberculosis; India and China are responsible for 40% of cases. In general, the highest incidence of disease is found in the coun- tries of sub-​Saharan Africa where HIV infection has contributed to extraordinary increases in case rates. The greatest number of cases arise in the populous nations of Asia, which have moderately high rates of disease per capita. The global incidence of tuberculosis is decreasing slightly, though population growth is resulting in higher numbers of cases each year. Declines in incidence in the devel- oped world have been offset by increasing rates in the HIV-​ravaged countries of Africa and by escalating incidence in Eastern Europe in the aftermath of the collapse of communism and its public health infrastructure. Effect of age Tuberculosis typically affects young adults, with peak incidence in those aged 25 to 44 years. The dynamics of tuberculosis within a particular country or region, however, reflect both historical trends in tuberculosis transmission and current risk factors and practices of disease control. For example, in Western Europe tuberculosis is seen in two demographic groups:  elderly native Europeans who were presumably infected many years ago and who experience reactivation of latent infections as they age or become immuno- compromised, and younger immigrants from high-​incidence countries in the developing world. Interestingly, increasing age is not a risk factor for developing active tuberculosis per se; among ageing populations infected with M.  tuberculosis earlier in life, the risk of developing disease decreases over time. In the United States of America tuberculosis is seen in young adults who have immigrated from endemic areas and in those with HIV infection, whereas reactivation tuberculosis in older people is increasingly uncommon. In the developing world, tuberculosis most com- monly occurs in young adults, with rapidly escalating rates in those with HIV infection. In all countries where tuberculosis is preva- lent, children who acquire tuberculosis from adults account for up to 10% of all cases. Interestingly, children between the ages of 5 and 15 years have extremely low rates of tuberculosis, even in areas with a high disease burden. Infection and disease The epidemiology of tuberculosis can be considered as a func- tion of two distinct but related phenomena:  the likelihood of section 8  Infectious diseases 1130 becoming infected with M. tuberculosis and the probability of developing disease once infection has occurred. Risk factors for becoming infected relate to exposure to infectious cases. Throughout the world, living with someone who has infectious tuberculosis is the most important risk factor for acquiring in- fection. The longer the duration of undiagnosed tuberculosis, the greater the severity of disease, and the more intimate the con- tact, the greater the chance of becoming infected. Exposure to infectious cases in other environments, including healthcare fa- cilities, prisons, and the workplace, is another important route of infection. In areas of the world where tuberculosis is relatively widespread, exposure in the community is commonplace and probably unavoidable. In low prevalence countries, community exposure is most likely to occur in distinct pockets of increased incidence, such as poorer areas of large cities or neighbourhoods with high HIV prevalence. Effect of host immunity After M. tuberculosis infection is acquired, the risk of developing disease is dependent on host immunity. As discussed next, several conditions have been identified that increase the risk of active disease in a person with latent tuberculosis infection, most notably HIV infection. Reactivation from latent tuberculosis infection is an important mechanism for the development of adult tuberculosis. However, studies using DNA fingerprinting techniques show that a significant proportion of tuberculosis cases thought to be due to reactivation are actually recently acquired due to reinfection or new infection, particularly in high HIV prevalence settings. Effect of M. tuberculosis strain Interestingly, strain differences in M. tuberculosis have not been associated with the risk of disease, although inoculum size is as- sociated with probability of becoming ill. For example, household contacts of heavily sputum acid-​fast bacilli smear-​positive cases of tuberculosis who become infected have a higher incidence of active disease than contacts of acid-​fast bacilli smear-​negative cases who become infected. On the other hand, while there is some evidence that specific strains of M. tuberculosis may more successfully infect contacts than other strains, the risk of disease in those infected with these transmissible strains is not elevated. Susceptibility Tuberculosis is a disease traditionally associated with specific population groups, notably the poor, alcohol and drug abusers, and, more recently, those with HIV infection. The increased in- cidence of tuberculosis in impoverished populations is probably Fig. 8.6.26.1  WHO-​estimated global tuberculosis incidence rates in 2016. From ‘Global tuberculosis report 2017’. Geneva: World Health Organization; 2017. 8.6.26  Tuberculosis 1131 multifactorial, involving increased risk of infection (e.g. due to crowded living conditions and a higher background prevalence of disease in the community) and increased risk of developing dis- ease after infection (e.g. due to malnutrition). Similar reasons may explain the higher rates of tuberculosis seen in cigarette smokers and alcohol and drug abusers, with suppression of host cellular immunity either directly or indirectly caused by substance abuse. The more recent association of tuberculosis and HIV infection is clearly related to development of cellular immunodeficiency in those with HIV, but in many settings those at highest risk for HIV infection are also more likely to be latently infected with M. tuber- culosis than others. Effect of the HIV epidemic The impact of HIV infection on the epidemiology of tubercu- losis is striking. As will be discussed next, HIV infection is the most potent known biological risk factor for tuberculosis. The relative risk of tuberculosis in an HIV-​infected person is 200 to 1000 times greater than in someone without HIV infection. The risk of tuberculosis increases shortly after HIV seroconversion, doubling within the first year. As a result of the extraordinary risk conferred from HIV infection, most tuberculosis patients in many sub-​Saharan countries are HIV seropositive. The incidence of active tuberculosis in HIV-​infected patients not receiving anti- retroviral therapy in the United States of America, with latent tuberculosis infection defined by a positive tuberculin skin test, is about 10% per year. Even when antiretroviral therapy is pro- vided to individuals with HIV infection, the risk of tuberculosis remains substantially higher than in HIV-​uninfected people from the same population. Of note, an annual incidence rate of about 10% is described in HIV-​infected patients in South Africa regard- less of tuberculin skin test status. In addition, HIV infection is the unifying theme in many nosocomial outbreaks of tuberculosis, as infection is spread among immunocompromised patients re- ceiving medical care at the same facility. It is increasingly apparent that control of tuberculosis will not be possible globally without control of HIV infection. Effect of drug resistance Another very important trend in tuberculosis epidemiology is the growing problem of drug-​resistant tuberculosis. Drug-​resistant tu- berculosis is reported as two types: ‘new’ and ‘previously treated’. New drug resistance is caused by transmission of a resistant strain of M. tuberculosis and previously treated implying the possibility of acquired drug resistance during previous treatment (e.g. non-​ adherence or inadequate treatment regimen), though recurrence of tuberculosis with drug resistance might also indicate previous in- appropriate treatment of an unrecognized resistant strain. A global survey of resistance performed by the WHO and the International Union Against Tuberculosis and Lung Disease found that the me- dian prevalence of primary drug resistance was 10%, and the median prevalence of acquired resistance was 36%. Moreover, ‘hot spots’ of drug-​resistant tuberculosis were identified on all continents. The most notable of these are in the former Soviet nations where multidrug-​resistant (MDR) tuberculosis, defined as resistance to at least rifampicin and isoniazid, is identified in 10 to 20% of all cases. Multidrug-​resistant tuberculosis treatment is exceedingly difficult, since the drugs used are less effective, costlier, and poorly toler- ated due to drug-​related side effects. Furthermore, failure to con- trol the spread of drug-​resistant tuberculosis has led to outbreaks of extensively drug-​resistant (XDR) tuberculosis, which is defined as MDR tuberculosis plus resistance to fluoroquinolones and at least one injectable second-​line agent (capreomycin, amikacin, or kana- mycin). XDR tuberculosis been associated with high rates of mor- tality in HIV-​infected individuals in South Africa and is reported in more than 70 countries globally. Drug-​resistant tuberculosis (MDR or XDR) will likely continue without effective implemen- tation of measures to rapidly diagnose drug resistance and treat it appropriately. Pathogenesis The development of active tuberculosis, like all infectious dis- eases, is a function of the quantity and virulence of the invading organism and the relative resistance or susceptibility of the host to the pathogen. Indeed, one lineage of tuberculosis known as the W/​Beijing family of strains is predominant in Southeast Asia, but widely distributed in India and South Africa. W/​Beijing strains of M. tuberculosis have been associated with outbreaks of drug-​ sensitive and drug-​resistant tuberculosis and may be more virulent than other strains. Genetic host factors also play a key role in innate non​immune resistance to M. tuberculosis. For example, the human gene SLC11A1, which has been mapped to chromosome 2q, may help determine susceptibility to tuberculosis, according to a study in Africa. But like many infectious diseases, it is likely that resist- ance to tuberculosis is polygenic. Transmission Tubercle bacilli are transmitted between people by aerosols gener- ated when an infectious person coughs or otherwise expels infec- tious pulmonary or laryngeal secretions into the air. M. tuberculosis bacilli excreted by this action are contained within droplet nuclei, extremely small particles (less than 1 µm) that remain airborne for long periods and are disseminated by diffusion and convection until they are deposited on surfaces, diluted, or inactivated by ultra- violet radiation. Individuals breathing air into which droplet nu- clei have been excreted are at risk of acquiring tubercle bacilli by inhaling these nuclei and having them deposited in their alveoli, where a productive infection may occur. Transmission of tubercu- lous infection by other routes, such as inoculation in laboratories and aerosolization of bacilli from tissues in hospitals, has been docu- mented, but these are an insignificant means of spread. M. bovis can be acquired from contaminated milk from tuberculous cows, but modern animal husbandry practices and the pasteurization of milk have substantially reduced this mode of infection throughout most of the world. Natural history of tuberculosis in humans People who are in contact with someone with infectious tuberculosis may acquire infection, as described earlier (see Fig. 8.6.26.2). Factors that affect the likelihood of infection being transmitted include the severity of the disease in the index case (e.g. extent of radiographic abnormalities, cavitation, frequency of cough), the duration and closeness of exposure and environmental factors such as humidity, ventilation, and ambient ultraviolet light. Several studies in diverse section 8  Infectious diseases 1132 locations and circumstances have shown that approximately 20–​ 30% of close contacts of an untreated tuberculosis patient become infected with M. tuberculosis, as demonstrated by the development of a reactive tuberculin skin test. Immune response Deposition of tubercle bacilli in the alveoli results in a series of pro- tective responses by the cellular immune system that forestall the development of disease in most infected people. Alveolar macro- phages ingest tubercle bacilli, which then multiply intracellularly and eventually cause cell lysis with release of organisms. Killing of M. tuberculosis within macrophages is prevented by inhibition of phagolysosome formation by the tubercle bacilli through a process that is not understood. Additional alveolar macrophages engulf progeny bacilli, resulting in further intracellular growth and cell death. Over a period of weeks as tubercle bacilli proliferate within macrophages and are released, infection spreads to regional lymph nodes, elsewhere in the lungs, and systemically. Foci of tubercle bacilli can be established in multiple organs, including the lymph nodes, brain, kidneys, and bones. In most people, specific im- munity is developed after several weeks and consists of activated T lymphocytes mediating a Th1 type response. Macrophages act as antigen-​presenting cells, interacting with CD4 lymphocytes primed for M. tuberculosis antigens. Activated CD4 lymphocytes produce both IL-​2, which promotes activation of additional T lymphocytes, and interferon-​γ, which binds with receptors on macrophages and promotes intracellular killing of organisms. Tumour necrosis factor-​α production is induced in macrophages, and this too pro- motes killing of intracellular bacilli. The specific role of CD8 cells in the control of tuberculosis has not been fully elaborated, although there is evidence that cytotoxic T lymphocytes may play a role in containing a tuberculous infection. In addition, CD8 lymphocytes also produce interferon-​γ and participate in granuloma formation. Recent evidence also supports a role of innate immunity in combat- ting tuberculosis infection. The classic immunological response to infection with tubercle bacilli is the walling off of viable bacilli in granulomas. Granulomas are collections of cells surrounding a focus of M. tuberculosis, usu- ally within macrophages but sometimes extracellularly, that serve to contain the infection. Granulomas consist of macrophages, CD4 and CD8 lymphocytes, fibroblasts, giant cells, and epithelioid cells that produce an extracellular matrix of collagenous and fibrotic materials which are continually remodelled and can become cal- cified. A calcified granuloma at the initial site of infection in the lung is referred to as a Ghon complex, while the combination of a Ghon complex and a calcified regional lymph node is called Ranke’s complex. The development of the cellular immune response to M. tu- berculosis is accompanied by the development of delayed-​type hypersensitivity to specific antigens from tubercle bacilli. While delayed-​type hypersensitivity is distinct from the cell-​mediated immunity that provides protection from disease, this sensitivity to tubercle-​derived proteins has proved enormously useful for diagnosing tuberculosis infection. The use of purified protein de- rivatives of tuberculin is the basis for estimating the prevalence of latent tuberculosis infection in populations, is essential in studying the natural history of tuberculosis infection, and is fre- quently helpful in evaluating patients with suspected tuberculosis disease. The difference between delayed-​type hypersensitivity and immunity to tuberculosis is underscored by the observation that 80–​90% of patients with active disease, and therefore clearly not immune, have positive tuberculin tests. For most people acquiring a new tuberculous infection, the de- velopment of cell-​mediated immunity to the organism is protective and holds the bacilli in check, though viability is often maintained. A small proportion of them will be unable to contain the infection and will progress to active tuberculosis disease, often referred to as primary tuberculosis. Factors associated with early progression of infection to disease include immunosuppression, particularly with HIV infection, a higher inoculum of organisms, malnutrition, and, perhaps, concomitant illness. While rates of active disease in chil- dren > 2 years of age who are contacts of cases are no higher than for older contacts, young children with primary tuberculosis do de- velop more severe forms of tuberculosis than adults, including dis- seminated disease and tuberculous meningitis. Reactivation Those who successfully contain the organism have a latent tuber- culosis infection that may reactivate later in life. Based on studies of latent tuberculosis infection acquired in childhood or adoles- cence, the lifetime risk of reactivation of M. tuberculosis is about 10%. Table 8.6.26.1 lists conditions that are associated with an increased risk of reactivating latent tuberculosis infection. The most potent of these is HIV infection, which increases the rate of reactivation by as much as 1000-​fold. Immunosuppression from malignancy, cytotoxic therapy, corticosteroids, and other agents that alter cellular immune responses also increase the likelihood that latent tuberculosis infection will reactivate. Other important factors that increase the risk of tuberculosis include diabetes and end-​stage renal disease, injection drug use (independent of HIV infection), low body weight, gastrointestinal surgery, and silicosis. Cigarette smoking is associated with increased tuberculosis inci- dence, as is alcohol abuse. Recently, the use of inhibitors of tumour necrosis factor-​α for the treatment of rheumatoid arthritis or in- flammatory bowel disease has been associated with increased risk of tuberculosis. Rates of tuberculosis are usually higher in older people than in younger adults in developed countries, but this might represent a higher prevalence of latent infection in older co- horts, rather than immunological senescence. Exposure (to infectious case) No infection c.70% Infection c.30% Inadequate host defences adequate Containment 90–95% Early progression (primary TB ≤2 years) 5–10% Late progression (reactivation TB) 5% Inadequate host defences adequate Continued containment 85–90% Fig. 8.6.26.2  Natural history of tuberculosis. 8.6.26  Tuberculosis 1133 Clinical features Classification of tuberculosis infection and disease Infection with M. tuberculosis can result in clinical manifestations ranging from asymptomatic carriage of tubercle bacilli to life-​ threatening pneumonia. Asymptomatic individuals with evidence of M. tuberculosis infection by tuberculin skin test or interferon-γ release assay are considered latently infected. In recent years the classification of the different stages of M. tuberculosis in humans has evolved as our understanding of the natural history of M. tu- berculosis has changed, and individuals can no longer be categor- ized simply as latently infected or actively diseased. Rather, the clinical manifestations of M. tuberculosis infection can be viewed as a spectrum, ranging from complete elimination of infection by host immune responses to truly latent infection with bacilli present but controlled by the host, to varying stages of subclinical infection with active bacterial replication but no symptoms, to active disease. Clinically, patients must still be considered to have either latent in- fection or active disease, but the status of latent infection can be further characterized by imaging studies, and research is underway evaluating the potential of gene expression signatures as predictors of subsequent disease. Current management of latent infection is based on an assessment of the risk of progression to active disease, as shown in Table 8.6.26.1. Clinical presentation of active tuberculosis This is highly variable, depending on the site and extent of disease and the immune status of the host. Historically, active tuberculosis has been classified as ‘primary’ or ‘post-​primary’ on the basis of both the presumed duration of infection and the clinical features of the disease. Recent studies using molecular epidemiological techniques, however, suggest that this classification may be unreliable. For ex- ample, the ‘classic’ presentation of reactivation tuberculosis has been seen in patients whose infection is clearly newly acquired, such as in nosocomial outbreaks where DNA fingerprinting confirms recent transmission. For practical purposes, tuberculosis is generally div- ided into pulmonary and extrapulmonary forms, with considerable clinical heterogeneity within these categories. Pulmonary tuberculosis Pulmonary tuberculosis is usually a subacute respiratory infec- tion with prominent constitutional symptoms. The most frequent symptoms of pulmonary tuberculosis are cough, fever, night sweats, and malaise. Cough in pulmonary tuberculosis is initially non-​ productive, but often progresses to sputum production and, in some instances, haemoptysis. The sputum is generally yellow in colour, and is neither malodorous nor thick. Haemoptysis can be seen in patients with untreated tuberculosis, but is also a feature of treated tuberculosis; damage from prior tuberculosis might result in bron- chiectasis or residual cavities that can either become superinfected or erode into blood vessels or airways, producing haemoptysis. Extremely advanced tuberculosis can also present with bloody sputum. Rarely, the bleeding is massive leading to shock, asphyxia, and death. Chest pain is not a prominent symptom in pulmonary tubercu- losis, although musculoskeletal pain from coughing might be noted. In patients with tuberculous pleurisy, however, chest pain may be present, particularly on inspiration. Radicular pain across the chest may be associated with spinal tuberculosis. Dyspnoea alone may be a sign of extensive parenchymal destruction, large pleural effusions, endobronchial obstruction, or pneumothorax. Patients with tuberculosis also experience loss of appetite and weight loss or cachexia, often out of proportion to their diminished intake of food. Increased tumour necrosis factor-​α is hypothesized to be the cause of cachexia in tuberculosis. Other symptoms with mild severity such as emotional liability, irritability, depression, and headache are frequent. The duration of symptoms varies greatly, but most patients will report weeks to months of feeling ill before presentation. In surveys of populations with high rates of disease and poor access to medical care, a history of cough for more than 3 weeks was strongly asso- ciated with a diagnosis of active tuberculosis, but in HIV-​infected patients any duration of cough predicts elevated risk for disease. Untreated tuberculosis is associated with high mortality, but many patients have persistent symptoms for years. A study of untreated pulmonary tuberculosis in the pretherapy era found that after 5 years 50% of patients had died, 25% had spontaneously healed, and 25% were chronically ill with pulmonary disease. A subset of patients has rapidly progressive disease, the so-​called ‘galloping consumption’ of old. Nowadays this is most often seen in patients with HIV infection or other forms of severe immunosuppression. These patients have an escalating course of severe pulmonary symptoms over a period of several weeks, often in the setting of disseminated disease. Failure promptly to diagnose and treat these patients results in death. Physical findings in pulmonary tuberculosis are limited and not generally helpful in making a diagnosis. Fever is an irregular and unreliable feature, and while most patients complain of fevers before Table 8.6.26.1  Incidence of active tuberculosis in people with a positive tuberculin skin test, by selected risk factors Risk factor Number of tuberculosis cases/​100 person-​years Recent tuberculosis infection:   Infection <1 year past 2–​8   Infection 1–​7 years past 0.2   HIV infection 3.5–​14 Injection drug use   HIV seropositive 4–​10   HIV seronegative 1 Silicosis 3–​7 Radiographic findings consistent with prior tuberculosis 0.2–​0.4 Weight deviation from standard:   Underweight by ≥15% 0.26   Underweight by 10–​14% 0.20   Underweight by 5–​9% 0.22   Weight within 5% of standard 0.11   Overweight by ≥5% 0.07 Diabetes mellitus 0.3 Renal failure 0.4–​0.9 None of the above factors 0.01–​0.1 section 8  Infectious diseases 1134 presentation, only one-​half to three-​quarters of patients with con- firmed tuberculosis have a documented fever. Examination of the chest may reveal dullness to percussion and crepitations, although these findings are highly variable and non​specific. Signs of consoli- dation are usually absent. The classic post-​tussive crepitations de- scribed in the last century are not often present and are not specific to tuberculosis. Patients with disseminated tuberculosis may have lymphadenopathy, hepatomegaly, or evidence of central nervous system involvement, but these are not generally seen in typical pul- monary tuberculosis. Finger clubbing and cyanosis are findings as- sociated with prolonged and advanced pulmonary disease. Thus, the diagnosis of tuberculosis almost always rests on the patient’s history and epidemiological characteristics, in conjunction with laboratory studies described next. The most important step in making a timely diagnosis of tuberculosis is to think of it in the first place. Radiological evaluations play a critical role in the diagnosis of pulmonary tuberculosis. Disease due to M. tuberculosis can involve any portion of the lungs, and radiographic findings are usually only suggestive, not diagnostic, of tuberculosis. The typical radiological manifestations of pulmonary tuberculosis are upper lobe infiltrates that may show cavitation. M. tuberculosis exhibits a unique predi- lection for the upper zones of the lungs for reasons that are not well understood. Latent infection characteristically reactivates in the ap- ical segments of the upper lobes, or the superior segments of the lower lobes. The infiltrates are often fibronodular and irregular, and can be diffuse and associated with volume loss. Cavities, when pre- sent, are rarely symmetrical and do not usually have air–​fluid levels, such as those seen in pyogenic lung abscesses. Several examples of the radiographic appearance of pulmonary tuberculosis are seen in Fig. 8.6.26.3. (a) (b) (c) (d) Fig. 8.6.26.3  Radiographic appearance of pulmonary tuberculosis. (a) Extensive tuberculosis with right upper lobe volume loss and multiple small cavities. This patient was the source of at least 14 secondary cases in contacts. (b) A 69-​year-​old man with right pleural tuberculosis. (c) Diffuse pulmonary nodules in an HIV-​infected man with pulmonary tuberculosis. (d) Cavitary upper lobe disease in an HIV-​infected woman. 8.6.26  Tuberculosis 1135 The classic radiographic presentation described here is neither pathognomonic nor highly sensitive for pulmonary tuberculosis. Several other lung infections, notably the pulmonary mycoses, can present with similar findings. More importantly, one-​third to one-​ half of patients with pulmonary tuberculosis lack the classic radio- graphic findings described. Lower lung zone infiltrates, mid-​lung focal infiltrates, pulmonary nodules, and infiltrates with mediastinal or hilar adenopathy are also seen. HIV-​infected tuberculosis pa- tients, in particular, most often present with these ‘atypical’ findings, and up to 5% of them might have a normal chest radiograph in the setting of sputum cultures that yield M. tuberculosis. The lack of typ- ical radiographic features should not, therefore, deter the clinician from considering the diagnosis in a patient with a clinical history compatible with and symptoms of tuberculosis. CT is increasingly used to evaluate pulmonary disorders, including tuberculosis. While the classic findings described earlier do not usually require confirmation with a more sensitive test, CT scanning is sometimes used to evaluate radiographic findings that are not readily explained after an initial assessment. CT scans of the chest in patients with tuberculosis may reveal a greater extent of involvement than conventional radiographs, including multiple nodules, small cavities, and multilobar infiltrates. However, CT scanning can only suggest the possibility of tuberculosis in a patient with other signs and symptoms consistent with the diagnosis, and further evaluation is still required. The laboratory diagnosis of pulmonary tuberculosis relies on the microbiological evaluation of sputum or other respiratory tract spe- cimens. A  definitive diagnosis requires growth of M. tuberculosis from respiratory secretions, while a probable diagnosis can be based on typical clinical and radiographic findings with either acid-​fast bacilli-​positive sputum or other specimens, or typical histopatho- logical findings on biopsy material. These latter approaches, how- ever, have a variable lack of specificity depending on the prevalence of disease due to non​tuberculosis mycobacteria in the population. Throughout most of the world, sputum acid-​fast staining is the sole test used to confirm the diagnosis of pulmonary tuberculosis. In the settings where it is utilized, the positive predictive value of the sputum acid-​fast smear is very high, as the likelihood of non-​ tuberculous mycobacterial disease is quite low. In industrialized countries, disease due to the non​tuberculous mycobacteria is rela- tively more common and reliance on smears without cultures is potentially misleading. Despite the best efforts of clinicians, a con- firmed diagnosis of tuberculosis cannot be established in some pa- tients who have the disease, and a response to presumptive therapy forms the basis for establishing the diagnosis. Further details on the microbiological approach to diagnosis are provided next. Extrapulmonary tuberculosis In the United States of America extrapulmonary tuberculosis is de- fined as disease outside the lung parenchyma; in the United Kingdom it is defined as disease outside the lungs and pleura. This seemingly subtle distinction has considerable epidemiological impact, how- ever, as pleural tuberculosis is the most common extrapulmonary site of disease in the United States of America. During the initial seeding of infection with M. tuberculosis, de- scribed earlier, haematogenous dissemination of bacilli to several organs can occur. These localized infections, as in the lung, can progress into primary tuberculosis or become walled off in small granulomas where bacteria may remain dormant if they are not killed by cell-​mediated immune responses. Extrapulmonary tuber- culosis, therefore, can either be a presentation of primary or reacti- vation tuberculosis. Extrapulmonary tuberculosis may be generalized or con- fined to a single organ. In otherwise immunocompetent adults, extrapulmonary tuberculosis is found in 15–​20% of all tuberculosis cases. In young children and immunosuppressed adults, rates of extrapulmonary disease are substantially higher, appearing in more than one-​half of HIV-​related tuberculosis cases and one-​quarter of tuberculosis cases under 15 years of age. Children less than 2 years old have high rates of miliary and meningeal disease. The organs most frequently involved in extrapulmonary tuber- culosis are listed in Table 8.6.26.2. To some extent the frequency with which specific organs are involved reflects the pathophysiology of the disease. Infection spreads from the lungs, the primary site of inoculation, by lymphatic and haematogenous routes. The tissues and organs most likely to be affected are the pleura, lymph nodes, kidneys and other genitourinary organs, bone, and central nervous system. Although infection is transiently spread in the blood, tu- berculosis bacteraemia is unusual and is seen most often in patients with HIV infection and low CD4 lymphocyte counts. The clinical presentation of extrapulmonary tuberculosis depends largely on the organ involved. Both pulmonary and extrapulmonary disease are found in up to 50% of patients with HIV-​related tubercu- losis, so it is important to consider the possibility of extrapulmonary pathology when pulmonary tuberculosis is diagnosed in an HIV-​ infected patient (and vice versa). Pulmonary involvement is seen in up to one-​quarter of patients with tuberculous meningitis and to lesser degrees with other sites of disease. Pleural tuberculosis This is the result of two distinct pathophysiological sequences, which present in strikingly different manners. Most pleural tu- berculosis is associated with primary infection and is the result of seeding of the visceral pleura with relatively small numbers of tu- bercle bacilli via direct extension from adjacent lung tissue. A large proportion of patients with this form of tuberculous pleurisy will have obvious pulmonary disease, although findings can be subtle. The duration of symptoms is generally brief (e.g. several weeks, and patients complain of fever, chest pain, and non​productive cough). Other constitutional and respiratory symptoms might be present. Unlike pneumococcal pneumonia, which presents abruptly, tuber- culous pleurisy starts more insidiously. Table 8.6.26.2  Common sites of extrapulmonary tuberculosis Site Percentage of extrapulmonary cases Pleura 20–​25 Lymphatics 20–​40 Genitourinary 5–​18 Bone/​joint 10 Central nervous system 5–​7 Abdominal 4 Disseminated 7–​11 section 8  Infectious diseases 1136 The second form of pleural tuberculosis occurs when larger numbers of bacilli invade the pleural space and multiply, producing frank empyema. Tuberculous empyema is seen in older patients, almost all of whom have extensive pulmonary disease. Patients present with prolonged symptoms of cough, chest pain, fever, cach- exia, and night sweats. Pneumothorax is a common complication of tuberculous empyema and may be associated with a more rapid disease course. The radiographic picture in tuberculous pleurisy reflects the underlying pathophysiology of the disease. Patients with the pri- mary type of pleurisy tend to have small unilateral effusions, and up to one-​half have visible parenchymal lesions on plain radiographs. In patients with tuberculous empyema, the effusions are larger and more likely to be loculated, and adjacent pulmonary involvement is often evident. The diagnosis of pleural tuberculosis can be approached along several lines. When pulmonary parenchymal involvement is mani- fest, sputum smears and cultures have a high yield, and the diagnosis of pleural disease can be inferred from the pulmonary findings. When pulmonary findings are minimal or the initial test results unrevealing, analysis of pleural fluid is essential. Acid-​fast stains of pleural fluid are usually negative in patients with primary tuber- culous pleurisy as the number of organisms in the pleural space is small. Repeated sampling will show organisms in less than one-​half of cases. Similarly, culture results might be negative. The pleural fluid is usually serous and exudative, with a protein concentration that is more than 50% of the serum level, normal or low glucose, and a slightly acidic pH. The pleural fluid white blood cell count is usually in the range of 1000 to 10 000 per µl with a lymphocytic predominance. Lactate dehydrogenase levels are generally elevated, as are adenosine deaminase levels. All of these tests are non​specific and cannot reliably distinguish tuberculosis pleurisy from other pleural diseases. Pleural biopsy is frequently useful in establishing a diagnosis of tuberculous pleurisy. Percutaneous biopsy of the pleura reveals granulomatous inflammation in up to 80% of patients, and cultures obtained at the time of biopsy are positive in over one-​half of pa- tients. If a first attempt fails to provide a diagnosis, a second biopsy might be successful. More recently, thoracoscopy has been utilized to improve the yield of biopsy by visualizing biopsy targets rather than blindly sampling with a percutaneous pleural needle. Lymphatic tuberculosis This can occur in any location, but classic scrofula involving the cervical or supraclavicular chains is the most common presenta- tion. Mediastinal and hilar lymphatic tuberculosis is a feature both of primary and disseminated disease, but discovery of these lesions is usually incidental. The pathophysiology of lymphatic tubercu- losis is thought to result from drainage of bacilli in the lungs into supraclavicular and posterior cervical lymph node chains. In con- trast, lymphatic disease caused by non​tuberculous mycobacteria usually involves anterior cervical, preauricular, or submandibular lymph nodes, suggesting acquisition through the oropharynx. In patients with HIV infection, multiple lymph node groups can be in- volved including axillary, inguinal, mesenteric, and retroperitoneal. Symptoms in lymphatic tuberculosis are generally limited, un- less the disease is disseminated. Painless swelling of a lymph node is the most common presentation. Constitutional symptoms are not prominent in most cases. Examination of the area may reveal several enlarged lymph nodes, as only about 20% of patients have disease of a solitary node. The diagnosis of lymphatic tuberculosis usually depends on cultures from affected nodes. Biopsies may show granulomatous changes and acid-​fast bacilli. Such findings are non​specific, how- ever, and cannot distinguish tuberculous from non​tuberculosis lymphadenitis. As discussed elsewhere, the presence of a positive tu- berculin skin test in the setting of typical biopsy findings is strongly suggestive of tuberculosis; in the setting of suspected lymphatic tu- berculosis, these findings warrant presumptive therapy. Genitourinary tuberculosis This encompasses a broad array of clinical entities, ranging from disease of the kidneys to endometrial, prostatic, and epididymal disease. The most common of these is renal tuberculosis, which re- sults from haematogenous seeding of the renal cortex during the primary infection. The pathogenesis of other genitourinary sites is either from downstream extension of renal infection over time or from haematogenous seeding at the time of the initial acquisition of M. tuberculosis. Renal tuberculosis is probably underdiagnosed because it is fre- quently asymptomatic. Many cases of genitourinary tuberculosis are diagnosed as a result of routine urinalyses that detect sterile py- uria. The development of symptoms reflects a more advanced stage of disease, associated with considerable tissue destruction. When genitourinary tuberculosis is symptomatic, the most common symptoms are localized and include urinary symptoms and flank pain. In men, tuberculosis can cause prostatitis and epididymitis, both of which can present with pain resulting from swelling. In women, genital tract tuberculosis may be symptomatic when it in- volves the ovaries and Fallopian tubes; pelvic pain is also a feature of endometrial tuberculosis. Menstrual abnormalities and infertility may be the only signs of genital disease, however. The diagnosis of genitourinary tuberculosis depends on the ana- tomical site of the disease. Renal tuberculosis, as noted, is suggested by sterile pyuria, and the diagnosis rests on isolation of organisms in the urine. Early morning urine samples are more likely to grow M. tuberculosis than spot samples obtained at other times. In pa- tients with symptoms of upper urinary tract illness, radiological studies are often helpful. The kidneys may appear calcified on ab- dominal radiographs. Intravenous pyelography may show distorted or dilated calyces or renal pelvis, papillary necrosis, cavitation, or abscesses of the renal parenchyma, or intrarenal or ureteral obstruc- tions. Use of renal ultrasonography or CT scanning may be more sensitive for identifying the abnormalities of renal tuberculosis, but contrast radiography is the technique with which the greatest ex- perience has accrued. When tuberculosis of the bladder is suspected, cystoscopy with biopsy may lead to the identification of granulomas before identification of organisms by culture. Diagnosis of prostatic, testicular, or epididymal tuberculosis is usually accomplished with cultures obtained by fine needle aspiration or transurethral resec- tion of the prostate. Cervical and endometrial tuberculosis can be diagnosed by biopsy with culture. Tuberculous meningitis This is the most common central nervous system manifestation of tuberculosis (see Chapter 24.11.1). It is much more likely to occur 8.6.26  Tuberculosis 1137 in children under the age of 15 years and in HIV-​infected patients than in immunocompetent adults. Although meningitis accounts for only a small fraction of all cases of tuberculosis, it is a devastating form of the disease that is uniformly fatal if left untreated. The pathogenesis of meningeal tuberculosis varies with the age and immunological status of the patient. Reactivation of micro- scopic granulomas in the meninges was found by Rich to cause diffuse meningeal infection. These foci of infection are probably implanted at the time of primary bacillaemia. When these lesions rupture into the subarachnoid space they invoke an inflammatory response leading to tuberculous meningitis. Meningeal disease can also complicate miliary disease, especially in children. Likewise, adults can acquire meningeal disease during bacillaemia of miliary disease, but this is not the usual pathogenesis of meningeal infection. Rarely, invasion into the spinal canal from a paraspinous or vertebral focus can also be the source of central nervous system involvement. The clinical features of tuberculous meningitis are the conse- quence of the pathophysiological process underlying the disease. Meningeal and leptomeningeal bacterial replication results in a ro- bust inflammatory reaction, often localized to the base of the brain. The number of bacilli present is usually limited, and the severity of illness is a function of the host response. Meningeal inflammation causes increases in cerebrospinal fluid pressure and can also cause cranial neuropathies. Patients complain of headache, neck stiff- ness, meningism, and an altered mental status, including irritability, clouded thinking, and malaise; as the disease progresses, symptoms worsen considerably. The clinical spectrum of tuberculous meningitis has historically been categorized in three stages, defined by the British Medical Research Council in 1948. Stage 1 consists of a prodrome lasting for 1 to 3 months. Non​specific symptoms such as fever, malaise, and headache predominate. In this stage, patients are conscious and ra- tional, but may have signs of meningism. Focal neurological signs are absent and there are no signs of hydrocephalus. In stage 2 dis- ease, single cranial nerve abnormalities such as ptosis or facial par- alysis appear, and paresis and focal seizures might occur. Kernig’s and Brudzinski’s signs have been noted as well as hyperactive deep tendon reflexes. Prominent signs include alterations in mentation, behavioural change, impaired cognitive ability, and increasing stupor. Headache and fever are also common features of this stage of disease. In stage 3, patients are comatose (Glasgow coma scale 8 or below) or stuporous and often have multiple cranial nerve palsies and hemi- plegia or paraplegia. By this stage, hydrocephalus is common and chronic inflammation in the enclosed space of the skull may result in significant intracranial hypertension. Seizures may be a prominent feature. Fever, headache, altered level of consciousness, and meningism are present in most patients in most large studies, although no one single sign or symptom has any reliable degree of sensitivity or spe- cificity. Children can be especially difficult to diagnose as symptoms such as fever, vomiting, drowsiness, or irritability are commonly seen in many minor viral illnesses. Transient tuberculous meningitis that presents as an aseptic men- ingitis and resolves without treatment has been described. Benign presentations of meningeal tuberculosis are uncommon in clinical practice, and when the diagnosis is made, treatment is mandatory, even in the patient with seemingly trivial symptoms. The diagnosis of tuberculous meningitis is often difficult and re- quires a high degree of suspicion. In the setting of disseminated disease, signs of tuberculosis in other organs, particularly the lungs, are often present. Between 25 and 50% of patients with meningitis in most series also have radiographic evidence of pulmonary tu- berculosis, either active or healed. The critical features of tuber- culous meningitis, however, are found in the cerebrospinal fluid. Patients with tuberculous meningitis usually have elevated cere- brospinal fluid pressure. An exudative fluid with a mononuclear cell pleocytosis is characteristic. Cerebrospinal fluid is usually clear and the protein is generally in the range of 100–​500 mg/​dl. Hypoglycorrhachia is typical, with cerebrospinal fluid glucose less than 50% of the serum value. The white blood cell count rarely ex- ceeds 1000 per µl, and cell counts below 500 are typical. In early meningitis, the cells may be predominantly neutrophils, but mono- nuclear cells predominate in most instances. Acid-​fast stains of concentrated cerebrospinal fluid are only positive in one-​third or fewer of patients, and cultures are positive in only one-​half, although repeated sampling increases the yield. The disastrous consequences of failing to diagnose tuberculous meningitis, coupled with the low yield of cerebrospinal fluid acid-​ fast stains and cultures, has prompted the development of additional tests for establishing a diagnosis. Adenosine deaminase was initially reported to be exceptionally accurate for tuberculous meningitis. Subsequent experience, however, has found it to be insufficiently specific to distinguish tuberculosis from a variety of other acute and chronic meningitides. Several other tests based on identification of mycobacterial antigens or specific antibodies have been evaluated, but none has been found to be reliable. Nucleic acid amplification tests such as polymerase chain reaction (PCR) have great appeal, but the sensitivity and specificity of available assays are only moderately good. Thus, the diagnosis of tuberculous meningitis often rests on the astute judgement of a clinician with a high degree of suspicion based on epidemiological and clinical clues. Presumptive therapy is frequently necessary. Central nervous system tuberculomas These are an unusual manifestation and are seen in a small pro- portion of patients with tuberculous meningitis. Tuberculomas are the result of enlarging tubercles that extend into brain paren- chyma rather than into the subarachnoid space. Patients with HIV infection appear to have an increased risk of central nervous system tuberculomas, but the disease is far less common than toxoplas- mosis, even in areas where tuberculosis is highly prevalent. Central nervous system tuberculomas may appear with clinical features of meningitis or of intracranial mass lesions. In the absence of menin- geal involvement, seizures or headaches may be the only symptoms. The diagnosis is suggested by brain imaging, with MRI scanning being more sensitive than CT scanning. Biopsy of the lesion is re- quired for diagnosis, and material should be submitted for histo- pathological staining and culture. Bone and joint tuberculosis These can affect several areas, but vertebral tuberculosis (Pott’s dis- ease) is the most common form, accounting for almost one-​half of cases. Haematogenous seeding of the anterior portion of vertebral bone during initial infection sets the stage for later development of Pott’s disease. Infection grows initially within the anterior vertebral section 8  Infectious diseases 1138 body, then may spread to the disc space and to paraspinous tissues. Destruction of the vertebral body causes wedging and eventual col- lapse. Patients usually complain of back pain, with constitutional symptoms less prominent. Neurological impairment is a late com- plication, but delays in diagnosis are common and many patients ex- perience neurological sequelae. Imaging studies of the spine usually reveal anterior wedging, collapse of vertebrae, and paraspinous ab- scesses. The diagnosis is established with bone biopsy or curettage, or by culture of the drainage from a paraspinous abscess. Abdominal tuberculosis Tuberculosis in the abdomen is relatively uncommon and can take two forms: (1) tuberculous enteritis and (2) peritoneal tuberculosis. The former was much more common in the era of unpasteurized milk and was due to M. bovis. Tuberculosis can occur in the intestines by a variety of means; swallowed sputum, haematogenous spread, or the ingestion of contaminated milk or food. The most common presentation is with non​specific abdominal pain. Diarrhoea occurs as a result of either bowel wall inflammation or partial obstruction, and sometimes rectal bleeding occurs. The ileo-​caecal region is most commonly af- fected and can result in a mass in the right iliac fossa. Tuberculous enteritis can mimic Crohn’s disease clinically, endoscopically, and radiographically. Bacilli are rarely seen in biopsies, so culture of tissue is essential in suspected cases. Peritoneal tuberculosis probably results from haematogenous spread from a pulmonary focus or, sometimes, spread from adja- cent enteric infection. Abdominal pain and fever, in association with other systemic symptoms of tuberculosis, are common but the main clinical presentation is with ascites. The ascitic fluid is lymphocytic with a high protein content, although the latter might not be seen in patients with cirrhosis (who are at increased risk of peritoneal tuber- culosis). Bacilli are rarely seen in the ascitic fluid so culture is essen- tial. Diagnosis is best made by biopsy of peritoneal tubercles under direct vision, either by laparoscopy or by mini-​laparotomy. One of the main differential diagnoses is ovarian cancer and it should be recognized that serum CA-​125 can be elevated in tuberculous peritonitis. Miliary tuberculosis and disseminated tuberculosis These are terms used interchangeably to describe widespread in- fection and the absence of minimal host immune responses. The term ‘miliary tuberculosis’ is derived from the classic radiographic appearance of haematogenous tuberculosis, in which tiny pul- monary infiltrates with the appearance of millet seeds are distrib- uted throughout the lungs. Miliary tuberculosis is a more common consequence of primary tuberculosis infection than reactivation, and is seen more frequently in children and immunocompromised adults. Primary miliary tuberculosis presents with fever and other constitutional symptoms over a period of several weeks. Clinical evaluation may reveal lymphadenopathy or splenomegaly and chor- oidal tubercles on retinoscopy. Laboratory tests might show only an- aemia. The chest radiograph is initially normal but later develops the typical miliary pattern. Involvement of multiple organ systems is the rule; usually liver, spleen, lymph nodes, central nervous system, and urinary tract. Patients with reactivation of latent infection who present with miliary disease may have a more fulminant course, al- though progression to severe disease without treatment is the rule in all patients. The diagnosis is made on tissue biopsy and culture, as sputum smears are usually negative, reflecting the small numbers of bacilli typically present in respiratory secretions. Other forms of extrapulmonary tuberculosis are less common than those listed earlier, and the diagnosis is based on a combination of clinical suspicion and the results of biopsies and cultures. Ocular, adrenal, and cutaneous tuberculosis are all rarely encountered in the modern era, even in immunocompromised patients. Diagnostic testing Evaluation of patients for M. tuberculosis infection or disease relies on both non​specific and specific tests. Imaging studies, body fluid chemistries and cell counts, and histochemical staining, as just de- scribed, are useful and important tests for the diagnosis of tuber- culosis. Specific studies for identifying mycobacterial infections include the tuberculin skin test, interferon-γ release assays, acid-​ fast microscopy, nucleic acid amplification tests, and mycobacterial culture. Tuberculin skin testing Tuberculin skin testing involves the intradermal injection of puri- fied proteins of M. tuberculosis (purified protein derivative, or PPD tuberculin) that provokes a cell-​mediated delayed-​type hypersen- sitivity reaction which produces a zone of induration. Tuberculin originated with Robert Koch who prepared a tubercle sensitin that he thought would cure tuberculosis. Administration of Koch’s tu- berculin, of course, did not cure the disease, and hypersensitivity reactions to the agent were sometimes severe or fatal, bringing Koch great discredit. Fortunately, it was recognized that because tuberculin induced reactions in people who were infected with tu- berculosis the substance might prove a better diagnostic test than treatment. Current tuberculin preparations are composed of pro- teins derived from culture filtrates and stabilized with a detergent to prevent precipitation. The standard dose of tuberculin is 5 tu- berculin units (TU) of PPD-​S, equivalent to 0.1 mg tuberculin in a volume of 0.1 ml. In recent years, a worldwide shortage of tuberculin has limited use of this technique. Tuberculin testing is used to identify people with M. tuberculosis infection, and the test cannot distinguish those who have disease from those with latent infection. Induration is the key feature of a tuberculin response, and the result of tuberculin testing is categor- ized according to the amount of induration measured. Because tuberculin contains antigens also found in non​tuberculous myco- bacteria, such as Bacille Calmette–​Guérin (BCG), false-​positive reactions occur. Tuberculin skin testing should be done by the Mantoux method of intradermal injection, as this is the only technique that has been standardized and extensively validated. Multipuncture devices should not be used. The amount of induration should be measured 2 to 7 days after the injection; measurements performed precisely 48 to 72 h later are not essential. The transverse diameter of induration should be measured in millimetres using a ruler. Criteria for the interpretation of tuberculin skin tests vary ac- cording to clinical and epidemiological circumstances. Cut-​off points for positive tests developed by the American Thoracic Society and the Centers for Disease Control and Prevention 8.6.26  Tuberculosis 1139 (CDC) are listed in Table 8.6.26.3. A cut-​off point of 5 mm indur- ation is used for individuals who are at high risk of tuberculosis infection, or at high risk of disease if infected. Such people include the close contacts of infectious patients and patients with radio- graphic abnormalities consistent with tuberculosis. The rationale for the 5-​mm cut-​off in these patients is that the prior probability of infection is high. A 5-​mm cut-​off is also used for HIV-​infected patients and those immunocompromised by corticosteroids or other agents. Failure to diagnose tuberculosis infection in these people could be calamitous, so a lower threshold is used to maxi- mize sensitivity. The use of control antigens such as candida or tetanus toxoid to aid the interpretation of tuberculin tests in HIV-​infected patients has been shown to be of no value and is not recommended. A cut-​off point of 10 mm induration is used for people from popu- lations with a high prevalence of tuberculosis or for individuals with conditions that increase the risk of developing active disease if in- fected. This would include immigrants from endemic areas, resi- dents of some inner cities, and healthcare workers, as well as patients with diabetes, renal disease, silicosis, and other medical conditions associated with an elevated risk of reactivation of latent tuberculosis. Finally, a cut-​off of 15 mm is used in people who have no risk fac- tors for tuberculosis infection or disease. In most instances, these patients would not be tested. Tuberculin testing does have limitations in both sensitivity and specificity. False-​negative tuberculin tests result from both errors in ap- plying and interpreting the test and from anergy. Errors in in- jection of tuberculin are common, and inter-​reader variability in measuring results is high. Fortunately, if there is doubt about the interpretation of a skin test, multiple readers can measure the result over a period of days, or the test can be repeated and re- interpreted. Specific anergy to tuberculin is seen in several situ- ations. Approximately 10–​20% of patients with culture-​confirmed pulmonary tuberculosis fail to respond to tuberculin as a result of anergy. These patients often will mount a response after their disease has been treated. HIV-​infected patients have a high preva- lence of anergy, both to tuberculin and other antigens. Only 10–​ 40% of patients with low CD4 counts and confirmed tuberculosis respond to tuberculin. Transient anergy is associated with acute viral infections such as measles, live virus vaccinations, and other acute medical illnesses. Interferon-​γ release assays Tuberculin skin testing is frustratingly crude and somewhat cum- bersome, but despite its limitations has proved superior to nu- merous more ‘modern’ assays including antibody tests and other in vitro immunodiagnostics. Recently, however, the use of assays to detect interferon-​γ production by sensitized T cells in response to challenge with specific antigens from the RD1 region of the M. tu- berculosis genome has shown promise as an alternative to tuberculin testing. Two commercial interferon-​γ release assays, one an enzyme-​ linked immunospot (T-​SPOT-​TB) and one an enzyme-​linked im- munosorbent assay (ELISA) (Quantiferon TB Gold-​In Tube or Quantiferon Plus) are now approved in several countries for in vitro diagnosis of tuberculosis infection. These assays are more than 90% sensitive for active tuberculosis and more specific than tuberculin testing in BCG-​vaccinated individuals, correlate better than tuber- culin skin testing with exposure to a point source of infection, and may not be compromised by immunosuppression related to HIV in- fection. In some studies, these assays have greater sensitivity than tuberculin skin testing and almost always have better specificity. In evaluating individuals with latent tuberculosis infection, however, the lack of a gold standard of diagnosis makes comparisons diffi- cult. However, emerging evidence suggests that interferon-​γ release assays may be more accurate than tuberculin testing in predicting which people are at greatest risk of developing subsequent active tuberculosis disease. Thus, the assays have enormous potential and Table 8.6.26.3  Criteria for tuberculin positivity, by risk group Reaction ≥5 mm induration Reaction ≥10 mm induration Reaction ≥15 mm induration HIV-​positive persons Recent immigrants (i.e. within the last 5 years) from high-​prevalence countries or regions Persons with no risk factors for tuberculosis Recent contacts of infectious tuberculosis patients Injection drug users Persons with fibrotic changes on chest radiograph consistent with prior tuberculosis Residents and employees of the following high-​risk congregate settings: Prisons and jails Nursing homes and other long-​term facilities for older people Hospitals and other healthcare facilities Residential facilities for patients with AIDS Homeless shelters Patients with organ transplants and other immunosuppressed patients (receiving the equivalent of ≥15 mg/​day prednisone for 1 month or more) Persons with the following clinical conditions that place them at high risk: Silicosis Diabetes mellitus Chronic renal failure Some haematological disorders (e.g. leukaemias and lymphomas) Other specific malignancies (e.g. carcinoma of the head or neck and lung) Weight loss of ≥10% of ideal body weight Gastrectomy Jejunoileal bypass Others Mycobacteriology laboratory personnel Children <4 years of age or infants, children, and adolescents exposed to adults at high risk section 8  Infectious diseases 1140 might contribute to improved detection of both active and latent tu- berculosis infections. One serious limitation of interferon-​γ release assays is high rates of reversion of positive tests in individuals undergoing serial testing, including healthcare workers. Several studies have shown that among healthcare workers in low incidence areas, between 40 and 70% of initially positive individuals become negative when re-​tested 6–​18 months later. Management of positive tests in people at low risk of infection, such as healthcare workers in settings with little or no exposure to the disease, is challenging and underscores the wisdom of the 2000 ATS/​CDC guidelines for tuberculin testing: a decision to test is a decision to treat. Only people felt to be at risk for M. tuberculosis infection and who should be treated, if positive, should be tested. Microscopic staining Microscopic staining of acid-​fast bacilli is the method most widely used to diagnose tuberculosis throughout the world. Acid-​fast staining is inexpensive, rapid, and technologically undemanding, making it an attractive technique for identifying mycobacterial in- fections. The waxy glycolipid matrix of the mycobacterial cell wall is resistant to acid–​alcohol decolorization after staining with carbol-​ fuchsin dyes, and red bacilli are visible after counterstaining. Both the Ziehl-​Neelsen method (which requires heat fixation) and the Kinyoun method utilize methylene blue or malachite green counter- stains, and have similar sensitivities for identifying acid-​fast bacilli in clinical specimens. The major limitation of acid-​fast staining is that a relatively large number of bacilli must be present to be seen microscopically. Acid-​fast smears are generally negative when there are fewer than 10 000 bacilli/​ml of sputum, and many microscope fields need to be examined to identify bacilli even when there are 10 000 to 50 000 bacilli/​ml. Thus, up to 50% of patients with sputum cultures positive for M. tuberculosis have negative acid-​fast smears. In settings where the sputum smear is the only test done to confirm tuberculosis, many smear-​negative cases go undetected. This is a serious problem for patients without cavitary tuberculosis, who tend to have fewer bacilli in their sputum, including many HIV-​infected tuberculosis patients in developing countries. Several techniques can be used to improve the yield of sputum smears. The most important method is enrichment of the spe- cimen through concentration of the sputum. Centrifugation of sputum allows examination of the bacilli-​rich pellet, which improves the sensitivity of smears substantially. Treatment of sputum with mucolytic agents is also helpful in identifying or- ganisms by both smear and culture. Use of fluorochrome proced- ures to identify mycobacteria is more sensitive, but less specific, than acid-​fast stains. Auramine O or auramine-​rhodamine dyes are used on concentrated smears and examined under a fluor- escence microscope. This technique allows much more rapid screening of slides than the traditional methods, but confirm- ation of positive results with Ziehl-​Neelsen or Kinyoun staining is essential, as false-​positive fluorochrome results are not un- common. Fluorescence microscopy has been limited historically to well-​equipped reference laboratories, but the introduction of light-​emitting diode (LED)-​based fluorescent microscopes has substantially lowered the cost of this technology and increased availability in resource-​limited areas. The proper collection of specimens is also important for opti- mizing the results of microscopy and culture. Early morning sputum specimens tend to have a higher yield than specimens collected at other times, and overnight sputum collections have provided even greater sensitivity. Morning gastric aspirates have a moderate yield for acid-​fast bacilli in children, who generally have a difficult time producing sputum. Sputum induction with hypertonic saline is useful in evaluating patients with minimal or no sputum produc- tion, and the use of fibreoptic bronchoscopy is often advocated for patients with negative sputum smears. Examination of multiple specimens increases the sensitivity of sputum microscopy for acid-​fast bacilli. The first smear identifies 70–​80% of patients, the second another 10–​15%, and the third an- other 5–​10%. Review of additional specimens has little value. In addition to the modest sensitivity of acid-​fast staining, the specificity of this technique can also present problems. The mor- phological properties of the mycobacteria are sufficiently similar to make distinguishing M. tuberculosis from non​tuberculous myco- bacteria impossible on the basis of acid-​fast smears. This is not a serious problem where tuberculosis is common and non​tuberculous mycobacterial infections are unusual. However, in many industrial- ized countries, disease due to the non​tuberculous mycobacteria is relatively common, and distinguishing these types of infections has important therapeutic and public health implications. Thus, while sputum microscopy is useful because of its rapidity and low cost, it should be supplemented with culture or other more sensitive and specific tests whenever feasible. Culture, nucleic acid amplification, and susceptibility testing Culture of M. tuberculosis This is the gold standard for confirming the diagnosis of tubercu- losis. A variety of media are available that support the growth of mycobacteria, including egg-​based and potato-​based solid media and several broth-​based media. The intrinsic growth rate of M. tu- berculosis makes the recovery of the organism in culture a slow pro- cess. In traditional egg-​based media such as Lowenstein–​Jensen, growth of colonies of M. tuberculosis takes between 3 and 6 weeks, and 7H11 agar requires an average of 3 to 4 weeks to show colonies. Obviously, the slow pace of these traditional culture systems inter- feres with optimal patient management, and more rapid techniques are required. Several faster (not rapid) systems for detection of mycobacteria in culture have been commercially developed. The Mycobacterial Growth Indicator Tube (MGIT) is a broth-​based system that uses fluorescence detection to monitor growth. Both manual and automated systems are available. Once growth is detected, staining to identify acid-​fast organisms and species identifica- tion need to be performed. The time to detection of mycobac- teria using MGIT is considerably faster than conventional solid media, and the yield can be appreciably higher. Contamination of cultures with bacteria and fungi is common, and laboratory cross-​contamination remains a concern. Nevertheless, the use of MGIT can increase case detection rates and speed the time to de- tection of tuberculosis. Many clinical laboratories use more than one culture system for mycobacteria, both to increase the overall recovery rate and to 8.6.26  Tuberculosis 1141 provide quality control. In addition, if one culture becomes contam- inated, alternative cultures can still be utilized. Preparation of specimens for mycobacterial culture This follows the same steps as outlined for acid-​fast smears. In addition, specimens being submitted for culture also require de- contamination to prevent overgrowth by more rapidly multiplying bacteria. Sodium hydroxide (NaOH) and N-​acetyl-​l-​cysteine are commonly used together for mucolysis and decontamination. By necessity, decontamination also inactivates more than 50% of mycobacteria in a specimen, thereby reducing the potential yield of the culture. Failure to decontaminate, however, leads to bacterial overgrowth and uninterpretable results. Lack of growth as a result of overdecontamination and bacterial overgrowth resulting from underdecontamination underscore the importance and utility of obtaining multiple specimens for culture, when possible. As with sputum smears, the yield of mycobacterial culture increases with evaluation of additional specimens. Speciation After mycobacterial growth has been identified, speciation of the organism is required. Conventional techniques for identification of mycobacterial species involve characterization of colony morph- ology, pigmentation, rate of growth, and biochemical tests. Niacin reduction, nitrate reduction, and lack of catalase activity at elevated temperatures are all characteristic of M. tuberculosis. Species iden- tification using these methods is time consuming and tedious, and further delays the diagnosis of tuberculosis. The use of nucleic acid probes has dramatically simplified speciation of mycobacteria in recent years. DNA probes that react with specific mycobacterial rRNA sequences to form DNA–​RNA hybrids that can be readily detected by chemoluminescence are commercially available for M. tuberculosis, M. avium complex, M. kansasii, and M. gordonae. These probes can be performed within hours of detection of myco- bacterial growth, and significantly accelerate the diagnosis of specific pathogens. The sensitivity of these probes is approximately 90–​95%, depending on the species, with specificities approaching 100%. Cultures that fail to respond to any of the DNA–​RNA probes are al- most always due to another mycobacterial species, but final identifica- tion depends on the laborious biochemical techniques of old. Nucleic acid amplification The difficulties of identifying mycobacteria in patient specimens ac- centuate the need for rapid and sensitive diagnostic methods for tu- berculosis. Recently, several commercial nucleic acid amplification tests have been introduced, including assays based on reverse tran- scription (RT)-​PCR, transcription-​mediated amplification, ligase chain reaction, and strand displacement amplification. All of these techniques use specific M. tuberculosis DNA sequences as targets for nucleic acid amplification. The great advantage of these assays is that they can provide results within 1 day of the collection of specimens. Their disadvantage is that they are uniformly less sensitive than cul- ture, particularly in sputum smear-​negative patients. Early studies also suggested that specificity was excellent overall but was reduced in smear-​positive samples; further refinements in these assays have resulted in improved sensitivity and specificity, and their diagnostic role in both smear-​negative sputum and extrapulmonary disease has grown accordingly. Recent advances in DNA amplification have made rapid detection of M. tuberculosis gene sequences more easily performed, marking a potentially revolutionary change in the diagnosis of tuberculosis. Line probe assays, using solid-​phase PCR techniques to identify signature sequences from M. tuberculosis can identify up to 99% of sputum smear-​positive specimens and between 50 and 70% of smear-​negative specimens. The turn-​around-​time for these assays is 4–​8 hours, and most results can be returned, in theory, within a few days, dramatically accelerating the diagnosis. In addition, as dis- cussed next, line probe assays can also be used to detect resistance mutations associated with isoniazid, rifampicin, ethambutol, and several second-​line antituberculosis drug resistance. Unfortunately, rapid genetic testing for pyrazinamide resistance, a key predictor of response to treatment of MDR and XDR tuberculosis, is not yet possible with these assays. Another new addition to the diagnostic armamentarium is the Xpert MTB/​RIF® assay. This commercial kit uses molecular bea- cons to identify both M. tuberculosis gene sequences and specific mutations responsible for more than 95% of all rifampicin resist- ance. It detects M. tuberculosis complex and rifampicin resistance by PCR amplification of rpoB gene sequences. The test kit is con- tained in a small cartridge into which sputum is placed, and the entire process is automated within the cartridge and a tabletop machine, with results returned in 90–​120 minutes. The sensitivity of the assay is more than 98% for smear-​positive sputum and 60–​ 70% for smear-​negative samples, with a sensitivity of more than 95% for rifampicin-​resistance. The ability to provide a diagnosis of tuberculosis and determine rifampicin-​susceptibility within 2 h is of enormous importance, and this assay is now being rolled out through much of the world. Conventional drug suscepti- bility testing is still required for patients found to have rifam- picin resistance, but from a clinical perspective the diagnosis of rifampicin-​resistant tuberculosis is tantamount to MDR tubercu- losis in most settings, and treatment decisions can be based on the Xpert ­result. A new version of the Xpert platform, Xpert Ultra, has a new cartridge design that has greatly improved sensitivity and retains good specificity. Antigen testing A rapid, point-​of-​care test that allows diagnosis of tuberculosis at the bedside is the Holy Grail of tuberculosis control, but such a tool has been challenging to develop. One approach evaluated in re- cent years is testing urine for the M. tuberculosis cell wall antigen lipoarabanomannin, using lateral flow technology similar to home pregnancy tests. Several early studies found lipoarabanomannin present in urine of a high proportion of hospitalized HIV-​infected patients with tuberculosis, but subsequent studies found very low yields (<15%) in ambulatory patients with or without HIV infec- tion, though specificity is high. The WHO currently recommends that bedside urine lipoarabanomannin testing be used in hospital- ized patients with HIV and CD4 cell counts less than 100 suspected of having tuberculosis. Here, sensitivity might be as high as 50–​60%, and when combined with Xpert testing can result in a diagnosis in up to 90% of patients. Drug susceptibility testing Susceptibility testing of M. tuberculosis isolates is essential for both clinical management and public health purposes. Susceptibility tests section 8  Infectious diseases 1142 for the first-​line antituberculosis drugs should be performed on at least one culture at the time of diagnosis for all patients. If the initial isolate is susceptible to the first-​line agents and treatment proceeds without incident, additional susceptibility tests are not required. Susceptibility testing should be performed for patients who relapse with tuberculosis and for patients who are treatment failures after 3–​4 months of therapy. Conventional susceptibility testing for M. tuberculosis uses standard concentrations of antituberculosis drugs to measure in- hibition of bacterial growth in culture. Drugs tested routinely include isoniazid, rifampicin, pyrazinamide, ethambutol, and streptomycin. Testing of second-​line antituberculosis drugs is only done when resistance to the first-​line agents is documented or strongly suspected. Susceptibility testing is generally performed on subcultures of the primary isolate, though direct inoculation of sputum or other spe- cimens can be performed in the case of a strongly positive acid-​fast bacilli smear. The standard method for measuring susceptibility to antituberculosis drugs is the proportions method. The organism is grown on agar plates in the presence of known concentrations of specific drugs. Growth on the plates is then compared with growth on control plates. By convention, if the test plate shows a colony count that is more than 1% of the control value, the isolate is re- sistant. Laboratories will report the isolate as being susceptible or resistant to the concentration of the drug used in the assay. Another method for susceptibility testing is to use the MGIT system, in which culture bottles contain antituberculosis drugs. Growth indices are compared to control cultures to determine sus- ceptibility. The MGIT system provides results more quickly than the proportions method, is automated, but is more expensive. Recently, the microscopic examination of growth in wells that are filled with liquid culture medium (MODS) has been reported to enable de- tection within about 10 days and permit rapid assessment of drug resistance. This technique has some promise in resource-​limited settings, but it is labour intensive and needs further validation in endemic countries. As noted earlier, the use of molecular methods to determine tuberculosis drug susceptibility is a major advance. Specific mu- tations in M. tuberculosis have been identified which confer resist- ance to antituberculosis drugs. For example, mutations in a small region of the rpoB gene of M. tuberculosis are responsible for more than 90% of all rifampicin resistance. Sequencing of this portion of the genome using a variety of techniques has been shown to be feasible in research laboratories. Rapid identification of rifam- picin resistance by molecular methods (line probe assay) would be of enormous clinical benefit, as almost all rifampicin-​resistant M. tuberculosis isolates are also resistant to isoniazid and are, by definition, multidrug resistant. Thus, early detection of resistance mutations would allow early initiation of appropriate treatment and infection control measures. A point-​of-​care nucleic acid amplifica- tion test, such as the Xpert MTB-​RIF, can detect at least rifampicin resistance, and a new cartridge design that detects mutations asso- ciated with resistance to other drugs has recently been introduced. Among culture-​positive patients, a single, Xpert MTB/​RIF test done on sputum directly had 98.2% and 72.5% sensitivity in smear-​ positive and negative tuberculosis, respectively; and a 99.2% speci- ficity in patients without tuberculosis. Molecular diagnosis of other types of resistance has been bolstered by whole genome sequencing, which allows a clearer understanding of associations between gen- etic polymorphisms and phenotypic resistance. Treatment of active tuberculosis The treatment of tuberculosis requires the use of a combination of antimycobacterial drugs active against the strain of M. tuberculosis causing the patient’s disease. The use of multiple agents is necessitated by the emergence of drug resistance when single agents are used. Mutations that confer resistance to antimycobacterial drugs arise spontaneously in wild-​type populations of M. tuberculosis in frequen- cies ranging from 1 in 105 to 1 in 108 bacilli. In the presence of large numbers of organisms, such as are present during active pulmonary disease, a single agent will kill susceptible bacilli, but naturally drug-​ resistant mutants will survive and eventually emerge to cause drug-​ resistant disease. Since the mechanisms of resistance are genetically distinct and arise independently, multiple drug resistance within a single organism is exceedingly rare in nature. The use of two or more agents with different mechanisms of action assures that populations of drug-​resistant bacilli are not selected for during therapy. Antituberculosis drugs These are divided into first-​line and second-​line agents. The first-​ line agents are widely available and used routinely in the treatment of tuberculosis, while the second-​line agents are generally less po- tent, more toxic, and less readily available. Exceptions are the newer fluoroquinolones, such as moxifloxacin and levofloxacin, which ap- pear to have good activity against M. tuberculosis. The addition of moxifloxacin to standard treatment has not been shown to shorten the duration of tuberculosis treatment, however. Second-​line drugs are reserved for the treatment of drug-​resistant tuberculosis. Table 8.6.26.4 lists the first-​line antituberculosis drugs, their activity in the treatment of tuberculosis, and common toxicities. Regimens currently used for the treatment of tuberculosis have been developed on the basis of trials conducted by the British Table 8.6.26.4  Drugs for the treatment of tuberculosis Agent Activity Toxicity Isoniazid Bactericidal Liver, peripheral nerve, hypersensitivity Rifampicin Bactericidal and sterilizing Liver, gastrointestinal, discoloration of body fluids, nausea, haematological Rifapentine Bactericidal and sterilizing Liver, gastrointestinal, discoloration of body fluids, nausea, haematological Pyrazinamide Sterilizing Liver, hyperuricaemia, gout, malaise, gastrointestinal Ethambutol Bacteriostatic (dose-​dependent) Liver, optic neuritis, skin 8.6.26  Tuberculosis 1143 Medical Research Council since the late 1970s. By combining drugs that target both rapidly growing bacillary populations and slow-​growing or semi-​dormant organisms within cells, modern short-​course chemotherapy can successfully cure drug-​susceptible pulmonary tuberculosis in 6 months. The regimens recommended for treatment of drug-​susceptible tuberculosis are shown in Table 8.6.26.5. Treatment of extrapulmonary tuberculosis is generally for the same duration as for pulmonary disease, with the exceptions of bone and joint and central nervous system tuberculosis, which are treated for 12 months. The dynamics of mycobacterial growth are such that treatment need be administered only once daily, and can be given as infre- quently as once a week in some circumstances. The long generation time of M. tuberculosis and a postantibiotic effect of antituberculosis drugs make more frequent drug dosing unnecessary. The dosages for drugs are listed in Table 8.6.26.6 according to the frequency with which they are administered. Isoniazid remains a key component of treatment because of its high bactericidal activity. Rifampicin is essential for short-​ course therapy because it is active against all populations of ba- cilli, both within and outside of cells. Pyrazinamide is uniquely active during the first 2 months of therapy, but appears to have no activity thereafter. The addition of pyrazinamide to the treatment regimen allows the duration to be reduced from 9 to 6 months, however. Ethambutol has bacteriostatic activity at lower doses and bactericidal activity at high doses. This agent is primarily given to prevent the emergence of drug resistance, as it appears to add little activity to combination regimens against drug-​susceptible tuberculosis. Although antituberculosis therapy is remarkably well tolerated and almost always given on an ambulatory basis, important drug toxicities do exist. The most serious adverse drug reaction during tuberculosis treatment is liver toxicity, which may occur in up to 5 to 10% of treated patients. Isoniazid, rifampicin, and pyrazinamide are all associated with liver toxicity and use of these agents to- gether increases the risk of a reaction. Isoniazid is the agent most frequently implicated when reactions occur. Isoniazid can produce an idiosyncratic hepatocellular injury, manifested by elevated liver enzymes and clinical hepatitis. Elevation of transaminases does not always portend the development of hepatitis, but may serve as an important signal to anticipate clinical toxicity. The development of signs and symptoms of hepatitis, such as abdominal pain, nausea, vomiting, or jaundice, requires immediate discontinuation of iso- niazid, as continuing treatment can result in death from hepatic Table 8.6.26.5  2016 American Thoracic Society/​Centers for Disease Control and Prevention/​Infectious Disease Society of America/​Treatment Guidelines for drug-​susceptible tuberculosis in children and adults Regimen Intensive Phase Continuation Phase Range of total doses Comment Drugs Interval and doses (minimum duration) Drugs Interval and doses (minimum duration) 1 INH RIF PZA EMB Seven days per week for 56 doses (8 weeks), or five days per week for 40 doses (8 weeks) INH RIF Seven days per week for 126 doses (18 weeks), or five days per week for 90 doses (18 weeks) 182 to 130 This is the preferred regimen for patients with newly diagnosed pulmonary tuberculosis. 2 INH RIF PZA EMB Seven days per week for 56 doses (8 weeks), or five days per week for 40 doses (8 weeks) INH RIF Three times weekly for 54 doses (18 weeks) 110 to 94 Preferred alternative regimen in situations in which more frequent DOT during continuation phase is difficult to achieve. 3 INH RIF PZA EMB Three times weekly for 24 doses (8 weeks) INH RIF Three times weekly for 54 doses (18 weeks) 78 Use regimen with caution in patients with HIV and/​or cavitary disease. Missed doses can lead to treatment failure, relapse, and acquired drug resistance. 4 INH RIF PZA EMB Seven days per week for 14 doses then twice weekly for 12 doses5 INH RIF Twice weekly for 36 doses (18 weeks) 62 Do not use twice weekly regimens in HIV-​ infected patients or patients with smear-​ positive and/​or cavitary disease. If doses are missed then therapy is equivalent to once weekly, which is inferior. Reproduced from Nahid P et al. (2016). 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(7), e147–​95 with permission from Oxford University Press. Table 8.6.26.6  Dosage recommendation for the initial treatment of tuberculosis in children and adults Drugs Daily dose Twice-​weekly dose Thrice-​weekly dose Children Adults Children Adults Children Adults Isoniazid (mg/​kg) 10–​20 (max. 300 mg) 5 (max. 300 mg) 20–​40 (max. 900 mg) 15 (max. 900 mg) 20–​40 (max. 900 mg) 15 (max. 900 mg) Rifampicin (mg/​kg) 10–​20 (max. 600 mg) 10 (max. 600 mg) 10–​20 (max. 600 mg) 10 (max. 600 mg) 10–​20 (max. 600 mg) 10 (max. 600 mg) Pyrazinamide (mg/​kg) 15–​30 (max. 2 g) 15–​30 (max. 2 g) 50–​70 (max. 4 g) 50–​70 (max. 3.5 g) 50–​60 (max. 3.5 g) 50–​60 (max. 3.5 g) Ethambutol (mg/​kg) 15–​25 (max. 1.5 g) 15–​25 (max. 1.5 g) 50 (max. 4 g) 50 (max. 4 g) 25–​30 25–​30 Streptomycin (mg/​kg) 20–​40 (max. 1.0 g) 15 (max. 1.0 g) 25–​30 (max. 1.5 g) 25–​30 (max. 1.5 g) 25–​?30 (max. 1.5 g) 25–​30 (max. 1.5 g) section 8  Infectious diseases 1144 failure. Risk factors for developing isoniazid hepatotoxicity include increasing age, chronic liver disease, alcohol abuse, daily dosing of isoniazid, and use of other hepatotoxic drugs, including rifampicin and pyrazinamide. In addition, individuals with a slow isoniazid acetylation genotype are significantly more likely to develop hep- atotoxicity from the drug than intermediate or rapid acetylators. Isoniazid interferes with metabolism of pyridoxine (vitamin B6) which can result in a sensory neuropathy. Coadministration of pyridoxine with isoniazid abrogates this effect without comprom- ising the antimicrobial activity. Rifampicin also causes hepatotoxicity, although the character- istic picture of liver disturbances due to rifampicin is cholestasis. However, the incidence of hepatotoxicity when rifampicin is given with isoniazid is substantially greater than when isoniazid is given alone. Rifampicin predictably causes a discoloration of body fluids, resulting in orange-​tinted tears, sweat, and urine. Haematological toxicity from rifampicin includes thrombocyto- penia and anaemia. Higher doses of rifampicin may produce a hypersensitivity reaction, with fever, rash, and joint swelling. It is for this reason that doses of rifampicin are not escalated during intermittent therapy, whereas the intermittent dosages of the other drugs are increased to deliver weekly doses that are equiva- lent to daily dosing. Pyrazinamide is often associated with arthralgias, and may pre- cipitate gout. Pyrazinamide inhibits renal tubular uric acid excre- tion, resulting in increased serum uric acid levels. Frank gouty arthritis is relatively uncommon with pyrazinamide use, and its fre- quency is reduced with intermittent dosing. Routine use of allopur- inol to prevent gout is not recommended. The major toxicity of ethambutol is optic neuritis, which is common at doses above 30 mg/​kg daily and unusual at doses below 25 mg/​kg daily. Patients receiving ethambutol should have baseline tests of visual acuity and colour discrimination, with monthly moni- toring while on treatment. Ethambutol use is discouraged in chil- dren under 8 years old because of their inability reliably to report visual disturbances. However, the incidence of optic neuritis with the doses of ethambutol typically used is so low that its use in young children is only relatively contraindicated. Monitoring of therapy Patients receiving therapy for tuberculosis require regular moni- toring to assess adherence with therapy, clinical response, and adverse reactions. In the initial phase of therapy, monitoring by a nurse or other trained clinician at least weekly is recommended, and supervision of every dose of medication is suggested by the WHO and other authorities (see next). Patients should be ob- served for clinical responses, including fever defervescence, im- provement in cough and appetite, and weight gain. Improvement in these symptoms and signs can take several weeks, but usually occurs within 3 weeks after starting treatment. Failure to improve suggests that the patient is not adhering to treatment, has drug-​ resistant tuberculosis, or has another illness in addition to, or in- stead of, tuberculosis. Treatment response should also be documented with repeated sputum smears and cultures and a follow-​up chest radiograph after 2 to 3 months (for pulmonary tuberculosis). All patients should have a repeat sputum smear and culture after 2 months of therapy; those who are smear or culture positive at 2 months should have another at 3 months. Failure to convert sputum smears and cultures to negative with 3 months of therapy is associated with a high risk of treatment failure; patients who are still smear or culture posi- tive at 4  months of treatment are considered treatment failures and should be evaluated for drug-​resistant disease. A culture at the end of therapy is recommended to document cure, while an end of therapy radiograph is not necessary. Because mycobacterial DNA can remain in pulmonary secretions long after the disease is effect- ively treated, monitoring with nucleic acid amplification tests is not recommended. Monitoring for drug toxicity is also required throughout therapy. At least monthly monitoring for symptoms and signs of liver toxicity is essential, and patients should be advised to stop therapy and seek care if evidence of hepatitis is noted. Routine liver enzyme moni- toring is recommended primarily for patients with underlying liver disease or baseline abnormalities in liver enzymes. Patients with symptoms of hepatitis, of course, should have liver studies obtained. As already noted here, monthly visual assessment is also recom- mended when ethambutol is given. Adherence to therapy and directly observed therapy Since the 1960s experts in tuberculosis have noted that the suc- cess of treatment depends largely on adherence to therapy. Poor adherence to therapy is responsible for treatment failures, early relapses, and the emergence of drug-​resistant disease. Two major interventions to improve adherence and prevent poor outcomes are directly observed therapy (DOT) and the use of fixed-​dose combination tablets. DOT was first promoted in the 1950s in India, and experience with DOT grew over the ensuing years. Intermittent dosing of tuberculosis therapy, along with the rela- tively short course of treatment, make supervision of treatment feasible in many settings. Ecological and programmatic studies of DOT programmes have shown that the introduction of DOT im- proves cure rates for tuberculosis, reduces non​adherence, and re- duces the emergence of drug-​resistant disease. Two observational studies have shown better survival of HIV-​infected tuberculosis patients who receive DOT. On the other hand, two randomized trials of DOT in developing countries have not found improved treatment completion rates compared with self-​administered treatment. These trials have been criticized for demonstrating only that DOT can be done badly, but the lack of randomized studies documenting that DOT per se leads to improved outcomes is of some concern. The data from observational studies are compelling, however, and DOT has been shown to be cost-​effective in resource-​limited settings and, therefore, is strongly encouraged by many experts and pro- fessional organizations. Use of wireless technologies, including secure video links, is a modern ­alternative to in-person supervi- sion of therapy. The use of fixed-​dose combination tablets is intended to reduce the risk of selecting for drug resistance, as opposed to improving adherence generally. By combining two, three, or four medica- tions in the same tablet, depending on the regimen being used, the opportunity for patients to receive partial treatment that would select for drug resistance is avoided. The bioequivalence of fixed-​dose combinations to individual medications has been established for some, but not all, of the combination products on the market. 8.6.26  Tuberculosis 1145 The catastrophic state of global tuberculosis control in the 1990s led the WHO to promulgate the directly observed therapy, short-​ course (DOTS) strategy. This strategy is a series of policies related to national tuberculosis control practices. The five elements of the DOTS strategy are: 1 Governmental commitment to tuberculosis control 2 A reliable supply of tuberculosis drugs 3 Diagnosis of tuberculosis cases microscopically 4 A registration system for tracking the outcomes of treatment 5 Supervision (DOT) of at least the first 8 weeks of treatment The DOTS strategy has been extremely successful in focusing attention on serious problems in tuberculosis treatment and con- trol, and implementation of the programme in several countries has produced remarkable improvements in clinical outcomes for patients with tuberculosis. There is strong evidence that the use of the DOTS strategy results in lower rates of drug-​resistant tu- berculosis. However, further expansion of the DOTS strategy and improvements in tuberculosis treatment programmes are clearly needed. Treatment of multidrug-​resistant tuberculosis This is beyond the scope of this chapter. Patients with drug-​ resistant tuberculosis should be managed by a physician who is a tuberculosis expert. Effective treatment and cure of multidrug-​ resistant tuberculosis (MDR-​TB) requires use of a combination of drugs that include second-​line drugs which are less effective than first-​line agents, have a greater toxicity, or demonstrate both disadvantages. In recent years, a shorter course of treatment with the so-​called ‘Bangladesh regimen’ of seven antituberculosis drugs has shown high MDR tuberculosis cure rates with just nine months of treatment. In addition, two new drugs, bedaquiline and delamanid, have been shown to speed the time to culture conversion and cure when given in combination with other second-​line drugs and will be a significant advance to further improve outcomes of such patients. Evidence from clinical trials and programmatic data from South Africa show that the use of bedaquiline to treat MDR-TB greatly improves survival and clin- ical outcomes. In 2018, the WHO recommended routine use of bedaquiline for treating MDR-TB and the elimination of inject- able agents. Supervised therapy is considered mandatory for pa- tients with resistant tuberculosis. Physician mistakes remain one of the leading causes of the emergence of multidrug-​resistant and extensively drug-​resistant tuberculosis (XDR-​TB), and the identi- fication of a drug-​resistant isolate of M. tuberculosis should result in immediate expert consultation. It is also clear that addressing drug-​resistant tuberculosis cannot be accomplished without ad- dressing the overall tuberculosis control effort. Treatment of tuberculosis in HIV-​infected people The United States (ATS/​CDC/​Infectious Disease Society of America) recommendations for the treatment of tuberculosis in HIV-​infected adults are, with a few exceptions, the same as those for HIV-​uninfected adults (i.e. standard 6-​month rifampicin-​ based therapy). The development of acquired rifampicin resist- ance has been noted among HIV-​infected patients with advanced immune suppression treated with twice weekly rifampicin-​based or rifabutin-​based regimens. Consequently, patients with HIV infection and tuberculosis should receive daily treatment. DOT and other adherence-​promoting strategies are especially important for patients with HIV-​related tuberculosis. A series of randomized clinical trials has shown that treatment outcomes, both survival and progression of HIV disease, are better among tuberculosis patients coinfected with HIV who start antiretroviral therapy within two to eight weeks of the initiation of tuberculosis treatment. For those in- dividuals with CD4 + T-​cell counts less than 50 cells/​mm3, survival is improved if treatment is initiated within two weeks, while for those with CD4 + counts more than 50 cells/​mm3 outcomes are better if antiretroviral therapy is started within eight to 12 weeks after tuber- culosis treatment begins. Drug interactions There are three possible complications that arise when tubercu- losis treatment and antiretroviral drugs are coadministered: shared side effects and toxicity, drug interactions arising from the in- duction of metabolism (cytochrome P450 enzymes) and efflux pumps by rifampicin, and the immune reconstitution inflamma- tory syndrome. Rifamycins induce the activity of cytochrome P450 enzymes that are important in drug metabolism. Several key anti- retroviral drug classes, protease inhibitors, non​nucleoside reverse transcriptase inhibitors and integrase inhibitors, are substrates of cytochrome P450 enzymes. Protease inhibitors are also substrates of P-​glycoprotein, which is also induced by rifamycins. The avail- able rifamycins differ in potency as P450 enzyme inducers, with ri- fampicin and rifapentine being the most potent and rifabutin the least. Coadministration with rifampicin reduces the concentrations of non​nucleoside reverse transcriptase inhibitors and integrase inhibitors to a moderate extent, but dramatically reduces the con- centrations of protease inhibitors. Rifabutin does not significantly affect the concentrations of ritonavir-​boosted protease inhibitors and is recommended when protease inhibitors have to be used. However, the use of rifabutin in low resource settings is currently limited due to its very high cost and the widespread use of fixed-​ dose combination antituberculosis drugs that include rifampicin. Patients diagnosed with tuberculosis while receiving antiretroviral therapy should continue the regimen without change with the ex- ception of those patients receiving a protease-​based regimen (e.g. lopinavir/​ritonavir) who should have their dose slowly doubled to 800/​200 mg twice daily during a 2-​week period. Such increase should continue for the duration of tuberculosis treatment and for an additional 2 weeks after the conclusion. While the patient is receiving the increased dose of lopinavir/​ritonavir, transamin- ases should be monitored on a regular basis, since however, that protease-​based regimen boosting has been associated with higher rates of toxicity. Immune reconstitution inflammatory syndrome Between 8 and 45% of patients with HIV infection commencing antiretroviral therapy while being treated for tuberculosis develop paradoxical deterioration of tuberculosis, the so-​called immune re- constitution inflammatory syndrome (IRIS). Paradoxical deterior- ation was well known in the pre-​HIV era, but occurs much more frequently in HIV-​infected patients starting antiretroviral therapy. The pathogenesis of IRIS is not completely understood. The most common manifestations of tuberculosis-​related IRIS are focal in- flammatory exacerbations of tuberculosis (lymphadenitis, serositis, section 8  Infectious diseases 1146 or abscesses, new infiltrates), ‘unmasking’ of tuberculosis or other subclinical diseases after antiretroviral therapy initiation, and so on. It typically occurs within 2–​4 weeks after antiretroviral initi- ation. Risk factors associated with an increased risk of IRIS include shorter intervals between antituberculosis therapy and antiretro- viral therapy initiation, low baseline CD4 counts and high baseline viral load, and vigorous CD4/​viral load response to antiretroviral therapy. However, new or worsening clinical features should be at- tributed to IRIS only after a thorough evaluation has excluded other possible causes, notably poor adherence to antituberculosis therapy, MDR tuberculosis, new opportunistic diseases, and systemic drug hypersensitivity reactions. The benefit of adjunctive corticosteroids in the management of patients with IRIS is suggested by results of at least one randomized controlled trial which showed that the use of 1.25 mg/​kg prednisone for 2 weeks followed by 0.75 mg over two weeks in non​severe IRIS was been associated with decreased hospi- talization and morbidity Adjunctive steroid treatment Corticosteroids are frequently advocated with tuberculosis treat- ment to reduce inflammation, but evidence for this practice is often lacking, particularly in HIV infection. Dexamethasone reduced mortality in a large study of Vietnamese adults with tuberculous meningitis, though rates of neurologic sequelae among survivors were unchanged. The HIV-​infected subgroup of the latter study appeared to gain a similar benefit, but this failed to achieve statis- tical significance. A Ugandan study of adjunctive prednisolone in HIV-​infected patients with pleural tuberculosis found faster reso- lution with prednisolone, but no mortality benefit. Of great concern, however, was their finding of excess cases of Kaposi’s sarcoma in the prednisolone arm. This sobering result is a reminder that the additive immunosuppressant effect of glucocorticoids can have severe con- sequences in HIV infection.. The recent multicentre Investigation of the Management of Pericarditis (IMPI) trial of the management of tuberculosis pericarditis enrolled African adults with definite and probable tuberculosis pericarditis and randomized them to re- ceive adjunctive prednisolone or placebo and to receive M indicus pranii vaccine or placebo in a 2-​by-​2 factorial design, together with standard TB treatment. Neither intervention had a significant effect on the primary composite endpoint of death, recurrent pericar- dial effusion with tamponade requiring pericardiocentesis, or con- strictive pericarditis. However, prednisolone reduced pericardial constriction by almost 50%, a finding that has critical implications in sub-​Saharan Africa where access to cardiothoracic surgery for peri- cardiectomy is limited. Patients assigned to adjunctive glucocortic- oids had an increased risk of developing cancer, notably Kaposi’s sarcoma, particularly if they also received M. indicus pranii vaccine. In general, glucocorticoids should only be used for patients with tu- berculous meningitis or in patients without HIV infection who have pericardial tuberculosis considered at high risk of developing con- strictive pericarditis. Treatment of latent tuberculosis infection Isoniazid chemoprophylaxis Prevention of tuberculosis with isoniazid therapy was first docu- mented in children in the mid-​1950s. Subsequently, several controlled trials of isoniazid chemoprophylaxis were under- taken, and its efficacy firmly established. A meta-​analysis of 11 placebo-​controlled trials of isoniazid, involving more than 70 000 persons, found that treatment reduced tuberculosis incidence by 63%. Among patients who adhered to more than 80% of the iso- niazid regimen, protection was 81%. These studies also showed that isoniazid chemoprophylaxis reduced tuberculosis deaths by 72%. The efficacy of isoniazid therapy to prevent tuberculosis in high-​risk persons is incontrovertible. Enthusiasm for isoniazid chemoprophylaxis was considerably dampened in the late 1960s and early 1970s when drug-​related hepatotoxicity, including deaths, was observed. Several studies based on decision analysis or modelling suggested that the risks of chemoprophylaxis might outweigh the benefits, and use of pre- ventive therapy was curtailed or ignored in many settings. Because the risk of isoniazid-​related hepatotoxicity increases with age, use of chemoprophylaxis in people older than 35 years was particularly discouraged. Preventive therapy in high-​risk individuals The resurgence of tuberculosis in the developed world, particularly HIV-​related tuberculosis, and the uncontrolled global epidemic have renewed interest in the use of preventive therapy in high-​risk individuals known or strongly suspected to be latently infected with M. tuberculosis. The ATS/​CDC guidelines on screening for latent tu- berculosis that stress the importance of targeting efforts on popu- lations and patients who would benefit from treatment to prevent active disease. In the past, screening for tuberculosis infection has been unfocused and often directed at patients who, if found to be in- fected, would have little risk of progressing to active disease. Current guidelines propose that only people with a high risk of disease or high prior probability of latent tuberculosis be tested, and that treat- ment be offered to infected individuals regardless of age. Individuals who should be targeted for tuberculin testing are those listed in the first two columns of Table 8.6.26.3 (i.e. those in whom a positive test is considered equal to or exceeding 5 or equal to or exceeding 10 mm induration). People without risk factors for tuberculosis (those in whom a positive test is equal to or exceeding 15 mm) should not be tested. Treatment regimens for latent tuberculosis are listed in Table 8.6.26.7, along with the rating given to the regimen by the ATS and CDC. Isoniazid remains a favoured drug for tuberculosis pre- ventive therapy because of its well-​documented efficacy, low cost, Table 8.6.26.7  Treatment regimens for latent tuberculosis Drug regimen Duration (months) Interval Rating (HIV−) Rating (HIV+) Isoniazid 9 Daily A II A II Isoniazid 9 Twice weekly B II B II Isoniazid 6 Daily B I C I Isoniazid 6 Twice weekly B II C II Rifampicin Rifapentine and Isoniazid 4 3 Daily Once weekly B II A I B III B I A, strongly recommended; B, recommended; C, optional; I, randomized trials; II, data from other scientific studies; III, expert opinion. 8.6.26  Tuberculosis 1147 and relatively low toxicity. The optimal duration of isoniazid therapy for latent tuberculosis has been the subject of extensive debate in re- cent years. The International Union Against Tuberculosis and Lung Disease conducted a landmark trial in Eastern Europe in the 1970s and 1980s that compared no treatment to 3, 6, or 12 months of iso- niazid in adults with fibrotic changes on radiographs. The results showed that, compared to placebo, 12 months of isoniazid reduced the incidence of tuberculosis by 75%, compared to 66% for 6 months and 20% for 3  months. In addition, patients who completed the 12 months of therapy and were judged to be compliant experienced a 92% reduction in tuberculosis risk, compared to a 69% decrease for compliant patients completing a 6-​month regimen. A meta-​analysis by the Cochrane Collaborative found that 12 months of isoniazid was more effective than 6 months for prevention of tuberculosis. An analysis of varying durations of isoniazid therapy in Alaskan natives revealed that the effectiveness of isoniazid therapy was optimal after 9 months, and that further treatment conferred no additional benefit. Several studies of isoniazid in HIV-​infected patients in Africa, how- ever, have found that prolonged treatment for three or more years is more efficacious than shorter durations, presumably by preventing disease due to reinfection in these highly susceptible patients. Several recent studies have demonstrated that shorter durations of preventive therapy using the combination of rifapentine and iso- niazid given once a week under direct observation is an acceptable alternative to longer courses of isoniazid alone. A large study spon- sored by the CDC’s TB Trials Consortium found that the rifapentine/​ isoniazid 3-​month regimen was not only non-​inferior to isoniazid for 9 months in high-​risk individuals, but almost reached superiority and was better tolerated, with significantly less hepatotoxicity. Another study in HIV-​infected adults in South Africa found rifapentine/​iso- niazid to be of similar efficacy to isoniazid alone for 6 months. A re- cent study by the AIDS Clinical Trials Group found that one month of daily rifapentine and isoniazid was noninferior to nine months of isoniazid in HIV-infected adults and adolescents. A one-month regimen has obvious clinical and public health advantages. Although isoniazid is a well-​tolerated drug, serious hepatotoxicity can occur in a small proportion of patients. Isoniazid may result in asymptomatic elevations in hepatic aminotransferase levels, but this does not always signal impending clinical toxicity. Hepatotoxicity is of concern when symptoms of hepatitis develop, including pain, nausea, vomiting, and jaundice. Continuing isoniazid in the pres- ence of symptoms can lead to death from fulminant hepatic necrosis and liver failure, with a case fatality rate of 10–​15%. Studies in the 1960s and 1970s found evidence of hepatotoxicity in 1–​5% of re- cipients of isoniazid, with higher rates among older patients. More recent experience with isoniazid therapy that is closely monitored shows a risk of hepatotoxicity in the range of 0.1–​0.3%. Thus, appro- priate patient screening and follow-​up makes the use of isoniazid for treating latent infection markedly safer. The use of rifampicin alone or rifapentine and isoniazid is better tolerated than isoniazid alone, but safety monitoring is still required. Other regimens In addition to the 3-​month rifapentine/​isoniazid treatment de- scribed earlier, other alternative regimens are sometimes used in selected situations. A  3-​month regimen of rifampicin alone was found to reduce the incidence of tuberculosis by about 65% in men with silicosis, and was more effective than 6 months of isoniazid. The combination of rifampicin and isoniazid given for three to four months is widely used for treatment of latent tuberculosis in chil- dren and improves completion rates. This regimen has also been found to be equally effective as isoniazid in studies in adults. The use of rifampicin does pose the risk of important drug inter- actions. For example, reduction in methadone concentrations caused by rifampicin can precipitate narcotic withdrawal. Moreover, rifam- picin can lower levels of protease inhibitors and non-​nucleoside reverse transcriptase inhibitors used to treat HIV infection. The efficacy of the oral contraceptive pill is also reduced. If multidrug-​ resistant tuberculosis is suspected, preventive therapy with etham- butol or pyrazinamide and a fluoroquinolone (e.g. moxifloxacin) for 6–​12 months may be used, but clinical data are absent. Candidates for treatment of latent tuberculosis are listed in Table 8.6.26.3. Criteria for treatment include a positive tuberculin test according to the categories in Table 8.6.26.3, elevated risk for developing active tuberculosis if untreated, and exclusion of ac- tive tuberculosis by clinical evaluation and chest radiograph. In addition, HIV-​infected and other severely immunocompromised persons who are contacts to an infectious tuberculosis patient should be treated for latent tuberculosis regardless of tuberculin skin test results. Monitoring treatment Patients receiving treatment for latent tuberculosis should be moni- tored for drug toxicity, as well as to promote adherence to therapy. As in treatment of active tuberculosis, patients receiving isoniazid should be warned about signs and symptoms of hepatotoxicity and advised to discontinue therapy and seek care if any of these occur. Patients with, or at risk of, chronic liver disease should have baseline liver enzymes obtained, with monthly monitoring if the results are abnormal. All patients should be clinically evaluated at least monthly to assess adherence and toxicity. Treatment using other preventive regimens and treatment of patients with mild transaminase eleva- tions (three times upper limits of normal or less) can proceed with regular clinical and laboratory monitoring. Higher elevations of transaminases, or the development of symptoms or signs of hepatitis should be managed with discontinuation of therapy at least tempor- arily. Patients who complete therapy for latent tuberculosis do not need periodic monitoring for tuberculosis subsequently. Prevention of tuberculosis Strategies to control tuberculosis are aimed at the prevention of the spread of M. tuberculosis infection and the development of clinical tu- berculosis. The principal approaches employed toward this end are: • identification and treatment of infectious tuberculosis cases • treatment of latent tuberculosis infection • prevention of exposure to infectious particles in air, especially in hospitals and other institutions • vaccination Identification and treatment of infectious tuberculosis cases Case identification and treatment reduces transmission by ren- dering patients with communicable tuberculosis non​infectious. Patients with pulmonary tuberculosis produce infectious aerosols section 8  Infectious diseases 1148 that can transmit tubercle bacilli to contacts breathing the same air. When cases are identified and treated, infectiousness is rapidly eliminated. The duration of treatment required to prevent further transmission of infection is not known precisely, but experimental, clinical, and microbiological data suggest that the level of infec- tiousness is reduced enormously within several days of beginning effective treatment. The number of secondary infections gener- ated by an infectious tuberculosis patient varies greatly depending on the duration of illness, the extent of pulmonary pathology, the amount of patient coughing, and the environment into which the patient expels infectious aerosols. Early diagnosis and treatment re- duces the number of secondary infections, while delays can result in ongoing transmission to large numbers of contacts. Failure to retain patients in treatment until they are cured also contributes to spread of infection. Treatment of latent tuberculosis infection This is discussed earlier on in this chapter. The benefit of treating latent infection is not only to the individual patient who does not fall ill with tuberculosis, but also accrues to the potential contacts of that patient, who might become secondarily infected were disease to develop. Targeting of high-​risk groups for screening and treatment of latent tuberculosis thereby reduces tuberculosis incidence within communities. Groups that should be targeted for screening are listed in the first two columns of Table 8.6.26.3. Prevention of exposure especially in hospitals and other institutions Control of exposure to infectious aerosols can have a major im- pact on the spread of tuberculosis. In the late 1980s and early 1990s, transmission of tuberculosis, including multidrug-​resistant tuber- culosis, was widespread in hospitals, homeless shelters, and cor- rectional facilities in New York City. More recently, the outbreak of XDR tuberculosis in the KwaZulu-​Natal province of South Africa is a tragic reminder of the importance of infection control measures in institutions. The congregation of large numbers of highly sus- ceptible people, especially HIV-​infected persons, in closed environ- ments with untreated tuberculosis patients has resulted in numerous microepidemics of both drug-​susceptible and drug-​resistant tuber- culosis. Reversal of the resurgence of tuberculosis in New York at that time was attributable in large part to strengthening of infection control practices. Identification and isolation of infected patients Tuberculosis infection control involves prompt identification and isolation of patients with suspected tuberculosis. The decision to isolate a patient in a hospital setting is a function of epidemiological and clinical factors. Patients with known tuberculosis risk factors who present with symptoms and signs characteristic of pulmonary tuberculosis should be placed in respiratory isolation. Local epi- demiological data should influence isolation practices. In settings where tuberculosis is prevalent, all HIV-​infected patients with pneu- monia may require isolation, whereas isolation can be more selective and based on individual patient features in low prevalence settings. Respiratory isolation requires placement of the patient in a room with negative air pressure relative to adjoining areas, ventilation to the room should provide at least six complete air changes per hour, and air should not be recirculated without filtering or irradiation. Patients should be instructed to cover their coughs at all times, and should wear surgical face masks when outside the room to reduce aerosol generation. Anyone entering the patient’s room should wear an appropriate face mask or respirator to prevent inhalation of droplet nuclei with tubercle bacilli. A considerable amount of debate has oc- curred in recent years in the United States of America regarding what constitutes appropriate protection for healthcare workers exposed to infectious tuberculosis. This debate is influenced as much by phil- osophy as by science, and will not be detailed here. Use of surgical masks for the protection against tuberculosis is clearly inappropriate, even though these masks are useful when placed on patients to pre- vent creation of infectious aerosols. Tightly fitting face masks that filter out more than 99.7% of particles less than 0.5 µm in size (high-​ efficiency particle air filters) are effective. Other devices, including positive air pressure respirators, are also effective. Use of ultraviolet germicidal irradiation can be useful for redu- cing the number of infectious particles in ambient air in settings where ventilation alone is not sufficient. Ultraviolet light must be concentrated in areas of rooms where exposure to people will not occur, such as upper air zones, in order to prevent skin and ocular toxicity. Areas where ultraviolet lights are often used include bron- choscopy suites, inside air circulation ducts, in emergency rooms, and in homeless shelters. Criteria for discontinuation of respiratory isolation are listed in Box 8.6.26.1. Guidelines for taking patients out of isolation in the hospital are strict and are intended to protect other vulner- able patients and hospital staff from any exposure to the disease. Respiratory isolation is not usually required or practical in the home setting, and patients with infectious tuberculosis do not need to be hospitalized solely for respiratory isolation. It is assumed that con- tacts in the home environment will already have had significant ex- posure to tuberculosis by the time a diagnosis is made, and isolation of the patient affords no measurable benefit. Exceptions to this may include patients living in congregate living facilities or other spe- cial situations. The primary protective measures for contacts of cases are a clinical evaluation to identify and evaluate symptoms of tuber- culosis and tuberculin skin testing with treatment of latent infec- tion, if present. Instituting infection control measures is likely to be challenging in developing countries where the healthcare system is already overburdened and where facilities often lack negative pres- sure isolation rooms and air filtration systems. In such settings, work practice and administrative control measures have been emphasized and are considered to be more effective and less expensive. These measures consist of policies and procedures intended to promptly identify infectious tuberculosis cases so that additional precautions and healthcare steps can be taken. Box 8.6.26.1  Criteria for discontinuing respiratory isolation for tuberculosis in hospital inpatients • Alternative diagnosis established • Infectious tuberculosis ruled out • Tuberculosis diagnosed and: -​ Treatment given for at least 14 days and -​ Clinical response to therapy documented, including improvement in fever and cough and -​ Acid-​fast smears of sputum negative or -​ Patient discharged to home 8.6.26  Tuberculosis 1149 BCG vaccination Vaccination against tuberculosis with the Bacille Calmette–​Guérin (BCG) vaccine is widely administered throughout the world but is a practice mired in controversy. BCG is an attenuated live bacterial vaccine developed in the early 20th century by Calmette and Guérin at the Institut Pasteur in France. After a series of uncontrolled and anecdotal assessments of the vaccine, a series of controlled trials of BCG was begun in the 1930s and continued through to the 1990s. The efficacy of BCG has varied greatly in these studies, ranging from more than 80% protection to complete lack of protection, with pos- sibly increased risk in vaccine recipients. Meta-​analyses of BCG trials find a protective benefit of BCG based on historical trials, but recent studies have not demonstrated efficacy. There is evidence that BCG diminishes haematogenous dissem- ination of primary tuberculosis infection and thereby reduces the incidence of miliary tuberculosis and tuberculous meningitis in children. It is primarily for this reason that BCG is included in the Expanded Programme on Immunization of the WHO. The current efficacy of BCG for preventing pulmonary tubercu- losis is debated on the basis of several recent trials which have failed to show protection. Several hypotheses have been proposed for the variation in efficacy reported in various studies, including differences in susceptibility within populations, environmental exposure to mycobacteria which masks vaccine effect, and attenuation of vaccine immunogenicity. This last explanation is very compelling and fits well with clinical trial data. Unlike most vaccines, BCG is not stand- ardized and there is no seedlot of vaccine from which new batches are derived. BCG is grown in several laboratories around the world and has not been re-​passaged in animals since it was derived from cattle a century ago. Multiple commercial and non​commercial BCG prod- ucts are in use presently, and comparative genomic analysis dem- onstrates considerable genetic heterogeneity in these strains, with many gene deletions and polymorphisms. One analysis of BCG trials found that protective efficacy was reduced in studies using multiply-​ passaged vaccine strains. The evidence supports the hypothesis that BCG has become further attenuated over time and no longer pro- motes immunity to M. tuberculosis infection and disease in adults. This position has not been universally accepted, however, and BCG remains one of the most widely administered vaccines in the world, largely for its perceived effects on paediatric tuberculosis. Areas for further research Effective global tuberculosis control will require a coordinated set of clinical and public health strategies that are based on a thorough understanding of the epidemiology, pathogenesis, and therapy of in- fection with M. tuberculosis. The WHO’s END TB strategy, which focuses on finding and effectively treating cases, has been aug- mented with additional strategies for intensified case-​finding, use of preventive therapy and infection control, particularly in countries with large HIV epidemics. Use of improved methods for the diag- nosis at point of care and treatment of tuberculosis infection and disease, particularly drug-​resistant tuberculosis, is urgently needed. Effective regimens for the treatment of multidrug-​resistant and ex- tensively drug-​resistant tuberculosis as well as shortening the total duration of drug-​susceptible tuberculosis (e.g. from 6 to 4 months), with both existing and new agents, need to be developed. A better understanding of the pathogenesis of and natural immunity to tu- berculosis may contribute to the development of a more effective vaccine. A recent trial of a subunit vaccine and adjuvant, which showed >50% efficacy in preventing tuberculosis disease in adults with latent infection, gives hope for future contributions to epidemic control from vaccines. The sequencing of the genome of M. tubercu- losis promises to open the door to a new generation of research on tuberculosis and its control. Scientific progress alone, however, will be insufficient to combat tuberculosis worldwide. The willingness of societies and nations to pay for the deployment of the fruits of bio- medical research, both past and future, to combat the disease where it is prevalent will be required for the conquest of tuberculosis. FURTHER READING Abdool Karim SS, et al. (2010). Timing of initiation of antiretroviral drugs during tuberculosis therapy. N Engl J Med, 362, 697–​706. Blanc FX, et al. (2011). Earlier versus later start of antiretroviral therapy in HIV-​infected adults with tuberculosis. CAMELIA (ANRS 1295–​ CIPRA KH001) study team. N Engl J Med, 365, 1471–​81. Boehme CC, et al. (2010). Rapid molecular detection of tuberculosis and rifampin resistant. N Engl J Med, 363, 1005–​15. Davies PDO, Barnes P, Gordon SB (eds) (2008). Clinical tuberculosis, 4th edition. Hodder Arnold, London. Dheda K, Barry CE 3rd, Maartens G (2016). Tuberculosis. Lancet, 387, 1211–​26. Diacon AH, et al. (2014). Multidrug-​resistant tuberculosis and culture conversion with bedaquiline. N Engl J Med, 371, 723–​32. Dorman SE, et  al. (2014). Interferon-​γ release assays and tuberculin skin testing for diagnosis of latent tuberculosis infection in healthcare workers in the United States. Am J Respir Crit Care Med, 189, 77–​87. Fox W, Ellard GA, Mitchison DA (1999). Studies on the treatment of tuberculosis undertaken by the British Medical Research Council tuberculosis units, 1946–​1986, with relevant subsequent publica- tions. Int J Tuberc Lung Dis, 3(10 Suppl 2), S231–​79. Gler MT, et al. (2012). Delamanid for multidrug-​resistant pulmonary tuberculosis. N Engl J Med, 366, 2151–​60. Mayosi BM, et al. (2014). Prednisolone and mycobacterium indicus pranii in tuberculous pericarditis. N Engl J Med, 371, 1121–​30. Meintjes GM, et al. (2018). Prednisone for the prevention of paradox- ical tuberculosis-associated IRIS. N Engl J Med, 379, 1915–25. Murphy RA, et  al. (2012). Coadministration of lopinavir/​ritonavir and rifampicin in HIV and tuberculosis co-​infected adults in South Africa. PLoS One, 7, e44793 Nahid P, et  al. (2016). 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–​95. Schnippel K, et al. (2018). Effect of bedaquiline on mortality in South African patients with drug-resistant tuberculosis: a retrospective cohort study. Lancet Respir Med, 6, 699–706. Sterling TR, et al. (2011). Three months of rifapentine and isoniazid for latent tuberculosis infection. N Engl J Med, 365, 2155–66. Uthman OA, et al. (2015). Optimal timing of antiretroviral therapy initi- ation for HIV-​infected adults with newly diagnosed pulmonary tubercu- losis: a systematic review and meta-​analysis. Ann Intern Med, 163, 32–​9. Van Der Meeren O, et al. (2018). 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