8.5.15 Dengue 845

8.5.15 Dengue 845

8.5.15  Dengue 845 FURTHER READING Aliota MT, et al. (2017). Zika in the Americas, year 2: What have we learned, What gaps remain? A report from the Global Virus Network. Antivir Res, 144, 223–46. Barrows N, et al. (2016). Repurposed drug candidates to treat ZIKV infection in pregnancy. Cell Host Microbe, 20, 259–​70. Brasil P, et al. (2016). Zika virus infection in pregnant women in Rio de Janeiro. N Engl J Med, 375, 2321–34. Beck AS, Barrett AD (2015). Current status and future prospects of yellow fever vaccines. Expert Rev Vaccines, 14, 1479–​92. Cao-​Lormeau et al. (2016). Guillain–​Barré syndrome outbreak asso- ciated with Zika virus infection in French Polynesia: a case-​control study. Lancet, 387, 1531–​9. Franca GVA et al. (2016). Congenital Zika virus syndrome in Brazil: a case series of the first 1501 livebirths with complete investigation. Lancet, 388, 891–​7. Griffiths MJ, Turtle L, Solomon T (2014). Japanese encephalitis virus infection. Handb Clin Neurol, 123, 561–​76. Halstead SB, Thomas SJ (2011). New Japanese encephalitis vaccines: alternatives to production in mouse brain. Expert Rev Vaccines, 10, 355–​64. Holbrook MR (2012). Kyasanur forest disease. Antiviral Res, 96, 353–​62. Kasabi G (2013). Coverage and effectiveness of Kyasanur forest disease (KFD) vaccine in Karnataka, south India, 2005–​10. PLoS Negl Trop Dis, 7, 1–​4. Madani TA (2005). Alkhumra (Alkhurma) virus infection, a new viral hemorrhagic fever in Saudi Arabia. J Infect, 51, 91–​7. Mead PS, et al. (2018). Zika virus shedding in semen of symptomatic infected men. N Engl J Med, 378, 1377–85. Patel H, et al. (2015). Long-​term sequelae of West Nile virus-​related illness: a systematic review. Lancet Infect Dis, 15, 951–​9. Piantadosi A, et al. (2016). Emerging cases of powassan virus enceph- alitis in New England: clinical presentation, imaging, and review of the literature. Clin Infect Dis, 62, 707–​13. Pierson TC, Diamond MS (2013). Flaviviruses. In: Knipe DM, Howley PM (eds) Fields virology, 6th edition. Wolters Kluwer/​Lippincott Williams & Wilkins, Philadelphia, PA. Vasilakis N, Weaver SC (2017). Flavivirus transmission cycles fo- cusing on Zika. Curr Opin Virol, 22, 30–​5. Weaver SC et al. (2015). Zika virus: history, emergence, and pro- spects for continued outbreaks in the Americas. Antiviral Res, 130, 69–​80. 8.5.15  Dengue Bridget Wills and Yee-Sin Leo ESSENTIALS Dengue is the most important mosquito-​borne viral infection to affect humans. It is an RNA virus in the Flavivirus genus, family Flaviviridae. There are four closely related but serologically distinct viral serotypes, all of which may cause disease. Following infec- tion with one serotype there is lifelong immunity to that serotype but the possibility of more severe disease during a subsequent in- fection with a different serotype. The primary mosquito vector is
Aedes aegypti. Recent estimates suggest around 100 million symptom- atic, and many more asymptomatic, infections occur annually world- wide. The disease is hyperendemic in many large Asian cities, and is also a significant problem in the Pacific region and in the Americas. Clinical features and diagnosis—​among symptomatic cases a wide variety of clinical manifestations are seen, ranging from mild febrile illness through to severe and potentially fatal disease. Of note, a systemic vascular leak syndrome may develop around the time of defervescence, with the resulting plasma leakage sometimes severe enough to cause life-​threatening dengue shock syndrome. Altered haemostasis and thrombocytopenia are typically present, some- times accompanied by clinically significant bleeding. Severe hepatic, neurological, and cardiac complications are also seen in some cases, but are less common. Diagnosis depends on viral isolation, detection of viral antigen or viral RNA, or serological testing. Management and prevention—​treatment is supportive, with particular emphasis on careful fluid management. In particular, prompt volume resuscitation is essential for patients with dengue shock syndrome, together with regular monitoring of the pulse rate, blood pressure, and haematocrit, to minimize the risk of fluid over- load. A vaccine with moderate efficacy has recently been licensed in a few countries, but current prevention efforts continue to rely on elimination of potential vector breeding sites, biological and chem- ical vector control strategies, and avoidance of mosquito bites. Introduction and aetiology Dengue is the most important mosquito-​borne viral infection of hu- mans. The name is originally a West Indian Spanish word, derived from Ki Swahili ‘ka dinga pepo’ (‘a kind of cramping plague’) that was brought from Africa to the Caribbean. In the British West Indies it was called ‘dandy fever’ because of the stiff posture of its victims. Later, in Cuba, dengue was termed ‘quebranta huesos’ or ‘break-​bone fever’ because of the severe myalgias and arthralgias that often occur. The dengue virus is a single stranded, positive-​sense RNA virus; four closely related but serologically distinct viral serotypes (DENV-​1, 2, 3, and 4) together constitute one subgroup of the genus Flavivirus, family Flaviviridae. The virus comprises three structural proteins, together with seven non​structural proteins that are ex- pressed in infected cells but do not form part of the mature virion. There is only transient cross-​protective immunity between the four serotypes, so people living in endemic areas can be infected several times during their lifetime. Epidemiology Humans are infected with dengue viruses by the bite of Aedes mos- quitoes, principally Ae. aegypti and Ae. albopictus. Other vertebrate

846 section 8  Infectious diseases species are generally not susceptible to infection. The most im- portant vector globally, Ae. aegypti, is a highly domesticated tropical mosquito that lays its eggs in artificial water containers commonly found in and around homes. The adult mosquitoes rest indoors and prefer to feed on humans during daylight hours, with peak biting ac- tivity in the early morning and late afternoon. A mosquito acquires DENV when it bites a viremic individual; the virus then replicates in the mid-​gut before disseminating to the saliva after 7–​12 days so that onward transmission can occur when the mosquito takes its next blood feed. Higher plasma viremia in the original infected in- dividual appears to be a significant factor in facilitating a productive infection in the mosquito. Aedes mosquitoes are nervous feeders, and if interrupted they will return to the same or different person(s) to continue feeding. Thus, during a single blood meal several indi- viduals may become infected, making Ae. aegypti a highly efficient epidemic vector. Epidemics of febrile illness attributed to dengue have been re- ported at intervals over the last 200 years across Asia, Africa, and North America, likely reflecting progressive expansion in the global distribution of the mosquito vectors. From the 1950s on- wards a new clinical syndrome, characterized by vascular leakage and bleeding and given the name dengue haemorrhagic fever (DHF), began to emerge in Southeast Asia. The first epidemic of DHF in the Americas appeared in 1981 in Cuba, associated with the arrival of a new Asian genotype of DENV-​2 different from the American strain. Since that time the geographic footprint of dengue, encompassing a range of clinical syndromes, has con- tinued to expand across the tropics and subtropics, a phenom- enon largely attributable to the effects of urbanization and climate change on mosquito ecology. Thus dengue is now hyperendemic in most Asian cities, with epidemics occurring every 3 to 5 years superimposed on background endemic transmission. It has also become established as a significant problem in the Pacific region and in the Americas, and outbreaks have been reported from Africa, the Arabian Peninsula, and the warmer parts of Europe. The spread of Ae. albopictus to new areas of the world including southern Europe, America, and Africa may explain the rise in dengue transmission in these areas. Today in excess of 2.5 billion people, or around 40% of the world’s population, live in tropical and subtropical areas of the world where they are at risk for infection, and approximately 100 million clinically apparent dengue infections are estimated to occur annually across more than 100 countries (Fig. 8.5.15.1). For more than 50 years dengue has been a leading cause of hospital- ization among children in many of these endemic countries, but in parallel with the changing epidemiology and transmission dy- namics described earlier, different clinical presentations are also emerging. A shift towards clinical disease in older age groups has been reported in Singapore, Indonesia, Bangladesh, and Thailand, and likewise many of the reported cases from the Americas are among adults rather than children. The nature and frequency of the complications now seen likely reflects age-​related differences in intrinsic physiology, as well as the greater likelihood for older patients to have underlying comorbidities. Thus, DSS remains more common in children and young adults, while clinically sig- nificant bleeding and/​or severe organ involvement typically occur in older adults. Although low mortality rates (below 1% for severe disease) are usual in experienced hands, higher rates are still re- ported from some regions, as well as among high-​risk groups such as infants and older people. Pathogenesis All four DENV serotypes can cause disease. Infection with one serotype elicits immunity to that serotype but does not provide long-​term cross-​protective immunity to the remaining serotypes. Severe disease occurs predominantly in patients experiencing a second or subsequent infection with a dengue serotype different from their first infection, or else in infants with transmitted ma- ternal antibody experiencing their first infection. The generally accepted antibody-​dependent enhancement hypothesis suggests that residual heterotypic non​neutralizing antibodies bind to the new virus, enhancing its infectivity by increasing the efficiency of binding and uptake of virus–​antibody complexes through Fc receptors on blood monocyte or tissue macrophage cells, thus amplifying viral replication. The resulting increase in viral load drives an immunopathogenic cascade that alters microvascular function in some way, resulting in the characteristic systemic vascular leak syndrome and coagulopathy. Rapid mobilization of serotype cross-​reactive memory T cells has been suggested as an alternative mechanism to trigger the inflammatory cascade. Other factors considered to influence disease severity include dif- ferences in viral virulence, molecular mimicry, and immune com- plex and/​or complement-​mediated dysregulation, as well as age, genetic predisposition, obesity, and pre-​existing or underlying comorbidities. However, the pathogenesis of the vascular leakage and coagulopathy associated with severe infections remains poorly understood and as yet no mechanism has been identified that links the established immunological derangements with a de- finitive effect on microvascular structure or function. The con- tribution of various viral proteins, in particular non​structural protein 1 (NS1), to pathogenesis is a major focus of current re- search efforts. Clinical manifestations Infection with dengue virus can cause a wide spectrum of symp- toms ranging from mild febrile illness to severe and fatal disease, although most infections are thought to be asymptomatic. In the past, symptomatic disease was conventionally separated into two major clinical syndromes, dengue fever and dengue haemorrhagic fever (DHF), with case definitions and management guidelines for these entities published by the World Health Organization (WHO). The pathognomonic feature of DHF is an increase in vascular permeability causing plasma leakage, which, in a small proportion of cases, is severe enough to result in hypovolaemic shock. In addition, to qualify for a diagnosis of DHF, a patient must have some evidence of bleeding and a platelet count below 100 × 109/​litre. Due to practical difficulties in using the old WHO scheme a revised classification system was adopted in the WHO

8.5.15  Dengue 847 dengue guidelines published in 2009. The new scheme classifies symptomatic disease into dengue and severe dengue, in line with several other complex diseases such as malaria and pneumonia. It is hoped that this simpler system will be useful for triage, aid clinical management, and improve the quality of surveillance and epidemiological data. Symptomatic dengue is primarily a disease of older children and adults. After an incubation period of around 4 to 7 days symptoms Fig. 8.5.15.1  Global evidence consensus, risk, and burden of dengue in 2010. Reprinted by permission from Macmillan Publishers Ltd: Nature (Bhatt S et al. (2013) The global distribution and burden of dengue. Nature 496, 504–​507), copyright © 2013.

848 section 8  Infectious diseases start suddenly and typically follow three phases—​an initial febrile phase, a critical phase around the time of defervescence, and a spon- taneous recovery phase. Febrile phase There is sudden onset of high fever often accompanied by facial flushing, headache, retro-​orbital pain, lumbosacral pain, severe malaise, myalgias, joint and bone pains, anorexia, altered sense of taste, mild sore throat, nausea, and vomiting. Young children may experience the high fever, but are generally less symptom- atic than older individuals. Some patients may have a transient rash or skin flush in early illness (Fig. 8.5.15.2a). Other find- ings associated with infection may include generalized lymph- adenopathy, mild haemorrhagic manifestations (e.g. petechiae or easy bruising, Fig. 8.5.15.3a, b), and palpable hepatomegaly but rarely splenomegaly. Jaundice is rare. Clinical laboratory find- ings during the first week include thrombocytopenia and leuko- penia, often with moderate elevation of hepatic transaminases. Creatine kinase may also be elevated, sometimes markedly so in patients with severe myalgia. Critical phase Most patients recover around the time of defervescence, usually between 3 and 7 days after fever onset. However, in a small propor- tion of cases a vasculopathy characterized by endothelial dysfunc- tion and a transient vascular leak syndrome becomes apparent at this time, marking the onset of the critical phase. The vascular leak syndrome manifests with increasing haemoconcentration, hypoproteinaemia, pleural effusions, and ascites; in severe cases the circulating plasma volume is so compromised that the patient develops hypovolaemic shock—​this is, DSS (Fig.  8.5.15.4a, b). When the pulse pressure narrows to less than 20 mm Hg with a rapid weak pulse and/​or impaired peripheral perfusion, or if the patient becomes hypotensive due to the reduced plasma volume, then he/​she is defined as having DSS. If fluid resuscitation is not instituted promptly the ongoing depletion of plasma becomes critical, the systolic pressure falls rapidly, and irreversible shock and death may follow. However, with judicious fluid manage- ment most patients make a full recovery. Warning signs that the patient may be developing severe disease include persistent vomiting, lethargy, worsening abdominal pain, and increasing hepatomegaly. Haemorrhagic manifestations are common during the crit- ical period but are often limited to the presence of skin petechiae or bruising, or a positive tourniquet test. Mucosal bleeding (e.g. epistaxis, gum bleeding, gastrointestinal bleeding, haematuria, menorrhagia) can occur, but is rarely clinically significant in chil- dren except in association with profound shock. However, adults tend to experience more severe bleeding problems than children (Fig. 8.5.15.5a, b), and gastrointestinal bleeding and/​or menor- rhagia may be significant even in patients with little evidence of vascular leakage. Moderate to severe thrombocytopenia is usual in all age groups, with nadirs below 20 × 109 /​litre not uncommon during the critical period followed by rapid improvement during the recovery phase. An increase in the activated partial thrombo- plastin time and a reduction in fibrinogen levels are also frequently noted. However, these findings are not indicative of classic dis- seminated intravascular coagulation and the true nature of the coagulopathy remains unclear. Other laboratory investigations show similar but usually more profound abnormalities to those seen in uncomplicated cases. Recovery phase The increase in vascular permeability is transient and reverts to normal after approximately 24–​48 hours. Fluid is reabsorbed quite rapidly, often with an obvious diuresis, and the patient generally improves. However, particular care is needed with parenteral fluid therapy at this time to minimize the risk for development of respira- tory compromise due to fluid overload. A second rash, varying in appearance from scarlatiniform to maculopapular, might be noted around day 6 to 7 of illness, typ- ically on the extremities although sometimes involving the trunk and face (Fig. 8.5.15.2b). The rash blanches on pressure and may be accompanied by intense pruritus. Florid rashes can take some weeks to resolve, often with marked skin desquamation on the extremities. Other complications, notably blurred vision and sometimes transient visual loss, are also reported infrequently during the recovery phase. These symptoms generally resolve with time. (a) (b) Fig. 8.5.15.2  (a) Blanching erythematous rash on the back of a young adult with dengue in the febrile phase. (b) Classic convalescent rash in two Singaporean adults with dengue: marked erythema highlighting islands of white skin, usually most obvious on the extremities. (a) Copyright Dinh The Trung. (b) Copyright IIDE-​TTSH.

8.5.15  Dengue 849 Other syndromes Moderate hepatitis is common across the spectrum of dengue dis- ease, but severe hepatitis and acute liver failure are also sometimes seen. Renal impairment in the absence of severe plasma leakage or shock develops occasionally, usually secondary to severe muscle involvement with rhabdomyolysis. Neurological involvement is usually attributed to an encephalopathy (e.g. hypoxic, hepatic), but true encephalitis does occur, albeit infrequently. Various cardiac manifestations have been described, ranging from rare fulminant myocarditis to relatively common functional myocardial impair- ment, arrhythmias, and subclinical myocarditis. Increasingly, with systematic application of appropriate diagnostic techniques, rec- ognition of these less common clinical manifestations of dengue is improving. Severe dengue Under the new WHO scheme, patients who recover without com- plications are classified as having dengue, while those who experi- ence any one of the following problems are classified as having severe dengue: plasma leakage resulting in shock, and/​or fluid accumula- tion sufficient to cause respiratory distress; severe bleeding; severe organ impairment (e.g. liver failure, fulminant myocarditis, and so on). In children most deaths from dengue occur due to profound shock, particularly if the situation is complicated by fluid overload. In such cases severe bleeding and organ involvement sometimes de- velop secondary to shock. However, major bleeding or severe organ impairment occurring in the absence of shock are seen almost exclu- sively in adults. Differential diagnosis The differential diagnosis during the acute phase of illness includes influenza, Epstein–​Barr virus, measles, rubella, typhoid, leptospir- osis, rickettsial infections, malaria, other viral haemorrhagic fevers, primary HIV infection, and meningococcaemia. Arboviral infec- tions such as Chikungunya and Zika must be considered, particu- larly since these viruses are also transmitted primarily by Aedes mosquito species. Laboratory diagnosis During the early febrile stage (up to about day 5 of illness) labora- tory confirmation of dengue relies either on viral isolation, or on detection of viral RNA or a viral antigen such as NS1 by RT-​PCR in blood. After this time IgM antibody capture enzyme-​linked (a) (b) Fig. 8.5.15.3  (a) Petechial rash on the leg of a Vietnamese child with dengue. (b) Conjunctival petechiae in a Vietnamese adult with dengue. (a) Copyright Dinh The Trung. (b) Copyright D. A. Warrell. (a) (b) Fig. 8.5.15.4  (a) Vietnamese child with severe DSS, pleural effusions, ascites, oedema, and bruising at venepuncture sites. He required crystalloid and colloid infusions, inotropic support, and nasal continuous positive airway pressure but made a good recovery. (b) Chest X-​ray of a Vietnamese teenager after resuscitation for DSS, showing a large right pleural effusion. (a) Copyright B. A. Wills. (b) Copyright Dinh The Trung.

850 section 8  Infectious diseases immunosorbent assay (MAC-​ELISA) is the most widely used serological test for dengue diagnosis; seroconversion or a rising titre of dengue-​specific IgM in paired samples indicates acute infection. Patients with a secondary infection (either dengue or another flavivirus infection) often develop high levels of IgG anti- bodies in the acute phase and the IgM response may be less in- tense. Serological diagnosis is also complicated by the existence of flavivirus cross-​reactivity, making it necessary to perform tests for other locally prevalent flaviviruses in parallel with dengue ser- ology. Because antidengue antibodies persist for several months, diagnosis based on a single positive MAC-​ELISA result should be considered provisional. Rapid serological tests are now available but, in common with conventional serological tests, might not be- come positive until towards the end of the first week of illness. By contrast ELISA tests that detect circulating NS1 can be used from the first few days after fever onset; these tests are generally very specific, although sensitivity is variable depending on the infecting viral serotype and the concurrent humoral immune response. In primary (first) infections the sensitivity of NS1 detection in the fe- brile phase typically exceeds 90%, but it may be considerably lower (60–​80%) in secondary infections when an anamnestic serologic response to the previous virus is present. Commercially available rapid diagnostic kits that detect both NS1 and IgM/​IgG exploit the differing evolution of these responses to extend the window of de- tection through the acute illness. They are easy to use with quick turnaround time and are often employed at the point-​of-​care; sensi- tivity is improved (>90%) compared to NS1 rapid testing alone, with only minor loss of specificity. Management Good supportive care, with a particular focus on careful fluid man- agement, is critical for a favourable outcome. For patients with mild disease, oral rehydration is usually sufficient. Fever should be con- trolled using conventional methods. If an antipyretic is indicated, paracetamol is the preferred agent, with both aspirin and non-​ steroidal anti-​inflammatory drugs being contraindicated. Given the likelihood of thrombocytopenia and deranged coagulation, intra- muscular injections should be avoided (Fig. 8.5.15.5c). In the absence of particular features of concern, most patients can be managed with daily review as outpatients. However, the threshold for admission for infants, older people, and individuals with chronic diseases should be low. Persistent vomiting, severe abdominal pain, mucosal bleeding, or severe bleeding into the skin or subcutaneous tissues, a rapidly rising haematocrit, or a marked drop in the platelet count, indicate the need for admission for close observation and frequent monitoring of vital signs and haemato- crit. Judicious parenteral fluid therapy is indicated for those with a rapidly rising haematocrit. For patients with established DSS, prompt restoration of circu- lating plasma volume is crucial, followed by carefully controlled maintenance fluid therapy to support the circulation at a level just sufficient to maintain critical organ perfusion until vascular per- meability reverts to normal. Unfortunately, fluid overload with re- spiratory compromise is a common complication and one of the major contributors to mortality. Thus, the volume of parenteral fluid must be kept to the minimum required to maintain cardio- vascular stability and adequate urine output during the phase of active leakage, and as soon as reabsorption begins, usually about 1 to 2 days later, intravenous fluids should be stopped. Isotonic crystalloid solutions should be used initially, with colloid solu- tions reserved for patients presenting with severe DSS and those (a) (b) (c) Fig. 8.5.15.5  (a) Major bleeding at the insertion site of a venous line in a Vietnamese teenager with severe DSS. (b) Extensive subconjunctival haemorrhages in a Vietnamese adult with dengue and markedly deranged coagulation. (c) Large haematoma developing in a Singaporean adult with dengue, at the site where an intramuscular injection of an antipyretic agent had been given. (a) and (b) Copyright Dinh The Trung. (c) Copyright IIDE-​TTSH.

8.5.15  Dengue 851 who fail to improve with crystalloid therapy. Correction of meta- bolic acidosis, electrolyte imbalances, and hypoglycaemia are also essential. As confirmed in a recent randomized trial, platelet con- centrates are not indicated, even for profound thrombocytopenia unless there is overt bleeding, as the thrombocytopenia usually improves rapidly during the recovery phase without intervention. However, in the event of significant bleeding, transfusion of fresh blood, platelets, and other blood products might be indicated, but should be undertaken with great care because of the risk of fluid overload. No specific pharmacologic agents are available as yet for the treatment of dengue. Current research is focused on two main therapeutic approaches: first, reduction in viraemia through use of antiviral drugs, and second, immune modulation to suppress the immunopathogenic cascade that is thought to be responsible for the severe manifestations. Unfortunately, the findings of sev- eral recent studies that investigated use of antiviral agents in adults with confirmed dengue have been disappointing—​separate ran- domized blinded trials of chloroquine, balapiravir, and celgosivir versus matched placebo, with treatment commenced during the febrile phase, failed to demonstrate either a clinical benefit, or to alter virological or host inflammatory kinetics. With respect to immune modulation, corticosteroid therapy showed no convincing benefit on mortality from shock in several small clinical trials during the 1980s. A more recent safety study of early prednisolone use during the viraemic phase indicated no effect of steroids on viral clearance mechanisms. In addition, although the study was not powered to assess efficacy, there was no reduction in the incidence of shock or other recognized complications of dengue, suggesting that any protective effect of early steroid use is small. In another recent randomized placebo-​controlled trial lovastatin, an agent thought to have pleiotropic immunomodulatory effects that could improve endothelial function, was investigated in adults with dengue. It proved to be safe and well tolerated, but again there was no evidence of a beneficial effect on clinical manifestations of the disease. Vaccines A vaccine that simultaneously induces sustained protective im- munity against all four serotypes is needed. The only vaccine currently licensed, Dengvaxia, is a tetravalent formulation of at- tenuated yellow fever 17D vaccine strains expressing dengue enve- lope proteins. Several large phase III trials of this vaccine showed modest protection, but with rather limited efficacy against disease caused by DENV-​2. Additional analyses, performed after deploy- ment of the vaccine in several countries, indicated that naïve vac- cine recipients had a significantly higher risk for hospitalization/ severe dengue upon exposure to a subsequent wild-type infec- tion, compared to the unvaccinated trial participants regardless of age. While acknowledging that the vaccine could still have sig- nificant population-level benefits in high seroprevalence settings, WHO’s current position is that Dengvaxia should only be adminis- tered to individuals with clear evidence of prior dengue infection. Alternative candidates in clinical development include two other live attenuated virus vaccines, noninfectious vaccines and recom- binant subunit vaccines. Outcome Most patients with dengue make a full recovery. Even those with DSS and/​or significant bleeding usually do well provided they re- ceive appropriate supportive care from experienced healthcare per- sonnel during the critical phase of the illness. For severe dengue mortality rates below 1% are usual in experienced centres. Adults may go on to experience several weeks of extreme tired- ness, weakness, skin desquamation, pruritus, and depression during convalescence after infection, but there are no permanent sequelae. In general, children recover more rapidly and do not experience such problems. Prevention Although major efforts are being directed towards development of safe and effective dengue vaccines, it seems unlikely that large-​scale deployment will occur in the near future. In the meantime, disease prevention efforts continue to rely on elimination of potential vector breeding sites together with biological and chemical vector control strategies. Community control of Ae aegypti by eradication of mos- quito larvae from standing water sources is recommended, but has proved difficult to achieve and sustain in contemporary tropical urban settings. Insecticide-​treated bed nets have limited use since Ae aegypti mosquitoes are primarily daytime feeders. An alternative approach involving release of biologically or genetically modified mosquitoes designed to invade and displace wild mosquito popu- lations is showing promise. When introduced into Aedes mosqui- toes the intracellular bacterium Wolbachia reduces dengue viral replication in the mosquito, thereby limiting its ability to transmit the virus. Large studies are currently underway in Australia and Vietnam to investigate the practicalities, effectiveness, and poten- tial limitations of introducing Wolbachia infected mosquitoes into wild local mosquito populations. Genetically engineered male Aedes mosquitoes are also entering field trials in the Cayman Islands, with promising results. A combination of host protective strategies and vector control initiatives are likely to be required to reduce dengue transmission. Avoidance of mosquito bites in areas infested with Ae aegypti by using repellents containing N,N-​diethyl-​3-​methylbenzamide or picaridin and protective clothing are the most effective preventive measures for the traveller. FURTHER READING Akey DL, et al. (2014). Flavivirus NS1 structures reveal surfaces for associations with membranes and the immune system. Science, 343, 881–​5. Bhatt S, et al. (2013). The global distribution and burden of dengue. Nature, 496, 504–​7.