The global distribution and burden of dengue

Dengue is a systemic viral infection transmitted between humans by Aedes mosquitoes. For some patients, dengue is a life-threatening illness. There are currently no licensed vaccines or specific therapeutics, and substantial vector control efforts have not stopped its rapid emergence and global spread. The contemporary worldwide distribution of the risk of dengue virus infection and its public health burden are poorly known. Here we undertake an exhaustive assembly of known records of dengue occurrence worldwide, and use a formal modelling framework to map the global distribution of dengue risk. We then pair the resulting risk map with detailed longitudinal information from dengue cohort studies and population surfaces to infer the public health burden of dengue in 2010. We predict dengue to be ubiquitous throughout the tropics, with local spatial variations in risk influenced strongly by rainfall, temperature and the degree of urbanization. Using cartographic approaches, we estimate there to be 390 million (95% credible interval 284-528) dengue infections per year, of which 96 million (67-136) manifest apparently (any level of disease severity). This infection total is more than three times the dengue burden estimate of the World Health Organization. Stratification of our estimates by country allows comparison with national dengue reporting, after taking into account the probability of an apparent infection being formally reported. The most notable differences are discussed. These new risk maps and infection estimates provide novel insights into the global, regional and national public health burden imposed by dengue. We anticipate that they will provide a starting point for a wider discussion about the global impact of this disease and will help to guide improvements in disease control strategies using vaccine, drug and vector control methods, and in their economic evaluation.

Dengue and dengue hemorrhagic fever

Dengue fever, a very old disease, has reemerged in the past 20 years with an expanded geographic distribution of both the viruses and the mosquito vectors, increased epidemic activity, the development of hyperendemicity (the cocirculation of multiple serotypes), and the emergence of dengue hemorrhagic fever in new geographic regions. In 1998 this mosquito-borne disease is the most important tropical infectious disease after malaria, with an estimated 100 million cases of dengue fever, 500,000 cases of dengue hemorrhagic fever, and 25,000 deaths annually. The reasons for this resurgence and emergence of dengue hemorrhagic fever in the waning years of the 20th century are complex and not fully understood, but demographic, societal, and public health infrastructure changes in the past 30 years have contributed greatly. This paper reviews the changing epidemiology of dengue and dengue hemorrhagic fever by geographic region, the natural history and transmission cycles, clinical diagnosis of both dengue fever and dengue hemorrhagic fever, serologic and virologic laboratory diagnoses, pathogenesis, surveillance, prevention, and control. A major challenge for public health officials in all tropical areas of the world is to develop and implement sustainable prevention and control programs that will reverse the trend of emergent dengue hemorrhagic fever.

Dengue viremia titer, antibody response pattern, and virus serotype correlate with disease severity

Viremia titers in serial plasma samples from 168 children with acute dengue virus infection who were enrolled in a prospective study at 2 hospitals in Thailand were examined to determine the role of virus load in the pathogenesis of dengue hemorrhagic fever (DHF). The infecting virus serotype was identified for 165 patients (DEN-1, 46 patients; DEN-2, 47 patients; DEN-3, 47 patients, DEN-4, 25 patients). Patients with DEN-2 infections experienced more severe disease than those infected with other serotypes. Eighty-one percent of patients experienced a secondary dengue virus infection that was associated with more severe disease. Viremia titers were determined for 41 DEN-1 and 46 DEN-2 patients. Higher peak titers were associated with increased disease severity for the 31 patients with a peak titer identified (mean titer of 107.6 for those with dengue fever vs. 108.5 for patients with DHF, P=.01). Increased dengue disease severity correlated with high viremia titer, secondary dengue virus infection, and DEN-2 virus type.

Refining the global spatial limits of dengue virus transmission by evidence-based consensus

BACKGROUND

Dengue is a growing problem both in its geographical spread and in its intensity, and yet current global distribution remains highly uncertain. Challenges in diagnosis and diagnostic methods as well as highly variable national health systems mean no single data source can reliably estimate the distribution of this disease. As such, there is a lack of agreement on national dengue status among international health organisations. Here we bring together all available information on dengue occurrence using a novel approach to produce an evidence consensus map of the disease range that highlights nations with an uncertain dengue status.

METHODS/PRINCIPAL FINDINGS

A baseline methodology was used to assess a range of evidence for each country. In regions where dengue status was uncertain, additional evidence types were included to either clarify dengue status or confirm that it is unknown at this time. An algorithm was developed that assesses evidence quality and consistency, giving each country an evidence consensus score. Using this approach, we were able to generate a contemporary global map of national-level dengue status that assigns a relative measure of certainty and identifies gaps in the available evidence.

CONCLUSION

The map produced here provides a list of 128 countries for which there is good evidence of dengue occurrence, including 36 countries that have previously been classified as dengue-free by the World Health Organization and/or the US Centers for Disease Control. It also identifies disease surveillance needs, which we list in full. The disease extents and limits determined here using evidence consensus, marks the beginning of a five-year study to advance the mapping of dengue virus transmission and disease risk. Completion of this first step has allowed us to produce a preliminary estimate of population at risk with an upper bound of 3.97 billion people. This figure will be refined in future work.

Dengue

The four dengue viruses are transmitted in tropical countries that circle the globe. All can cause syndromes that are self-limited or severe. The common severe syndrome--dengue haemorrhagic fever/dengue shock syndrome (DHF/DSS)--is characterised by sudden vascular permeability generated by cytokines released when T cells attack dengue-infected cells. Dengue 1 virus became prevalent in Hawaii where it was transmitted by Aedes albopictus, producing a classic virgin soil epidemic, with clinical disease seen largely in adults. In Cuba and Singapore, sequential dengue infections at long intervals produced unusually severe disease in adults. Evidence suggests that enhancing and cross-reactive neutralising antibodies regulate dengue epidemics and disease severity. Classic DHF/DSS arises during initial dengue infections in infants with low circulating amounts of maternal dengue antibodies, an observation that precludes an exclusive causal role for secondary T-cell responses. Here, I review and discuss data on clinical diagnosis and pathophysiology of vascular permeability and coagulopathy, parenteral treatment of DHF/DSS, and new laboratory tests.

Epidemic dengue/dengue hemorrhagic fever as a public health, social and economic problem in the 21st century

Dengue fever/dengue hemorrhagic fever is now one of the most important public health problems in tropical developing countries and also has major economic and societal consequences.

Pathogenesis of dengue: challenges to molecular biology

Dengue viruses occur as four antigenically related but distinct serotypes transmitted to humans by Aedes aegypti mosquitoes. These viruses generally cause a benign syndrome, dengue fever, in the American and African tropics, and a severe syndrome, dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS), in Southeast Asian children. This severe syndrome, which recently has also been identified in children infected with the virus in Puerto Rico, is characterized by increased vascular permeability and abnormal hemostasis. It occurs in infants less than 1 year of age born to dengue-immune mothers and in children 1 year and older who are immune to one serotype of dengue virus and are experiencing infection with a second serotype. Dengue viruses replicate in cells of mononuclear phagocyte lineage, and subneutralizing concentrations of dengue antibody enhance dengue virus infection in these cells. This antibody-dependent enhancement of infection regulates dengue disease in human beings, although disease severity may also be controlled genetically, possibly by permitting and restricting the growth of virus in monocytes. Monoclonal antibodies show heterogeneous distribution of antigenic epitopes on dengue viruses. These epitopes serve to regulate disease: when antibodies to shared antigens partially neutralize heterotypic virus, infection and disease are dampened; enhancing antibodies alone result in heightened disease response. Further knowledge of the structure of dengue genomes should permit rapid advances in understanding the pathogenetic mechanisms of dengue.

Efficacy and Long-Term Safety of a Dengue Vaccine in Regions of Endemic Disease

BACKGROUND

A candidate tetravalent dengue vaccine is being assessed in three clinical trials involving more than 35,000 children between the ages of 2 and 16 years in Asian-Pacific and Latin American countries. We report the results of long-term follow-up interim analyses and integrated efficacy analyses.

METHODS

We are assessing the incidence of hospitalization for virologically confirmed dengue as a surrogate safety end point during follow-up in years 3 to 6 of two phase 3 trials, CYD14 and CYD15, and a phase 2b trial, CYD23/57. We estimated vaccine efficacy using pooled data from the first 25 months of CYD14 and CYD15.

RESULTS

Follow-up data were available for 10,165 of 10,275 participants (99%) in CYD14 and 19,898 of 20,869 participants (95%) in CYD15. Data were available for 3203 of the 4002 participants (80%) in the CYD23 trial included in CYD57. During year 3 in the CYD14, CYD15, and CYD57 trials combined, hospitalization for virologically confirmed dengue occurred in 65 of 22,177 participants in the vaccine group and 39 of 11,089 participants in the control group. Pooled relative risks of hospitalization for dengue were 0.84 (95% confidence interval [CI], 0.56 to 1.24) among all participants, 1.58 (95% CI, 0.83 to 3.02) among those under the age of 9 years, and 0.50 (95% CI, 0.29 to 0.86) among those 9 years of age or older. During year 3, hospitalization for severe dengue, as defined by the independent data monitoring committee criteria, occurred in 18 of 22,177 participants in the vaccine group and 6 of 11,089 participants in the control group. Pooled rates of efficacy for symptomatic dengue during the first 25 months were 60.3% (95% CI, 55.7 to 64.5) for all participants, 65.6% (95% CI, 60.7 to 69.9) for those 9 years of age or older, and 44.6% (95% CI, 31.6 to 55.0) for those younger than 9 years of age.

CONCLUSIONS

Although the unexplained higher incidence of hospitalization for dengue in year 3 among children younger than 9 years of age needs to be carefully monitored during long-term follow-up, the risk among children 2 to 16 years of age was lower in the vaccine group than in the control group. (Funded by Sanofi Pasteur; ClinicalTrials.gov numbers, NCT00842530, NCT01983553, NCT01373281, and NCT01374516.).

Dengue: an update

This review is an update of dengue and dengue haemorrhagic fever (DHF) based on international and Cuban experience. We describe the virus characteristics and risk factors for dengue and DHF, and compare incidence and the case fatality rates in endemic regions (southeast Asia, western Pacific, and the Americas). The clinical picture and the pathogenesis of the severe disease are explained. We also discuss the viral, individual, and environmental factors that determine severe disease. Much more research is necessary to clarify these mechanisms. Also reviewed are methods for viral isolation and the serological, immunohistochemical, and molecular methods applied in the diagnosis of the disease. We describe the status of vaccine development and emphasise that the only alternative that we have today to control the disease is through control of its vector Aedes aegypti.

Critical review of the vector status of Aedes albopictus

The mosquito Aedes (Stegomyia) albopictus (Skuse) (Diptera: Culicidae), originally indigenous to South-east Asia, islands of the Western Pacific and Indian Ocean, has spread during recent decades to Africa, the mid-east, Europe and the Americas (north and south) after extending its range eastwards across Pacific islands during the early 20th century. The majority of introductions are apparently due to transportation of dormant eggs in tyres. Among public health authorities in the newly infested countries and those threatened with the introduction, there has been much concern that Ae. albopictus would lead to serious outbreaks of arbovirus diseases (Ae. albopictus is a competent vector for at least 22 arboviruses), notably dengue (all four serotypes) more commonly transmitted by Aedes (Stegomyia) aegypti (L.). Results of many laboratory studies have shown that many arboviruses are readily transmitted by Ae. albopictus to laboratory animals and birds, and have frequently been isolated from wild-caught mosquitoes of this species, particularly in the Americas. As Ae. albopictus continues to spread, displacing Ae. aegypti in some areas, and is anthropophilic throughout its range, it is important to review the literature and attempt to predict whether the medical risks are as great as have been expressed in scientific journals and the popular press. Examination of the extensive literature indicates that Ae. albopictus probably serves as a maintenance vector of dengue in rural areas of dengue-endemic countries of South-east Asia and Pacific islands. Also Ae. albopictus transmits dog heartworm Dirofilaria immitis (Leidy) (Spirurida: Onchocercidae) in South-east Asia, south-eastern U.S.A. and both D. immitis and Dirofilaria repens (Raillet & Henry) in Italy. Despite the frequent isolation of dengue viruses from wild-caught mosquitoes, there is no evidence that Ae. albopictus is an important urban vector of dengue, except in a limited number of countries where Ae. aegypti is absent, i.e. parts of China, the Seychelles, historically in Japan and most recently in Hawaii. Further research is needed on the dynamics of the interaction between Ae. albopictus and other Stegomyia species. Surveillance must also be maintained on the vectorial role of Ae. albopictus in countries endemic for dengue and other arboviruses (e.g. Chikungunya, EEE, Ross River, WNV, LaCrosse and other California group viruses), for which it would be competent and ecologically suited to serve as a bridge vector.

Spectrum of disease and relation to place of exposure among ill returned travelers

BACKGROUND

Approximately 8 percent of travelers to the developing world require medical care during or after travel. Current understanding of morbidity profiles among ill returned travelers is based on limited data from the 1980s.

METHODS

Thirty GeoSentinel sites, which are specialized travel or tropical-medicine clinics on six continents, contributed clinician-based sentinel surveillance data for 17,353 ill returned travelers. We compared the frequency of occurrence of each diagnosis among travelers returning from six developing regions of the world.

RESULTS

Significant regional differences in proportionate morbidity were detected in 16 of 21 broad syndromic categories. Among travelers presenting to GeoSentinel sites, systemic febrile illness without localizing findings occurred disproportionately among those returning from sub-Saharan Africa or Southeast Asia, acute diarrhea among those returning from south central Asia, and dermatologic problems among those returning from the Caribbean or Central or South America. With respect to specific diagnoses, malaria was one of the three most frequent causes of systemic febrile illness among travelers from every region, although travelers from every region except sub-Saharan Africa and Central America had confirmed or probable dengue more frequently than malaria. Among travelers returning from sub-Saharan Africa, rickettsial infection, primarily tick-borne spotted fever, occurred more frequently than typhoid or dengue. Travelers from all regions except Southeast Asia presented with parasite-induced diarrhea more often than with bacterial diarrhea.

CONCLUSIONS

When patients present to specialized clinics after travel to the developing world, travel destinations are associated with the probability of the diagnosis of certain diseases. Diagnostic approaches and empiric therapies can be guided by these destination-specific differences.

A prospective study of dengue infections in Bangkok

Dengue infections were prospectively studied among 4- to 16-year-old students at a Bangkok school. Blood samples were obtained from 1,757 students in June 1980, before the dengue season, and in January 1981, after the season, and tested for dengue antibodies by the hemagglutination inhibition method. Classrooms were monitored daily for school absences. Fifty percent of the children had antibodies to, and were presumably immune to, at least 1 dengue serotype by the age of 7 years. Most (90/103, 87%) students who became infected by dengue viruses during the study period were either asymptomatic or minimally symptomatic (absent only 1 day). Most (7/13, 53%) of the symptomatic dengue infections (absent with fever for greater than or equal to 2 days) were clinically recognized as cases of dengue hemorrhagic fever which required hospitalization. None of 47 primary dengue infections required hospitalization, whereas 7 of 56 secondary infections did (P = 0.012). Preexistent dengue immunity, as detected by conventional serologic techniques, was a significant (odds ratio greater than or equal to 6.5) risk factor for development of dengue hemorrhagic fever.

Effect of Dengue Serostatus on Dengue Vaccine Safety and Efficacy

BACKGROUND

In efficacy trials of a tetravalent dengue vaccine (CYD-TDV), excess hospitalizations for dengue were observed among vaccine recipients 2 to 5 years of age. Precise risk estimates according to observed dengue serostatus could not be ascertained because of the limited numbers of samples collected at baseline. We developed a dengue anti-nonstructural protein 1 (NS1) IgG enzyme-linked immunosorbent assay and used samples from month 13 to infer serostatus for a post hoc analysis of safety and efficacy.

METHODS

In a case-cohort study, we reanalyzed data from three efficacy trials. For the principal analyses, we used baseline serostatus determined on the basis of measured (when baseline values were available) or imputed (when baseline values were missing) titers from a 50% plaque-reduction neutralization test (PRNT₅₀), with imputation conducted with the use of covariates that included the month 13 anti-NS1 assay results. The risk of hospitalization for virologically confirmed dengue (VCD), of severe VCD, and of symptomatic VCD according to dengue serostatus was estimated by weighted Cox regression and targeted minimum loss-based estimation.

RESULTS

Among dengue-seronegative participants 2 to 16 years of age, the cumulative 5-year incidence of hospitalization for VCD was 3.06% among vaccine recipients and 1.87% among controls, with a hazard ratio (vaccine vs. control) through data cutoff of 1.75 (95% confidence interval [CI], 1.14 to 2.70). Among dengue-seronegative participants 9 to 16 years of age, the cumulative incidence of hospitalization for VCD was 1.57% among vaccine recipients and 1.09% among controls, with a hazard ratio of 1.41 (95% CI, 0.74 to 2.68). Similar trends toward a higher risk among seronegative vaccine recipients than among seronegative controls were also found for severe VCD. Among dengue-seropositive participants 2 to 16 years of age and those 9 to 16 years of age, the cumulative incidence of hospitalization for VCD was 0.75% and 0.38%, respectively, among vaccine recipients and 2.47% and 1.88% among controls, with hazard ratios of 0.32 (95% CI, 0.23 to 0.45) and 0.21 (95% CI, 0.14 to 0.31). The risk of severe VCD was also lower among seropositive vaccine recipients than among seropositive controls.

CONCLUSIONS

CYD-TDV protected against severe VCD and hospitalization for VCD for 5 years in persons who had exposure to dengue before vaccination, and there was evidence of a higher risk of these outcomes in vaccinated persons who had not been exposed to dengue. (Funded by Sanofi Pasteur; ClinicalTrials.gov numbers, NCT00842530 , NCT01983553 , NCT01373281 , and NCT01374516 .).

Dengue and dengue haemorrhagic fever

The incidence and geographical distribution of dengue have greatly increased in recent years. Dengue is an acute mosquito-transmitted viral disease characterised by fever, headache, muscle and joint pains, rash, nausea, and vomiting. Some infections result in dengue haemorrhagic fever (DHF), a syndrome that in its most severe form can threaten the patient's life, primarily through increased vascular permeability and shock. The case fatality rate in patients with dengue shock syndrome can be as high as 44%. For decades, two distinct hypotheses to explain the mechanism of DHF have been debated-secondary infection or viral virulence. However, a combination of both now seems to be the plausible explanation. The geographical expansion of DHF presents the need for well-documented clinical, epidemiological, and virological descriptions of the syndrome in the Americas. Biological and social research are essential to develop effective mosquito control, medications to reduce capillary leakage, and a safe tetravalent vaccine.

Consequences of the expanding global distribution of Aedes albopictus for dengue virus transmission

The dramatic global expansion of Aedes albopictus in the last three decades has increased public health concern because it is a potential vector of numerous arthropod-borne viruses (arboviruses), including the most prevalent arboviral pathogen of humans, dengue virus (DENV). Ae. aegypti is considered the primary DENV vector and has repeatedly been incriminated as a driving force in dengue's worldwide emergence. What remains unresolved is the extent to which Ae. albopictus contributes to DENV transmission and whether an improved understanding of its vector status would enhance dengue surveillance and prevention. To assess the relative public health importance of Ae. albopictus for dengue, we carried out two complementary analyses. We reviewed its role in past dengue epidemics and compared its DENV vector competence with that of Ae. aegypti. Observations from "natural experiments" indicate that, despite seemingly favorable conditions, places where Ae. albopictus predominates over Ae. aegypti have never experienced a typical explosive dengue epidemic with severe cases of the disease. Results from a meta-analysis of experimental laboratory studies reveal that although Ae. albopictus is overall more susceptible to DENV midgut infection, rates of virus dissemination from the midgut to other tissues are significantly lower in Ae. albopictus than in Ae. aegypti. For both indices of vector competence, a few generations of mosquito colonization appear to result in a relative increase of Ae. albopictus susceptibility, which may have been a confounding factor in the literature. Our results lead to the conclusion that Ae. albopictus plays a relatively minor role compared to Ae. aegypti in DENV transmission, at least in part due to differences in host preferences and reduced vector competence. Recent examples of rapid arboviral adaptation to alternative mosquito vectors, however, call for cautious extrapolation of our conclusion. Vector status is a dynamic process that in the future could change in epidemiologically important ways.

Risk factors in dengue shock syndrome: a prospective epidemiologic study in Rayong, Thailand. I. The 1980 outbreak

In January 1980, the municipal area of Rayong, Thailand, and contiguous suburban villages were chosen for a long-term study on dengue epidemiology. From 3,185 children randomly sampled in schools and households, the population prevalence of neutralizing antibody to the four dengue serotypes was estimated. To estimate the incidence of infection with each dengue virus serotype (dengue seroconversions), first grade children were re-bled in January 1981 (cohort study). Children admitted to hospital were studied for dengue virus isolation and antibody responses in paired sera. An epidemic of dengue occurred in 1980. Plaque reduction neutralization tests of 1,009 pre-epidemic sera from children aged less than 1-10 years of age determined that 3.3% were immune to dengue 1, 13.2% to dengue 2, 6.4% to dengue 3, and 5.8% to dengue 4. Examination of pre- and post-epidemic cohort blood samples revealed that the incidence of dengue infection in 251 seronegative children was 39.4% (15.1% dengue 1, 11.1% dengue 2, 2.0% dengue 3, 4.8% dengue 4, and 6.4% two or more dengue viruses). Among the 52,935 residents of the study area, there were 22 cases of virologically and clinically confirmed dengue shock syndrome, in children 15 years or younger. All 22 shock syndrome cases had secondary type antibody responses. Eight of 22 had been included in the random serologic sample prior to onset of shock; five had been immune to dengue 1, two to dengue 3, one to dengue 4, and none to dengue 2. Despite the high rate of dengue 1 infections in 1980, only dengue 2 viruses were recovered from dengue shock syndrome cases, including two dengue 1 immune children with pre-illness serum specimens. Although the pre-epidemic prevalence of antibodies to dengue 1 was the lowest to any type, children with this immunologic background contributed disproportionately to shock cases. In descending order of magnitude, risk factors for dengue shock syndrome in Rayong were secondary infections with dengue 2 which followed primary infections with dengue 1, dengue 3, or dengue 4.

Dengue: the risk to developed and developing countries

Dengue viruses are members of the Flaviviridae, transmitted principally in a cycle involving humans and mosquito vectors. In the last 20 years the incidence of dengue fever epidemics has increased and hyperendemic transmission has been established over a geographically expanding area. A severe form, dengue hemorrhagic fever (DHF), is an immunopathologic disease occurring in persons who experience sequential dengue infections. The risk of sequential infections, and consequently the incidence of DHF, has risen dramatically, first in Asia and now in the Americas. At the root of the emergence of dengue as a major health problem are changes in human demography and behavior, leading to unchecked populations of and increased exposure to the principal domestic mosquito vector, Aedes aegypti. Virus-specified factors also influence the epidemiology of dengue. Speculations on future events in the epidemiology, evolution, and biological expression of dengue are presented.

Prospects for a dengue virus vaccine

The number of cases of severe dengue disease continues to grow in endemic areas of southeast Asia, Central and South America, and other subtropical regions. Children bear the greatest burden of disease, and the development of an effective vaccine remains a global public health priority. A tetravalent vaccine is urgently needed and must be effective against all four dengue virus serotypes, be cost-effective and provide long-term protection. In this Review we discuss the unique immunological concerns in dengue virus vaccine development and the current prospects for the development of an acceptable vaccine, a goal that is likely to be reached in the near future.

Dengue

Mortality from severe dengue is low, but the economic and resource burden on health services remains substantial in endemic settings. Unfortunately, progress towards development of effective therapeutics has been slow, despite notable advances in the understanding of disease pathogenesis and considerable investment in antiviral drug discovery. For decades antibody-dependent enhancement has been the prevalent model to explain dengue pathogenesis, but it was only recently demonstrated in vivo and in clinical studies. At present, the current mainstay of management for most symptomatic dengue patients remains careful observation and prompt but judicious use of intravenous hydration therapy for those with substantial vascular leakage. Various new promising technologies for diagnosis of dengue are currently in the pipeline. New sample-in, answer-out nucleic acid amplification technologies for point-of-care use are being developed to improve performance over current technologies, with the potential to test for multiple pathogens using a single specimen. The search for biomarkers that reliably predict development of severe dengue among symptomatic individuals is also a major focus of current research efforts. The first dengue vaccine was licensed in 2015 but its performance depends on serostatus. There is an urgent need to identify correlates of both vaccine protection and disease enhancement. A crucial assessment of vector control tools should guide a research agenda for determining the most effective interventions, and how to best combine state-of-the-art vector control with vaccination.

Detecting the impact of temperature on transmission of Zika, dengue, and chikungunya using mechanistic models

Recent epidemics of Zika, dengue, and chikungunya have heightened the need to understand the seasonal and geographic range of transmission by Aedes aegypti and Ae. albopictus mosquitoes. We use mechanistic transmission models to derive predictions for how the probability and magnitude of transmission for Zika, chikungunya, and dengue change with mean temperature, and we show that these predictions are well matched by human case data. Across all three viruses, models and human case data both show that transmission occurs between 18-34°C with maximal transmission occurring in a range from 26-29°C. Controlling for population size and two socioeconomic factors, temperature-dependent transmission based on our mechanistic model is an important predictor of human transmission occurrence and incidence. Risk maps indicate that tropical and subtropical regions are suitable for extended seasonal or year-round transmission, but transmission in temperate areas is limited to at most three months per year even if vectors are present. Such brief transmission windows limit the likelihood of major epidemics following disease introduction in temperate zones.

The role of human movement in the transmission of vector-borne pathogens

Background

Human movement is a key behavioral factor in many vector-borne disease systems because it influences exposure to vectors and thus the transmission of pathogens. Human movement transcends spatial and temporal scales with different influences on disease dynamics. Here we develop a conceptual model to evaluate the importance of variation in exposure due to individual human movements for pathogen transmission, focusing on mosquito-borne dengue virus.

Methodology and Principal Findings

We develop a model showing that the relevance of human movement at a particular scale depends on vector behavior. Focusing on the day-biting Aedes aegypti, we illustrate how vector biting behavior combined with fine-scale movements of individual humans engaged in their regular daily routine can influence transmission. Using a simple example, we estimate a transmission rate (R₀) of 1.3 when exposure is assumed to occur only in the home versus 3.75 when exposure at multiple locations—e.g., market, friend's—due to movement is considered. Movement also influences for which sites and individuals risk is greatest. For the example considered, intriguingly, our model predicts little correspondence between vector abundance in a site and estimated R₀ for that site when movement is considered. This illustrates the importance of human movement for understanding and predicting the dynamics of a disease like dengue. To encourage investigation of human movement and disease, we review methods currently available to study human movement and, based on our experience studying dengue in Peru, discuss several important questions to address when designing a study.

Conclusions/Significance

Human movement is a critical, understudied behavioral component underlying the transmission dynamics of many vector-borne pathogens. Understanding movement will facilitate identification of key individuals and sites in the transmission of pathogens such as dengue, which then may provide targets for surveillance, intervention, and improved disease prevention.

Vector-borne diseases constitute a largely neglected and enormous burden on public health in many resource-challenged environments, demanding efficient control strategies that could be developed through improved understanding of pathogen transmission. Human movement—which determines exposure to vectors—is a key behavioral component of vector-borne disease epidemiology that is poorly understood. We develop a conceptual framework to organize past studies by the scale of movement and then examine movements at fine-scale—i.e., people going through their regular, daily routine—that determine exposure to insect vectors for their role in the dynamics of pathogen transmission. We develop a model to quantify risk of vector contact across locations people visit, with emphasis on mosquito-borne dengue virus in the Amazonian city of Iquitos, Peru. An example scenario illustrates how movement generates variation in exposure risk across individuals, how transmission rates within sites can be increased, and that risk within sites is not solely determined by vector density, as is commonly assumed. Our analysis illustrates the importance of human movement for pathogen transmission, yet little is known—especially for populations most at risk to vector-borne diseases (e.g., dengue, leishmaniasis, etc.). We outline several important considerations for designing epidemiological studies to encourage investigation of individual human movement, based on experience studying dengue.

House-to-house human movement drives dengue virus transmission

Dengue is a mosquito-borne disease of growing global health importance. Prevention efforts focus on mosquito control, with limited success. New insights into the spatiotemporal drivers of dengue dynamics are needed to design improved disease-prevention strategies. Given the restricted range of movement of the primary mosquito vector, Aedes aegypti, local human movements may be an important driver of dengue virus (DENV) amplification and spread. Using contact-site cluster investigations in a case-control design, we demonstrate that, at an individual level, risk for human infection is defined by visits to places where contact with infected mosquitoes is likely, independent of distance from the home. Our data indicate that house-to-house human movements underlie spatial patterns of DENV incidence, causing marked heterogeneity in transmission rates. At a collective level, transmission appears to be shaped by social connections because routine movements among the same places, such as the homes of family and friends, are often similar for the infected individual and their contacts. Thus, routine, house-to-house human movements do play a key role in spread of this vector-borne pathogen at fine spatial scales. This finding has important implications for dengue prevention, challenging the appropriateness of current approaches to vector control. We argue that reexamination of existing paradigms regarding the spatiotemporal dynamics of DENV and other vector-borne pathogens, especially the importance of human movement, will lead to improvements in disease prevention.

Suppression of a Field Population of Aedes aegypti in Brazil by Sustained Release of Transgenic Male Mosquitoes

The increasing burden of dengue, and the relative failure of traditional vector control programs highlight the need to develop new control methods. SIT using self-limiting genetic technology is one such promising method. A self-limiting strain of Aedes aegypti, OX513A, has already reached the stage of field evaluation. Sustained releases of OX513A Ae. aegypti males led to 80% suppression of a target wild Ae. aegypti population in the Cayman Islands in 2010. Here we describe sustained series of field releases of OX513A Ae. aegypti males in a suburb of Juazeiro, Bahia, Brazil. This study spanned over a year and reduced the local Ae. aegypti population by 95% (95% CI: 92.2%-97.5%) based on adult trap data and 81% (95% CI: 74.9-85.2%) based on ovitrap indices compared to the adjacent no-release control area. The mating competitiveness of the released males (0.031; 95% CI: 0.025-0.036) was similar to that estimated in the Cayman trials (0.059; 95% CI: 0.011-0.210), indicating that environmental and target-strain differences had little impact on the mating success of the OX513A males. We conclude that sustained release of OX513A males may be an effective and widely useful method for suppression of the key dengue vector Ae. aegypti. The observed level of suppression would likely be sufficient to prevent dengue epidemics in the locality tested and other areas with similar or lower transmission.