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      Spatial and Temporal Clustering of Dengue Virus Transmission in Thai Villages

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          Abstract

          Background

          Transmission of dengue viruses (DENV), the leading cause of arboviral disease worldwide, is known to vary through time and space, likely owing to a combination of factors related to the human host, virus, mosquito vector, and environment. An improved understanding of variation in transmission patterns is fundamental to conducting surveillance and implementing disease prevention strategies. To test the hypothesis that DENV transmission is spatially and temporally focal, we compared geographic and temporal characteristics within Thai villages where DENV are and are not being actively transmitted.

          Methods and Findings

          Cluster investigations were conducted within 100 m of homes where febrile index children with (positive clusters) and without (negative clusters) acute dengue lived during two seasons of peak DENV transmission. Data on human infection and mosquito infection/density were examined to precisely (1) define the spatial and temporal dimensions of DENV transmission, (2) correlate these factors with variation in DENV transmission, and (3) determine the burden of inapparent and symptomatic infections. Among 556 village children enrolled as neighbors of 12 dengue-positive and 22 dengue-negative index cases, all 27 DENV infections (4.9% of enrollees) occurred in positive clusters ( p < 0.01; attributable risk [AR] = 10.4 per 100; 95% confidence interval 1–19.8 per 100]. In positive clusters, 12.4% of enrollees became infected in a 15-d period and DENV infections were aggregated centrally near homes of index cases. As only 1 of 217 pairs of serologic specimens tested in positive clusters revealed a recent DENV infection that occurred prior to cluster initiation, we attribute the observed DENV transmission subsequent to cluster investigation to recent DENV transmission activity. Of the 1,022 female adult Ae. aegypti collected, all eight (0.8%) dengue-infected mosquitoes came from houses in positive clusters; none from control clusters or schools. Distinguishing features between positive and negative clusters were greater availability of piped water in negative clusters ( p < 0.01) and greater number of Ae. aegypti pupae per person in positive clusters ( p = 0.04). During primarily DENV-4 transmission seasons, the ratio of inapparent to symptomatic infections was nearly 1:1 among child enrollees. Study limitations included inability to sample all children and mosquitoes within each cluster and our reliance on serologic rather than virologic evidence of interval infections in enrollees given restrictions on the frequency of blood collections in children.

          Conclusions

          Our data reveal the remarkably focal nature of DENV transmission within a hyperendemic rural area of Thailand. These data suggest that active school-based dengue case detection prompting local spraying could contain recent virus introductions and reduce the longitudinal risk of virus spread within rural areas. Our results should prompt future cluster studies to explore how host immune and behavioral aspects may impact DENV transmission and prevention strategies. Cluster methodology could serve as a useful research tool for investigation of other temporally and spatially clustered infectious diseases.

          Abstract

          Investigating dengue cases identified by testing febrile schoolchildren in rural Thai villages, Mammen P. Mammen and colleagues find a pattern of focal spread to houses neighboring those of case patients.

          Editors' Summary

          Background.

          Every year, over 50 million people living in tropical and subtropical urban and semi-urban areas become infected with dengue (a mosquito-borne viral infection) and several hundred thousand develop a potentially lethal complication called dengue hemorrhagic fever. Dengue is caused by four closely related viruses that are transmitted to people through the bites of infected female Aedes aegypti mosquitoes. These day-biting insects, which breed in household water containers and in the water that collects in used tires and other discarded containers, acquire dengue virus through feeding on the blood of an infected person. Some people who become infected with dengue virus have no symptoms but others develop high fever, a rash, and severe headache that lasts two to seven days. In dengue hemorrhagic fever, small blood vessels become leaky, which causes nose and gum bleeds, bruising and, in the worst cases, failure of the circulatory system and death. There is no specific treatment for dengue fever or dengue hemorrhagic fever but standard medical care—in particular, replacement of lost blood fluids—helps most people survive the latter condition.

          Why Was This Study Done?

          There is no vaccine to prevent dengue. As a result the only way to minimize dengue outbreaks is to control mosquito numbers through environmental management—providing piped water, encouraging people not to store water in open containers, and removing other sources of standing water—and by applying insecticides to areas where mosquitoes breed. During outbreaks, because Ae. aegypti mosquitoes rest in houses, insecticides are also often sprayed in dwellings in the affected areas. However, to improve dengue prevention and surveillance, public-health officials need to know much more about the patterns of dengue virus transmission and about the factors that underlie these patterns. In this study, therefore, the researchers test the idea that dengue virus transmission occurs in localized neighborhood clusters over short periods of time.

          What Did the Researchers Do and Find?

          The researchers used “cluster investigations” to examine the pattern of dengue virus transmission among school children in several rural villages in Thailand, a country where dengue is very common (hyperendemic). Primary school children with fever were identified during two seasons of peak dengue virus transmission. Each child was characterized as a dengue-positive index case (by finding dengue virus in their blood) or as a dengue-negative index case. Data on human infection and mosquito infection and density were then collected within 100 meters of the homes of each index case—the “cluster area.” Not all the neighbors of the index cases participated in the study but among the 556 village children who did participate, there were 27 dengue infections, all of which occurred in clusters centered on the homes of the dengue-positive index cases. In the positive clusters, one in eight of the enrolled children became infected within 15 days of the index case becoming ill. Among 1,000 Ae. aegypti mosquitoes collected inside and around the houses in each cluster, only eight were infected with dengue and these were all collected from houses in positive clusters. Finally, there was a greater availability of piped water and fewer Ae. aegypti pupae in the negative clusters than in the positive clusters.

          What Do These Findings Mean?

          Although this study did not sample all the children or mosquitoes within each cluster area, these findings show that in an area where dengue is hyperendemic, dengue virus transmission among children occurs in localized areas and over short time periods. The findings also suggest that focal transmission is associated with recent dengue virus introductions and that one or a few mosquitoes are likely responsible for all the transmission in each cluster. Although it would be impractical to set up surveillance of all the school children in Thailand for dengue infections, these findings suggest that improved detection of cases within schools combined with local spraying inside the homes in the immediate vicinity of any affected children could help to halt dengue virus transmission. Future cluster studies could explore how human behavior and human immunity affect dengue virus transmission and could also be used to investigate other temporally and spatially clustered infectious diseases, including malaria.

          Additional Information.

          Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0050205.

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          Most cited references 40

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          Categorical Data Analysis

           Alan Agresti (2002)
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            Dispersal of the dengue vector Aedes aegypti within and between rural communities.

            Knowledge of mosquito dispersal is critical for vector-borne disease control and prevention strategies and for understanding population structure and pathogen dissemination. We determined Aedes aegypti flight range and dispersal patterns from 21 mark-release-recapture experiments conducted over 11 years (1991-2002) in Puerto Rico and Thailand. Dispersal was compared by release location, sex, age, season, and village. For all experiments, the majority of mosquitoes were collected from their release house or adjacent house. Inter-village movement was detected rarely, with a few mosquitoes moving a maximum of 512 meters from one Thai village to the next. Average dispersal distances were similar for males and females and females released indoors versus outdoors. The movement of Ae. aegypti was not influenced by season or age, but differed by village. Results demonstrate that adult Ae. aegypti disperse relatively short distances, suggesting that people rather than mosquitoes are the primary mode of dengue virus dissemination within and among communities.
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              Differing influences of virus burden and immune activation on disease severity in secondary dengue-3 virus infections.

              Dengue hemorrhagic fever (DHF), the most severe form of illness following infection with a dengue virus, is characterized by plasma leakage, thrombocytopenia, and hepatic inflammation. The interrelationships among virus burden, immune activation, and development of DHF were examined in 54 children with secondary dengue-3 virus infections participating in a prospective, hospital-based study. DHF was associated with higher mean plasma viremia early in illness and earlier peak plasma interferon-gamma levels. Maximum plasma viremia levels correlated with the degree of plasma leakage and thrombocytopenia. Maximum plasma levels of interleukin (IL)-10 and soluble tumor necrosis factor receptor-II correlated with the degree of thrombocytopenia, independently of viremia levels. Hepatic transaminase elevation correlated with plasma soluble IL-2 receptor levels and not with viremia levels. Quantitative differences in virus burden and host immune responses, and the timing of type 1 cytokine responses, have differing influences on the severity of disease manifestations during secondary dengue-3 virus infections.
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                Author and article information

                Contributors
                Role: Academic Editor
                Journal
                PLoS Med
                pmed
                plme
                plosmed
                PLoS Medicine
                Public Library of Science (San Francisco, USA )
                1549-1277
                1549-1676
                November 2008
                4 November 2008
                : 5
                : 11
                Affiliations
                [1 ] Department of Virology, Armed Forces Research Institute of Medical Sciences (AFRIMS), Bangkok, Thailand
                [2 ] Department of Entomology, University of California, Davis, California, United States of America
                [3 ] Center for Infectious Disease and Vaccine Research, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America
                [4 ] Department of Geography, San Diego State University, San Diego, California, United States of America
                [5 ] Department of Entomology, AFRIMS, Bangkok, Thailand
                [6 ] Office of Disease Prevention and Control, Ministry of Public Health, Nonthaburi, Thailand
                Hong Kong University, Hong Kong
                Author notes
                * To whom correspondence should be addressed. E-mail: mammen.mammen@ 123456us.army.mil
                Article
                08-PLME-RA-0417R2 plme-05-11-01
                10.1371/journal.pmed.0050205
                2577695
                18986209
                This is an open-access article distributed under the terms of the Creative Commons Public Domain declaration, which stipulates that, once placed in the public domain, this work may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose.
                Page count
                Pages: 12
                Categories
                Research Article
                Ecology
                Infectious Diseases
                Public Health and Epidemiology
                Virology
                Infectious Diseases
                Epidemiology
                Public Health
                Global Health
                Medicine in Developing Countries
                Custom metadata
                Mammen MP Jr, Pimgate C, Koenraadt CJM, Rothman AL, Aldstadt J, et al. (2008) Spatial and temporal clustering of dengue virus transmission in Thai villages. PLoS Med 5(11): e205. doi: 10.1371/journal.pmed.0050205

                Medicine

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