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.
Read the related PLoS Medicine Perspective by Steven Riley
The US Centers for Disease Control and Prevention provides detailed
information about dengue fever, including a questions and answers
section in English and Spanish
The World Health Organization provides information on dengue and
dengue hemorrhagic fever around the world (in several languages)
Links to additional information about deng