A Prospective Study of Spatial Clusters Gives Valuable Insights into Dengue Transmission

Perspective I f a school-age child contracts dengue in rural Thailand, it is difficult to predict whether that child was infected at home, at school, or somewhere else. Therefore it is difficult to know how best to allocate scarce resources towards public health measures to reduce the chance of further infections. However, the potential for transmission at home relative to the potential at school must depend mainly on two features of those settings: (1) the density of effective vectors compared with the human population, and (2) the previous infection history of the human population. Unfortunately, despite the wealth of knowledge that is available on many detailed aspects of the biology of dengue, firm ecological evidence with which to estimate key public health outcomes (such as the relative risk of transmission in different settings) remains surprisingly scarce. A prospective spatial cluster study of dengue transmission in rural Thailand by Mammen P. Mammen and colleagues reported in this issue of PLoS Medicine represents an important step forward in addressing this shortfall [1]. The global impact of dengue grew dramatically over the latter part of the 20th century, with the annual incidence currently estimated to be 50 million cases of dengue fever and 500,000 hospitalizations due to dengue hemorrhagic fever [2]. The pathogen is found throughout the tropics and subtropics (almost everywhere that viable vectors are found), with 2.5 billion people thought to be at risk [3]. No safe effective vaccine is currently available to prevent dengue infections. Therefore, public health strategies to reduce the adverse effects of the virus on human populations are designed (1) to reduce transmission by reducing vector abundance and (2) to mitigate morbidity using active surveillance and rapid supportive therapy [4]. Both strategies require substantial human resources that could be more effectively deployed if the patterns of dengue transmission at the local level were better understood. For example, although it is widely accepted that children are at an excess risk of infection with dengue, it is not clear if these extra infections occur at home, at school, or elsewhere. Mammen and colleagues used a prospective spatial cluster study to resolve some of these uncertainties for dengue transmission in a rural population in Thailand [1]. They started from a simple premise: during the time of year when dengue transmission is common, a substantial proportion of children who were absent from school because of illness (index cases) may have been …