Date Published: November 4, 2008
Publisher: Public Library of Science
Author(s): Steven Riley
Abstract: Steven Riley discusses the public health implications of a new prospective spatial cluster study of dengue transmission in a rural population in Thailand.
Partial Text: If 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 .
Mammen and colleagues used a prospective spatial cluster study to resolve some of these uncertainties for dengue transmission in a rural population in Thailand . 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 infected recently with dengue. Therefore, absent children found to be infected with dengue (positive index cases) provided an ideal opportunity to directly observe possible spatial clusters of dengue infection in and around homes and schools. Children absent from school who were subsequently found not to be infected with dengue formed a perfect control group (negative index cases). The researchers recruited 33 index cases from 11 schools and 556 children from the households of index cases or nearby households (neighbors). Repeat blood samples were obtained (over 15 days of follow-up), and symptoms were recorded. The study team also tested adult mosquitoes in and around homes and schools, estimated numbers of Aedes aegypti pupae, and recorded key statistics of water usage.
For the total resources used, the design of Mammen and colleagues’ study was excellent. Their results give greater insight into the transmission dynamics of dengue than an earlier similar study in Indonesia (designed primarily to recruit early-stage infections for a pathogenesis investigation) . For example, the researchers in Thailand recruited from schools rather than hospitals, included a control group, and conducted the study in a rural rather than an urban area (giving a greater chance of distinct spatial clusters) . However, their study could certainly have been larger. As the authors note and discuss, their quantitative results are sensitive to the presence of one very heavily infected cluster and to the fact that one school was over-represented in the positive index cases. A high variance in the number of infections per cluster is an inherent characteristic of infectious disease transmission: for a given expected number of total cases, more clusters are required in infectious disease studies than would be required for chronic disease studies. With more clusters, the variations between clusters can be accounted for robustly when reporting key outcomes such as the underlying relationship between infection risk and distance.