Trends in Cholera Epidemiology

0016 C holera has been scrutinized since the birth of epidemiology, and it is still a subject of intense interest for modern-day epidemiologists. Studying cholera has led to the development of new epidemiological methods that have helped to illuminate not only cholera transmission but the whole science of infectious disease epidemiology. It was John Snow in London in the 1800s who originally established a causal link between cholera transmission and exposure to contaminated water (Figure 1). His work on cholera was fundamental in many ways: he proposed methods and ideas that are still part of the basic toolkit of modern epidemiology, such as time–spatial analysis and notions of source of exposure and incubation periods [1]. More recently, researchers have begun to understand more about the mechanisms of infectiousness of the cholera pathogen Vibrio cholerae. And in a new study in PLoS Medicine , David Hartley and colleagues have adjusted existing approaches to modeling cholera to evaluate how these recently found mechanisms of infectiousness can help us better explain the observed epidemic pattern of the disease [2]. Cholera is caused by the toxin-producing bacterium V. cholerae. In endemic regions, such as South Asia, cholera is seasonal, with explosive outbreaks occurring once or twice a year, depending on the region. Periodically, pandemic waves of cholera roll across the world causing a heavy death toll. Two features of cholera outbreaks are puzzling: their almost simultaneous appearance in distinct areas (suggesting an environmental trigger) and their explosive nature. Until the 1970s, V. cholerae was thought to be a human-specialized parasite, incapable of persisting outside its host. But in the 1990s, it became clear that V. cholerae was a successful member of the brackish water microbial community, living in association with plankton in an unculturable but viable state [3]. This fi nding sparked a debate on the relative importance of human-to-human transmission versus transmission from environment to humans. The fi nding that V. cholerae lived in brackish water shifted the balance toward the environmental hypothesis, that is, the hypothesis that seasonal outbreaks are triggered by seasonal blooming of aquatic V. cholerae [4]. But the other aspect of cholera outbreaks—its explosive nature—was still unexplained. Volunteer studies suggest that cholera infection requires consumption of a heavy infectious dose, which is unlikely to be found in the environment in the beginning of the epidemic season, even considering the blooming of aquatic V. cholerae. An important part of the …