Red flag. Satellite images of warmer water (red) may indicate when cholera epidemics are likely to strike.

Forecasting Cholera Outbreaks

SEATTLE--Between outbreaks, cholera hides in coastal waters and rivers. Although that means it's impossible to eradicate the disease, scientists now think they may be able to predict cholera epidemics. A new study shows that climatic changes, such as warmer water, can provide an early warning.

Cholera reappeared in Latin America in 1991, initially in Peru, after more than a century's hiatus. One of the places that the bacterium Vibrio cholerae hides is on minuscule crustaceans called copepods. These critters can contaminate the water supply by moving into estuaries and rivers. When drinking water is not filtered properly, they can transmit cholera bacteria directly to humans.

The authors of the new report--including microbiologists Ana Gil of the Institute of Nutritional Investigation in Lima and Valerie Louis of the Center of Marine Biotechnology in Baltimore--investigated the connection among cholera, copepods, and climate. Each month between October 1997 and June 2000 they mapped the distribution of V. cholerae in seawater and in zooplankton (including copepods) in four sites off the coast of Peru and also measured many environmental factors. The results, presented here at the AAAS annual meeting on 14 February, showed that when the sea surface warmed off the coast of Peru in 1997-98, during El Niño, there was a marked increase in rates of cholera infection in Lima and nearby cities. By culturing the bacteria from plankton samples, the team also confirmed that certain species of copepods act as a reservoir for the disease between outbreaks.

The strong correlation among climate, copepods, and cholera suggests that satellite monitoring of sea surface temperatures and phytoplankton (eaten by zooplankton such as copepods) may enable predictions of disease outbreaks, says lead investigator and microbiologist Rita Colwell of the University of Maryland, College Park. The team is currently developing computer models that can predict disease outbreaks in both Latin America and Bangladesh, Colwell says. The findings will be published in an upcoming issue of Environmental Microbiology.

"Linking environmental conditions to cycles of illness is a powerful shift in approach, especially since marine invertebrates are such sensitive sensors of climatic stresses," says marine biology graduate student James Cervino of the University of South Carolina, Columbia. "It is not what we have been doing as a research community, but it's what we should be doing," he says.

Related sites
Climate change and infectious disease
NSF publication on cholera work
Valérie Louis's Web site, with an animation of El Niño from satellite data
Information on cholera from the CDC