El Niño cycles linked to cholera outbreaks

September 06, 2000

ITHACA, N.Y. -- About 11 months after the start of an El Niño event in the equatorial Pacific, hospitals thousands of miles away in Bangladesh can expect a surge of cholera cases, according to the first mathematical model to link climatic cycles with subsequent cholera outbreaks.

Details of the climate-disease model are reported in the latest issue (Sept. 8, 2000) of the journal Science by ecologists at Cornell University and the universities of Barcelona, Maryland and London.

"So far we aren't seeing a return to the time when cholera was such a scourge on humanity," says Stephen P. Ellner, professor of ecology and evolutionary biology at Cornell. "But we are getting an explanation for outbreaks of cholera and diarrheal diseases in South America and the recent, higher-than-historic levels of cholera in South America and Asia." The Cornell biomathematician provided the model for the study.

Besides Ellner, other authors of the Science report, "Cholera dynamics and the El Niño Southern Oscillation," are Mercedes Pascual of the University of Maryland's Center of Marine Biotechnology; Xavier Rodo, Climate Research Group, University of Barcelona; Rita Colwell, director of the National Science Foundation and professor of cell and molecular biology at University of Maryland; and Menno J. Bouma of the University of London's School of Hygiene and Tropical Medicine.

Cholera is caused by the pathogenic microorganism Vibrio cholerae , a bacterium that lives among zooplankton in brackish waters and in estuaries where rivers meet the sea, and infects humans through contaminated water. Colwell previously had proposed a link between cholera outbreaks and distant El Niño Southern Oscillation (ENSO) events that were mediated, she suggested, by increased sea-surface temperatures and higher numbers of bacteria-bearing zooplankton. But it took a complex model that could account for all contributing factors while discounting insignificant correlations -- as well as month-by-month records of cholera cases -- to prove the link. Data on cholera incidence, which normally can rise and fall twice each year with local influences such as monsoons and seasonal temperature changes, came from a hospital in Bangladesh that had tested all incoming patients for cholera for 18 years, from January 1980 to March 1998. The Ellner model took into consideration recent frequencies of cholera cases, an ENSO index based on sea-surface temperatures in the Pacific and seasonal variation in local climates.

Peaks in cholera incidence at the Bangladesh hospital were found to occur every 3.7 years -- exactly the same frequency as of ENSO events between 1980 and 1998. But the cholera outbreaks and ENSO events did not precisely coincide: The scientists found an 11-month time lag from the start of an ENSO event near the equator and a peak in cholera incidence.

A separate analysis of climate variables by co-authors Pascual and Rodo -- including humidity in the troposphere layer of the Earth's atmosphere, cloud cover and the amount of absorbed solar radiation -- suggested that the 11-month lag results from a six-month lag between an ENSO and increases in sea-surface temperatures off the coast of Bangladesh, plus a five-month lag between increased sea-surface temperatures and a peak in cholera.

"These results indicate that the dynamics of cholera in Bangladesh are consistent with a remote forcing by ENSO," the ecologists say in their Science report. But they caution that the climate-disease story might be even more complicated and call for further research. In the case of Bangladesh, "another mediating factor in the ENSO-cholera relation might be the melting of the snowpack in the Himalayas and its effect on monsoons, precipitation and river discharge." Floods and drought, the ecologists write, "can affect not only human interactions with water resources and therefore exposure to the pathogen, but also sanitary conditions and disease susceptibility."

The discovery of a remote link between El Niño events and cholera outbreaks comes at a time when some ecologists, including David Pimentel at Cornell, are predicting major increases in disease and death as global climate change provides ideal conditions for disease-causing organisms. Yet, the El Niño -cholera model should not be used to predict cholera outbreaks far into the future until climatologists learn more about the frequency of ENSOs over the long term, Ellner notes.

Climatologists' understanding of El Niño events is just in its infancy, says the Cornell biomathematician, whose model previously was applied to measles outbreaks. It is uncertain whether the 3.7-year frequency of ENSO events will hold up over the long run, and whether, as some scientists suggest, ENSOs are becoming more intense. Although public-health authorities will have an 11-month warning, beginning with the start of an ENSO, Ellner says, "this model will be more useful when somebody figures out how to predict El Niño."
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The study was supported, in part, by grants and fellowships from the James S. McDonnell Foundation, the Knut and Alice Wallenberg Foundation and the Mellon Foundation.

Cornell University

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