El Niño cycles linked to cholera outbreaks

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 of the journal Science by ecologists at Cornell University and the universities of Barcelona, Maryland and London. 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.

Researchers previously had proposed a link between cholera outbreaks and distant El Niño Southern Oscillation (ENSO) events that were mediated 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 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 climate-disease story might be even more complicated and needs 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 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, 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.