Their research will be published in the online version of the Proceedings of the National Academy of Sciences this week.
In a previous study published in the journal Science, the researchers found evidence that El Nino-Southern Oscillation (ENSO), a major source of climate variability from year to year, influences cycles of cholera. In that work, they looked only at climate and disease data from Bangladesh for the past two decades. In the new research, they compared those results with data from Bangladesh for the periods 1893-1920 and 1920-1940 to see whether the coupling between climate variability and cholera cycles has become stronger in recent decades. Their examination of the data, which relied on a suite of techniques called time series analysis, suggests that it has.
"What is new in this work is not showing that ENSO plays a role in the variability of cholera, but that the role of ENSO has intensified," says Mercedes Pascual, an assistant professor in the department of Ecology and Evolutionary Biology at the University of Michigan. In addition, the link is strongest following ENSO events, with cholera increasing after warm events and decreasing after cold events. In the years between events, the climate-cholera connection breaks down.
Scientists who study climate change predict that ENSO will become stronger and more variable in coming years under a global warming scenario, so understanding how its connection to human disease changes will be increasingly important, says Pascual.
Cholera, an intestinal infection with symptoms that may include diarrhea, vomiting and leg cramps, is caused by the bacterium Vibrio cholerae. People usually get the disease by eating or drinking contaminated food or water.
The greater role of ENSO in cholera dynamics probably reflects known changes in ENSO itself, the researchers believe. Since the late 1970s, there has been a tendency toward warmer ENSO events, in conjunction with global warming. Because the disease-causing bacterium lives in brackish water and thrives in warm temperatures, it may be particularly sensitive to climate patterns. People also may be more likely to come in contact with contaminated water in warmer weather.
Other diseases, such as malaria and dengue, may be similarly affected by climate variability, says Pascual. But because other factors, such as patterns of immunity, also lead to cycles in disease dynamics, Pascual and her colleagues are working on methods to sort out the relative roles of climate and intrinsic factors such as temporary immunity.
The University of Michigan
Ann Arbor, MI 48109-1399