Ecological effects of climate change include human epidemics

The link between climate and cholera, a serious health problem in many parts of the world, has become stronger in recent decades, according to a University of Michigan scientist who takes an ecological approach to understanding disease patterns. Mercedes Pascual, an assistant professor of ecology and evolutionary biology, discussed her work during a symposium Feb. 17 on the ecology of infectious diseases at the annual meeting of the American Association for the Advancement of Science.

In work published over the past three years, Pascual and coworkers at the University of Barcelona and the International Center for Diarrhoeal Disease Research in Bangladesh found evidence that El Nino-Southern Oscillation (ENSO), a major source of climate variability from year to year, influences cycles of cholera. They looked initially at climate and disease data from Bangladesh for the past two decades; more recently 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.

“We had known that ENSO plays a role in the variability of cholera, but our work revealed that the role of ENSO has intensified,” says Pascual, who was named one of “The 50 Most Important Women in Science” by Discover magazine in November 2002. What’s more, the link is strongest during ENSO events, with cholera increasing after warm events and decreasing after cold events. In the years between events, the climate-cholera link breaks down.

With predictions that ENSO will become stronger and more variable in coming years under a global warming scenario, understanding how its connection to human disease changes will be increasingly important, says Pascual. Often, it’s difficult to tell whether disease cycles are driven by environmental factors or by processes intrinsic to disease transmission. Pascual and coworkers recently developed a method that makes it possible to distinguish between the two possibilities.

Pascual says her work is just one example of how principles and tools of ecology and evolutionary biology can aid understanding of disease patterns. For example, using techniques developed to study the movement of species or populations, researchers are studying the spread of diseases such as rabies. In addition, interactions among diseases can be analyzed similarly to interactions among species. And even such classic ecological subjects as competition have applications to disease, Pascual said. “When you have diseases that share hosts, it’s similar to having species that compete for a resource. One big question is how do they coexist?”

Another sign of growing interest in the ecology and evolutionary biology of disease is the trend for universities to offer courses on the subject, said Pascual, who is co-teaching a new undergraduate course, Evolutionary Biology and Human Disease, at the University of Michigan this semester.

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