Sensitivity to seasonal changes affects disease-causing bugs' success
ANN ARBOR, Mich.—As concerns about global warming and erratic climate fluctuations grow, scientists wonder how human disease patterns will be affected. They're especially concerned about diseases such as cholera, which seem to be strongly influenced by environmental changes.
An analysis by University of Michigan graduate student Katia Koelle, to be presented Aug. 5 at the annual meeting of the Ecological Society of America in Portland, Ore., offers new insights into the relationship. Koelle used game theory to study interactions between strains of disease-causing organisms with different sensitivities to seasonal climate fluctuations. Her work helps explain why a mutant strain of cholera has replaced the original strain in Bangladesh, and the same approach might be used to predict the outcome of competition between other disease strains.
Cholera, an intestinal infection with symptoms that may include diarrhea, vomiting and leg cramps, is a serious health problem in many parts of the world. Scientists who study climate change predict that temperatures will increase worldwide and climate will become more variable in some regions, so understanding connections between human health and climate variability is more important than ever, Koelle said.
In her study, Koelle classified disease-causing organisms as seasonal generalists or seasonal specialists. Seasonal specialists can function only within narrow temperature and rainfall ranges. Seasonal generalists may not be quite as effective as specialists in the best of conditions, but they're able to function over a wider range of environmental conditions.
Koelle showed that when climate conditions are far from optimal, pathogens are under evolutionary pressure to become more generalist than specialist. Factoring in such evolutionary changes may yield more accurate long-range predictions of disease trends than just looking at the immediate ecological effects of climate change, she said.
Koelle also analyzed cholera dynamics using disease incidence data collected between 1966 and 2002 by a surveillance program of the International Center for Diarrhoeal Disease Research in Bangladesh. One strain, known as the Classical strain, has been in the area "for as long as anyone knows," Koelle said. A mutant strain, El Tor, appeared in the 1970s and has virtually replaced the Classical strain.
"I wanted to know what's different about the El Tor strain that allowed it to survive and replace the Classical strain," Koelle said. "Why is it more fit, in the evolutionary sense, than the strain that's been around for hundreds or thousands of years?" Her analysis showed that El Tor, which is more of a generalist, may have an advantage over Classical in dealing with the hot, wet summers that have been occurring in Bangladesh.
With collaborators in a working group from the National Center for Ecological Analysis and Synthesis (NCEAS), Koelle also is looking into what makes previously harmless strains of bacteria start infecting humans and causing disease, and the researchers are especially interested in the role environmental changes may play in the process.
"With escalating climate change and habitat destruction, we need to consider how these changes may affect the evolution of pathogen life history strategies, and the ultimate effects on human health," Koelle said.
Koelle did the research under the direction of Mercedes Pascual, associate professor of ecology and evolutionary biology, with funding from the University of Michigan and a grant to Pascual from a joint program on Climate Variability and Human Health funded by governmental agencies including the National Oceanic and Atmospheric Administration.
Contact: Nancy Ross-Flanigan