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Extreme weather events can unleash a "perfect storm" of infectious diseases

June 25, 2008

June 25, 2008

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An international research team has found the first clear example of how climate extremes, such as increased frequency of droughts and floods expected with global warming, can create conditions in which diseases that are tolerated one at a time may converge and cause mass die-offs of livestock or wildlife.

The study, published June 25 by PloS (Public Library of Science) ONE, an online peer-reviewed  research journal, suggests that extreme climatic conditions are  capable of altering normal host-pathogen relationships and causing a "perfect storm" of multiple infectious outbreaks that could trigger epidemics with catastrophic mortality.

Led by scientists at the University of California, Davis, School of Veterinary Medicine, the University of Illinois, and the University of Minnesota, the research team examined outbreaks of canine distemper virus (CDV) in 1994 and 2001 that resulted in unusually high mortality of lions of Tanzania's Serengeti National Park and Ngorongoro Crater. The virus cycles periodically within these ecosystems; numerous epidemics have occurred without decreasing lion populations.

But the canine distemper virus outbreaks of 1994 and 2001 were both preceded by extreme drought conditions, which led to debilitated populations of Cape buffalo, a major prey of lions. After the rains returned, the buffalo suffered heavy tick infestations, resulting in high levels of a tick- borne blood parasites in the lion population. (The parasites are normally present in lions at harmless levels.)

The canine distemper virus further suppressed the lions’ immunity, which was already challenged by the high level of blood parasites. This allowed the tick-borne disease to reach fatally high levels, leading to mass die-offs of lions. In 1994, the number of lions in the Serengeti study area dropped by over 35 percent after the double infection. Similar losses occurred in the Crater die-off in 2001.

Lion populations recovered quickly -- within years after each event, but most climate change models predict increasing frequency of droughts in East Africa.

"This study disclosed a key relationship between climate extremes and wildlife die-offs that had previously been unexplained," says Linda Munson, professor of veterinary pathology and principal investigator. "As climate extremes become more frequent, wildlife may be less able to cope with infectious agents that were formerly tolerated."

This work was supported by the Morris Animal Foundation, University of Illinois Research Board, National Science Foundation Grants for Ecology of Infectious Diseases, Long-Term Research in Environmental Biology and Biocomplexity, and the Messerli Foundation, Zurich, Switzerland.

Linda Munson, DVM, PhD, is a professor in the Department of Pathology, Microbiology and Immunology. She earned her DVM in 1980 and her PhD in experimental pathology in 1988, both from Cornell University. She has been a faculty member at the school since 1997. She is the 2003 recipient of the UC Davis Faculty Development Award. She is the pathology adviser for the cheetah, red wolf, lion, and jaguar species survival plans and president of the American College of Veterinary Pathologists. She was also the 2001 recipient of the American Association of Zoological Veterinarians' Emil Dolensek Award. Her research interests include diseases of wild and captive cheetahs, pathogenesis of endometrial growth disorders and effects of progestins in carnivores, and infectious diseases of wild canids and felids.