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Study Casts New Light on Malaria Transmission

Research shows transmission peaks at lower temperature than previously believed

Dr. Sadie Ryan

A researcher from the SUNY College of Environmental Science and Forestry has contributed to a study that shows the most deadly form of malaria is most effectively transmitted at temperatures six degrees cooler than previously believed, a finding that could lead to more effective disease control.

Past estimates put the optimal temperature for malaria transmission at 88 degrees F. But according to a new mathematical model based on mosquito thermal physiology, the temperature for peak transmission is lower. The research indicates transmission of malaria is likely to peak at 77 degrees F and dramatically decrease above 82 degrees F.

The findings were published in the journal, "Ecology Letters."

"This has huge implications for all sorts of disease transmission," said Dr. Sadie Ryan, an assistant professor in ESF's Department of Environmental and Forest Biology, who was a member of the research team. "It changes our ability to predict where malaria can occur. We can develop more accurate models of when and where there might be an outbreak; it allows us to do more effective intervention."

Ryan said the findings go beyond malaria, which occurs in the tropics and subtropics.

"This is relevant for all vector-borne diseases, things like encephalitis and West Nile virus, anything that's caused by an insect that bites, like a mosquito or tick or midge," she said.

According to the Center for Disease Control and Prevention, about 1,500 malaria cases are diagnosed in the United States each year, most in travelers and immigrants coming from regions where malaria is more common, such as sub-Saharan Africa and South Asia. The World Health Organization reported that up to 300 million cases of malaria occurred in 2008 causing as many as one million fatalities, most of them children in Africa.

Ryan's role in the study was to analyze data from previous scientific studies to determine how temperature affects mosquitoes. It had been believed that malaria was likely to worsen with increasing temperatures but the study showed the transmission drops off dramatically above 82 degrees. She is now using the information to create maps of where malaria is likely to occur.

"The old maps did not reflect reality," she said.

The study could also help researchers learn more about how climate change might affect the transmission of such illnesses.

"Clarifying the response of malaria transmission to temperature helps us anticipate how climate change might affect disease risk," says U.S. Geological Survey parasite ecologist and senior author Kevin Lafferty.

"This study has discovered important new temperature thresholds that govern the relationship between humans and insect-borne parasites in the environment," said USGS Director Marcia McNutt. "With hundreds of millions of cases of malaria reported in the tropics and subtropics each year, each new scientific finding brings us closer to more effectively combating this deadly disease."

Erin Mordecai, Lafferty's doctoral student at University of California, Santa Barbara, was the lead author of the study. In addition to Ryan, coauthors include other scientists from UCSB, Pennsylvania State University, University of Chicago and University of California, Los Angeles.

The research was conducted as part of the Malaria and Climate Change Working Group supported by the Luce Environmental Science to Solutions Fellowship and the National Center for Ecological Analysis and Synthesis. NCEAS is supported by the National Science Foundation, UCSB and the State of California. Additional support came from NSF, USGS and the UCSB Michael J. Connell Trust.