Contrary to a widespread assumption, global warming is unlikely to expand the range of malaria because of malaria control, development and other factors that are at work to corral the disease.
So concludes a team of scientists including two University of Florida researchers in a paper appearing May 20 in the journal Nature.
Scientists and public policy makers have been concerned that warming temperatures would create conditions that would either push malaria into new areas or make it worse in existing ones. But the team of six scientists, including David Smith and Andy Tatem, faculty members with UF's biology and geography departments and both at UF's Emerging Pathogens Institute, analyzed a historical contraction of the geographic range and general reduction in the intensity of malaria -- a contraction that occurred over a century during which the globe warmed. They determined that if the future trends are like past ones, the contraction is likely to continue under the most likely warming scenarios.
"If we continue to fund malaria control, we can certainly be prepared to counteract the risk that warming could expand the global distribution of malaria," Smith said.
The team, part of the Wellcome Trust's multinational Malaria Atlas Project, noted that malaria control efforts over the past century have shrunk the prevalence of the disease from most of the world to a region including Sub-Saharan Africa, Southeast Asia and South America, with the bulk of fatalities confined to Africa. This has occurred despite a global temperature rise of about 1 degree Fahrenheit, on average, during the same period.
"The globe warmed over the past century, but the range of malaria contracted substantially," Tatem said. "Warming isn't the only factor that affects malaria."
The reasons why malaria has shrunk are varied and in some countries mysterious, but they usually include mosquito control efforts, better access to health care, urbanization and economic development. The banned pesticide DDT was instrumental in ridding the disease from 24 countries in Southern Europe, the former Soviet Union and elsewhere in the world between 1955 and 1969, Smith said. Researchers debate how the U.S. defeated malaria, but the reduction of mosquito breeding grounds, improved housing and reduced emphasis on agriculture that comes with development -- and the reduced risk of bites that accompanies urbanization -- probably played a role, Smith said.
"There is no one tale that seems to determine the story globally," Tatem said. "If we had to choose one thing, we would guess economic development, but that's kind of a cop out" because the specific mechanisms may still remain unclear, and controlling malaria might also help to kick-start development.
In any case, current malaria control efforts such as insecticide-treated bed nets, modern low-cost diagnostic kits and new anti-malarial drugs, have proved remarkably effective, with more and more countries achieving control or outright elimination. Unless current control efforts were to suddenly stop, they are likely to counteract the spread of mosquitoes or other malaria-spreading effects from anticipated temperature increases, Smith said.
Simon Hay, an author of the Nature paper and one of the chief architects of the Malaria Atlas Project, noted that modern malaria control efforts "reduce transmission massively and counteract the much smaller effects of rising temperatures."
"Malaria remains a huge public health problem, and the international community has an unprecedented opportunity to relieve this burden with existing interventions," he said. "Any failure in meeting this challenge will be very difficult to attribute to climate change."
- Peter W. Gething, David L. Smith, Anand P. Patil, Andrew J. Tatem, Robert W. Snow, Simon I. Hay. Climate change and the global malaria recession. Nature, 2010; 465 (7296): 342 DOI: 10.1038/nature09098
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