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As Temperatures Climb, So Does Malaria

The risk area for malaria expanded between 1990 and 2005
Simien Mountains in Ethiopia


43 percent of Ethiopia's population live at elevations between 1,600 and 2,400 meters. Pictured: Simien Mountains in Ethiopia.
Credit: A.Davey/Wikimedia Commons

Warming temperatures expand the risk area for malaria, pushing the disease farther uphill in afflicted regions, according to a new study.

Infecting more than 300 million people each year, malaria emerges from a tapestry of temperature, rainfall, vectors, parasites, human movement, public health and economics. Fighting the disease involves pulling on all of these threads, but scientists have a hard time figuring out which ones are the most important to predicting where the disease will go.

Temperature has been especially contentious. Some previous research indicated that warmer weather only plays a minor role in this mosquito-borne illness, with human factors being the major influence on disease risk (ClimateWire, Feb. 4). Other studies concluded that climate change will cause no net increase in the disease in some parts of the world (ClimateWire, Sept. 20, 2013).

"Part of the controversy has to do with how to attribute particular causes to a long-term trend," said Mercedes Pascual, a professor in the Department of Ecology and Evolutionary Biology at the University of Michigan.

In a paper published online yesterday in the journal Science, Pascual and her collaborators looked at how malaria moved up in elevation with temperature in Ethiopia and Colombia. Tracking year-to-year temperature variations from 1990 to 2005, researchers observed how malaria's range shifted.

Infection rates tend to increase as temperatures go up, since the Plasmodium parasite that causes the disease reproduces faster inside vector mosquitoes when it's warmer, increasing the infection likelihood when the mosquito bites someone, Pascual explained. The Anopheles mosquitoes that spread the disease also thrive in the heat.

The results confirmed for the first time that malaria creeps uphill during warmer years and recedes as temperatures cool, a dangerous effect as the climate warms. "The implication is this will, without any mitigation, result in the increase of the malaria burden," Pascual said.

The findings hold promise for better forecasting. In previous work, Pascual found she could predict malaria up to four months in advance in parts of India by monitoring monsoons (ClimateWire, March 4, 2013)

"What we hope to have done is eliminate lingering doubts about climate influencing malaria," added Menno Bouma, a co-author and a lecturer at the London School of Hygiene & Tropical Medicine.

Highland cities could face severe outbreaks
He noted that some previous studies suggested that as malaria risk expanded in some areas, it would contract in others, so the net burden of disease would stay the same. The theory was that some areas would get too hot for the vectors to survive, so the infections would taper off (ClimateWire, Oct. 26, 2012).

But that doesn't account for where people reside relative to the disease. Much fewer people live in the regions where malaria is endemic compared to the metropolises at higher elevations, many that emerged in part to avoid the disease, according to Bouma. About 37 million people -- 43 percent of Ethiopia's population -- live at elevations between 1,600 and 2,400 meters.

People who live in these highland cities, whether in Africa or South America, have much lower rates of disease resistance and don't have a history of fighting malaria. If the disease encroaches on these regions, overall morbidity would increase and the results could be devastating to millions.

"If you have a [disease range] contraction due to temperature increases in the drier parts of the world and an increase in the cooler parts of the world, the population affected in the cooler ends of the malaria distribution would be larger," Bouma said.

Nonetheless, human interventions can still hold back the rising tide of the disease, if vaccine research, for example, delivers on its promises (ClimateWire, Feb. 26)

Kevin Lafferty, an ecologist at the U.S. Geological Survey who was not involved in the research, said the study is important because it isolates how temperature affects malaria. "Normally, if we were to compare a cool country and a hot country, there could easily be a lot of economic differences," he said. "The value of going up an altitudinal gradient is you're controlling for a lot of those things."

Pascual said the researchers are now looking for better data on other variables like rainfall and migration to develop tailored malaria interventions.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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