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February 15, 2009

Malaria Forecast Varies by Temperature

Credit: CDCMalaria grips many of the impoverished regions throughout the world, killing between 700,000 and 2.7 million people each year. And as Earth warms, researchers only expect the problem to get worse. But at a symposium here yesterday, entomologist Matthew Thomas of Penn State University, University Park, said the story may not be that simple. Researchers have been ignoring the fact that temperatures vary throughout the day, he says, and when these daily fluctuations are taken into account, they show that predicted incidences of malaria can be off by 50% to more than 100%.

Temperature is critical to malaria's spread. A mosquito picks up the Plasmodium parasite that carries the disease when it bites an infected human, and it carries the parasite in its gut, where it matures--a process that takes about 10 to 14 days at about 25°C. At temperatures below about 16°C, the parasite doesn't mature fast enough, and the mosquito dies before it can pass it on. Temperatures above 40°C kill the parasite.

So where did scientists go wrong?

As climate change warms the world, scientists are concerned that relatively cool areas such as the East African highlands, which have been largely malaria-free and whose people have no immunity to the disease, will see increased outbreaks. That's true, says Thomas, but scientists have been underestimating the problem in some cases. That's because they've only thought about an increase from, say, 15° to 17°C. But associated with that, says Thomas, are variations in temperature throughout the day: An average temperature of 17°C means that temperatures could rise higher than 20°C during the hottest parts of the day, helping Plasmodium mature and malaria spread. Conversely, scientists may have overestimated the increase in malaria throughout warmer regions of the world, because variations in temperature at higher degrees Celsius slow Plasmodium maturation.

Thomas's findings were borne out by fellow panelist Mercedes Pascual of the University of Michigan, Ann Arbor. Her recent studies in the highlands of western Kenya, a relatively cool region, have revealed that a 0.5°C increase in temperature increases malaria cases an average of threefold. That means a small increase in temperature can have a large impact on the spread of malaria, she said. Pascual also said that this temperature increase could account for about one-half to two-thirds of the observed outbreaks in western Kenya since the 1970s.

Thomas's work can also improve malaria transmission models and high-resolution hazard maps, such as one presented by panelist Christopher Thomas of Aberystwyth University in the U.K. The maps use remote sensing and field data to produce site-specific estimates for infectious bites per year.

And Thomas himself hopes his data will help stem the tide of malaria transmission. "If we can better understand disease risk and the intensity of transmission," he says, "it allows us to develop a much more effective [way] to target our limited toolbox of intervention more effectively."

--Jackie Grom