Climate change is expanding the disease-causing pathogen’s comfort zone.

The mosquito Anopheles gambiae spreads the malaria parasite in Africa.

As temperatures rise in the East African highlands, so does the number of malaria cases. This conclusion of a new study by HHMI scientists suggests that areas not historically affected by malaria epidemics are at increasing risk of outbreaks due to rising global temperatures.

The retrospective study, led by HHMI investigator Mercedes Pascual of the University of Michigan, focused on a highland region of western Kenya. Here, a tea plantation made up of several estates, with a combined population of 50,000, kept detailed hospital admissions data, including confirmed monthly malaria cases, spanning 1966–2002.

"There aren't many data sets on malaria that span such a long time period and are of such good quality," says Pascual. The wealth of data combined with local temperature records made the new analysis possible.

At a physiological level, the effect of temperature on the malaria parasite has been well-studied: the parasite reproduces more quickly at higher temperatures, and the mosquitoes that carry the pathogen develop faster and bite more often. Because of this link, the Kenyan highlands are considered a "fringe region" for malaria transmission—their cooler climate has historically kept malaria levels much lower than in nearby warmer lowlands.

Pascual wondered whether long-term shifts in temperature had already changed the incidence of malaria in this fringe region. Her team isolated the effect of temperature—as opposed to drug resistance and land-use changes, among other factors—and revealed that warmer temperatures explain a significant fraction of the increase in malaria cases from the 1970s through the 1990s. The work appeared online in Proceedings of the Royal Society B on November 10, 2010.

The researchers are now looking at fringe regions of eastern Africa and India—including deserts, where low rainfall has typically limited malaria—to study the changing prevalence of the disease not just in the long term but also from year to year. Establishing a link between climate and malaria cases at these interannual time scales could help predict epidemics.

"Climate's effect on epidemic malaria does not mean that interventions cannot be effective," stresses Pascual. "It might just mean a bigger control problem." And understanding climate's effect, she says, might be key to this control.

Photo: Dan Salaman / London School of Hygiene and Tropical Medicine