Encompassing mountainous regions of Kenya, Tanzania, Rwanda, and Ethiopia, the East African highlands have traditionally avoided malaria. Mosquitoes don't mind cool, high-altitude climates, but cooler temperatures slow the development of the malaria parasites, limiting disease transmission. Malaria incidence in several East African highland regions, however, has been rising since the late 1970s. What's more, time-series data generated by the Climatic Research Unit at the University of East Anglia in the United Kingdom show that ambient temperatures in the region climbed approximately 0.5°C from 1950 to 2002.

Experts argue about what's driving the rise in cases; some point to growing drug resistance among malaria parasites and changing demographics as probable causes. Pascual doesn't discount those possibilities. But she adds that her modeling results—which suggest that mosquito populations under current warming trends could have doubled in some regions—indicate that local climate change can't be ruled out as a contributing factor.

Andrew Dobson in the Department of Ecology and Evolutionary Biology at Princeton University concurs. He adds that it's difficult to validate Pascual's modeled findings, given the paucity of mosquito population data from the East African highlands and other areas in the developing world. “Scientists are only now beginning to collect the data that allow us to make more predictive models,” he says. “But you're starting to see the shadow on the wall—we've got more people with malaria in the highlands and it's my impression that this is the result of more transmission, which is in turn exacerbated by greater numbers of mosquitoes.”

That conclusion, Pascual says, has implications for how scientists might consider the influence of global warming on infectious disease. Many current efforts on climate change and malaria focus on scenarios for spatial distribution of the disease in the future. But her ongoing work considers the recent past—the last three decades within her own lifetime—and shows that the effects of warming on disease transmission may already be under way in this highland region, providing an indication of what's to come in the years ahead.

But even as society absorbs information and predictions about the influence of warming on infectious disease, it must accept that modeled estimates are never perfect. Roberto Bertollini, director of the World Health Organization Department of Public Health and Environment, in Geneva, Switzerland, says that climate models need more development and systematic validation before they can be widely used as warning systems for disease outbreaks. “I see them as a very positive development for future applications in public health,” he says. “For too long, our approach to public health has been reactionary—we need to become more proactive, more anticipatory when it comes to disease. We're not using these models now, but we encourage their development, we see them as useful tools to managing the effects of climate change.”

“If we're going to understand the way global climate change modifies disease exposure conditions, we're going to have to bring a lot more mechanistic rigor into our modeling,” says Jonathan Patz, professor of environmental studies and population health sciences at the University of Wisconsin-Madison and a leading expert on climate change and infectious disease. “That's what Mercedes brings to the table—an understanding of how to model climate change's influence on disease ecology and its impact on public health. And that's a component that we really need.”

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