Since the 1970s, the highlands of East Africa have witnessed a surge in malaria outbreaks. Because the mosquitoes that carry the disease do not thrive in cooler climes some researchers have suggested a link between this rise and climate change. A 2002 study found no such connection, but a new analysis of the data--including five more years of records--seems to show that a modest increase in temperature could lead to a population boom in mosquitoes and, therefore, malaria.

Mercedes Pascual of the University of Michigan--whose previous research helped link cholera outbreaks and the El Nino Oscillation--and her colleagues used temperature data from the Climate Research Unit in the U.K. stretching back to 1950, the same data used by the 2002 study but with five more years of records subsequently added. Looking at the records for four highland spots where malaria has been on the rise-- Gikongoro in Rwanda, Kabale in Uganda, Kericho in Kenya and Muhanga in Burundi--they found an overall warming trend of 0.5 degree Celsius since the late 1970s.

The team then fed these temperatures, along with rainfall information, into a computer model built to simulate mosquito population dynamics in Hawaii. Even though the temperature change was quite small, its effect on mosquito abundance was quite large. For example, although the temperature rise represented just a 3 percent change for these areas, it led to as much as a 40 percent change in mosquito abundance in Kericho and more than 100 percent in Kabale in the simulation.

Kabale showed an especially large increase in mosquito population because it was among the highest, and therefore coldest sites they modeled. The number of mosquitoes in these areas is typically small, owing to the chilly conditions. But even a small increase in temperature in these locales can quickly tip the balance in the insects' favor, leading to more mosquitoes and, hence, more vectors for the malaria parasite. On the other hand, Pascual and her colleagues point out, warming could dry out the pools in which the mosquitoes breed if temperatures continue to climb or rainfall patterns change.

Although this study does not prove that climate change is responsible for the increase in the malaria plaguing African highlands, the authors say it should be taken into account along with other factors such as treatment resistance and land-use changes. The findings appear in a paper published online this week by the Proceedings of the National Academy of Sciences.

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