A rise in temperature leads to the spread of malaria, according to a review of conflicting studies.
The review, which assessed over 70 studies that set out to untangle the link between rising temperatures and malaria incidence, says those studies that have found a link are based on more robust statistical methods than those that have not.
It is widely believed that vector-borne diseases are set to worsen with climate change. But the links are complex and some argue that vector-borne diseases in certain areas might actually decrease with climate change (see Climate complexities stoke disease controversies).
Focusing exclusively on malaria, Luis Chaves of Emory University in the United States and Constantianus Koenraadt of Wageningen University in the Netherlands, analysed the studies using new modelling techniques. They found a clear link between temperature rises and the spread of malaria — even in studies where no previous association was found.
Writing in The Quarterly Review of Biology, the scientists conclude that "evidence for a role of climate in the dynamics [of malaria] is robust".
In particular, the authors criticised studies that had found temperatures to be stable but malaria to be spreading – and had concluded that there could therefore be no link between the two. Temperature changes are surprisingly hard to measure in a relevant way but Chaves and Koenraadt said that more robust models revealed that they were, in fact, increasing.
But Simon Hay, an infectious disease epidemiologist at Oxford University and author of one of the studies criticised in the review, said many factors exert a more dramatic effect on malaria than temperature — if indeed temperature exerts any effect at all. He said that studies conducted subsequent to the Chaves-Koenraadt review are finding that the incidence of malaria is decreasing in some areas.
"I think the primary drivers of malaria prevalence in most regions are intervention coverage," Hay told SciDev.Net. In the African highlands, the site of many studies of temperature and malaria, this has involved "the introduction of an effective replacement to chloroquine and more latterly high ITN [insecticide treated nets] coverage".
Chaves and Koenraadt acknowledge that climate is not the only factor affecting malaria's spread. Changes in agricultural practice, human migration, population density, poverty and access to health services are other important factors, they say.
But scientists must continue to investigate the effects of climate on malaria as part of their efforts to develop effective mitigation strategies in the future, Chaves and Koenraadt insist in their report.
The Quarterly Review of Biology Volume 85, Issue 1, Page 27-55, March 2010 doi: 10.1086/650284