Malaria may not rise as world warms

Heidi Ledford | 19 May 2010
Nature News
Of the many climate-change catastrophes facing humankind, the anticipated spread of infectious tropical diseases is one of the most frequently cited — and most alarming. But a paper in this week's Nature adds to the growing voice of dissent from epidemiologists who challenge the prediction that global warming will fuel a worldwide increase in malaria.

On the surface, the connection between malaria and climate change is intuitive: higher temperatures are known to boost mosquito populations and the frequency with which they bite. And more mosquito bites mean more malaria.

Yet when epidemiologists Peter Gething and Simon Hay of the Malaria Atlas Project at the University of Oxford, UK, and their colleagues compiled data on the incidence of malaria in 1900 and 2007 (see page 342), they found the opposite: despite rising temperatures during the twentieth century, malaria has lost ground. According to the models the researchers used to tease out the factors affecting the incidence of malaria, the impact of public-health measures such as improved medications, widespread insecticide use and bed nets have overwhelmed the influence of climate change. "Malaria is still a huge problem," says Gething. "But climate change per se is not something that should be central to the discussion. The risks have been overstated."

Some earlier analyses painted a dire picture of a malaria-ridden future, but these models often exclusively evaluated the impact of warmer temperatures without taking other factors into consideration, says Paul Reiter, an entomologist at the Pasteur Institute in Paris. The latest assessment of the Intergovernmental Panel on Climate Change noted these concerns: "Despite the known causal links between climate and malaria transmission dynamics, there is still much uncertainty about the potential impact of climate change on malaria at local and global scales."

Gething and colleagues' study is the first of its kind to provide a detailed statistical model of global trends over the twentieth century, but it does have limitations. For instance, the data used to generate a global map of malaria in 1900 sum up all malarial infections, including those by a malaria parasite named Plasmodium vivax, whereas the data in the 2007 map track infections by just one malaria parasite, P. falciparum, which carries the highest disease burden. And the analysis does not take into account some parameters that are likely to change as a result of global warming, such as rainfall patterns and human migrations.

Nevertheless, the results largely match those of several other recent studies, including one published last year by Kevin Lafferty of the US Geological Survey in Santa Barbara, California, which also concluded that rising temperatures over the last century had no net impact on the incidence of malaria (K. D. Lafferty Ecology 90, 888-900; 2009). In 2000, models designed by David Rogers and Sarah Randolph at the University of Oxford predicted that although some parts of the world would gain malaria because of climate change, large areas would see a drop in disease due to reductions in rainfall and humidity (D. J. Rogers and S. E. Randolph Science 289, 1763-1766; 2000). The result: no net difference.

"The complexity of malaria and the other vector-borne diseases is astonishing," says Reiter. "To bring it down to just one factor — climate change — is totally unjustifiable."

The same could hold true for other diseases that rely on intermediate vectors like mosquitoes. Dengue fever, for example, has also been touted as an infectious disease that could get a boost from climate change. The virus, which is carried by mosquitoes, is already on the march, and is spreading more rapidly into the southern United States. But the impact it will have there is likely to be minimal compared with that in less developed regions of the world because of lifestyle differences, says Reiter. Americans spend more time indoors, he says, and are more likely to have window screens to keep mosquitoes out.

Laura Harrington, who studies mosquito-borne diseases at Cornell University in Ithaca, New York, agrees. "In the context of vector-borne diseases, climate change is probably going to have a minimal role in comparison to other factors," she says. Even so, Gething's results are likely to be controversial, cautions Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York. Although global analyses such as Gething's are useful, he says, they may miss important regional trends — such as the spread of mosquitoes from the lowlands to the highlands of eastern Africa, which some argue is the result of rising temperatures. "There's a pretty strong spatial disconnect between areas where there is strong economic development and increasing control of mosquitoes, versus those areas where the risk of climate-induced disease is highest," he says.

Others worry that the results of the study will be misinterpreted. "On smaller scales, climate change certainly has big effects on disease," says Harrington. "This does not diminish the importance of climate change at all."