Cross one item off the list of the potentially harmful effects of climate warming. New research suggests that malaria's range and outbreak intensity should be affected minimally, if at all, by a rising thermometer. Researchers have compared the extent of malarial incidence from the beginning of the 20th century with that of today. Despite the observed warming during that time, they found that economic development and disease-control measures appear to have combated malaria very effectively.
Malaria, a parasitic disease transmitted by the bite of a mosquito, kills an estimated 1 million people worldwide each year, and some 2.4 billion people—nearly one-third of the world's population—are considered at risk. The most lethal form of malaria is caused by Plasmodium falciparum, and the most potent vector, or carrier mosquito, is Anopheles gambiae. Because warmer temperatures are more conducive to malaria transmission, it has become common wisdom, in some circles at least, that a warming planet will increase the range and intensity of the disease, leading to even more deaths and infections.
A prominent group of malaria researchers has questioned that link. Several years ago, they formed a collaboration called the Malaria Atlas Project (MAP). Their first effort was to create a current-day world map of malaria endemicity—that is, a map of the degree and extent of risk for the disease. In a more recent project, MAP acquired similar data for malaria transmission during about 1900, compiled in a Russian study published in the 1960s. Then, they compared the centurywide spread for changes (see illustration). The result, they report in today's issue of Nature, is that malaria's reach has actually contracted dramatically, despite the fact that temperatures rose on average about 0.6˚C during the 20th century. The most likely factors in that contraction are vector-control measures such as insecticide spraying and bed nets, as well as effective antimalarial drugs. That's why the "boost that warming temperatures may give to malaria is measly compared to the declines we've seen in the last century," says epidemiologist and lead author Peter Gething of the University of Oxford in the United Kingdom.
The team's analysis, Gething says, shows that these disease-control measures should be more than enough to counteract the influence of rising temperatures in the future. Specifically, the researchers found that using the countermeasures worldwide can reduce the extent of malarial infections up to 100 times more than temperature changes might increase them. "We can't fully unpick the causes of the decline of malaria in the past," he explains, "but our results have a clear message for the future." He says world health officials "have at our disposal effective control measures that can easily overwhelm the effects of rising temperatures—but they must reach the people most in need."
The challenge, Gething says, is motivating the international community to scale up the relatively simple interventions known to reduce malaria deaths and disease. Any failure to meet that challenge, he adds, "will be very difficult to attribute to climate change."
"At long last we have an article with tangible data written by professionals in the field," says medical entomologist Paul Reiter of the Pasteur Institute in Paris. Reiter calls some of the predictions of climate change–caused malaria epidemics "irresponsible" and "based on intuition and the misapplication of elementary models."
Epidemiologist Mark Woolhouse of the University of Edinburgh in the U.K. calls the study "a welcome contribution to an important debate on the impact of climate change on vector-borne diseases such as malaria." As the data make clear, he says, "the burden of malaria has much more to do with the effectiveness of public health measures to contain it" than with rising mercury.