The only thing protecting you from malaria may be a cool climate. And with climate change raising temperatures in some regions, that protection may be over. Researchers today published the first hard evidence that malaria moves into new regions as they warm up.
Specifically, they found that malaria creeps to higher elevations during warmer years and retreats to lower altitudes when temperatures cool. The results reveal how global warming could spell trouble for the highlands of Africa, South America and other elevated regions that have long benefited from the protection afforded by cooler climes.
Above: A photo captured in the Antioquia Department of western Colombia, a highland region vulnerable to malaria in the face of global warming, via Wikimedia Commons
"This is indisputable evidence of a climate effect," said HHMI Investigator Mercedes Pascual, a theoretical ecologist from the University of Michigan and co-author of the study, which appears in today's issue of Science.
In an interview with io9, Pascual went on to explain that the causal link between climate change and the expansion of Malaria's domain has not always been so clear. For years, it's been ambiguous what effect, if any, future global warming might have on the incidence of malaria around the world. While it's true that the Plasmodium parasites that cause the disease and the Anopheles mosquitoes that spread it both thrive in warmer climates, malaria's success hinges on a lot more than temperature. Population density; increases or decreases in drug resistance; changes in rainfall and land use; and the implementation of mosquito-control measures all affect the prevalence of the disease over time. "It's very difficult, when examining long-term trends, to attribute cause to a specific factor like average temperature," Pascual explains.
So rather than analyze long-term trends, Pascual and her team decided to compare malaria cases on a year-to-year basis. The researchers identified two mountainous regions in South America and East Africa where local officials have maintained meticulous records of malaria infections for more than thirty years. These records allowed Pascual and her team to examine the altitudinal distribution of the disease, "not over the course of several decades," she says, "but from one year to the next, in response to climate variability, rather than long term changes in climate."
The first place the team examined was the Antioquia region of western Colombia, where Pascual and her colleagues mapped the geographical distribution of malaria cases from 1990 to 2005. The second was the Debre Zeit area of central Ethiopia, where the researchers investigated disease incidence from 1993 to 2005. Pascual says the researchers excluded data from 2005 on, as mosquito control measures have ramped up considerably in that time and could interfere with the researchers' ability to distinguish the effects of prophylactic interventions from variations in climate.
The researchers found that that the average altitude of malaria cases shifted to higher elevations in warmer years and back to lower elevations in cooler years. Did Pascual expect to find this link? "Yes," she tells io9, "but I never thought the signal would be this clear."
That signal is relevant to epidemiologists faced with confronting malaria as effectively as possible in the coming decades. "We know there some regions of the world that will be more sensitive to climate change and variability that others," Pascual says; "highlands, the edges of deserts, coastal regions, and so forth." These are areas where climate plays a limiting role, be it in the distribution of animals, plant life, sea levels or disease, she explains – factors that underscore the need for localized intervention.
Consider, for example, that populations in regions like Antioquia and Debre Zeit currently lack protective immunity to malaria. This makes them more vulnerable to the disease. And yet, the fact that these high-risk populations currently exist at the fringes of malaria's altitudinal range could make them easier to protect – at least for the time being – than those in highly endemic, lower-elevation regions.
"In my opinion," says Pascual, we need to deal with [Malaria] regionally, not globally." By understanding climate's effect on malaria incidence at the local level, she reasons, epidemiologists can be more effective in alleviating the disease's global burden.
The researchers' findings are published in the latest issue of Science.