In the tropical highlands of South America and East Africa, cool temperatures have historically kept mosquito-borne diseases, such as malaria, at bay. New research by Howard Hughes Medical Institute (HHMI) scientists shows that as annual temperatures rise in these areas, malaria can spread to populations in higher elevations that had historically not been at as much risk of being infected by malaria parasites.

HHMI scientists have compared the yearly distribution of malaria cases in two mountainous regions in South America and East Africa, and found that in warmer years, populations at high elevations experience more infections than they do in cooler years. Their findings, published March 7, 2014, in the journal Science, suggest that without increased control measures, climate change will increase the burden of malaria, particularly in areas that are densely populated at higher elevations.

"The pattern is very clear, and the implication is that warmer temperatures cause an expansion in altitudes [where malaria infections occur]," says Mercedes Pascual, an HHMI investigator at the University of Michigan who led the study. "Long-term trends should see an increase in cases as the disease both expands to higher altitude and causes more cases at higher altitudes."

Temperature has long been known to affect the spread of malaria. The mosquito that transmits the malaria-causing parasites, Plasmodium falciparum and Plasmodium vivax, thrives at warmer temperatures. Furthermore, the parasite itself matures into its infectious form more rapidly when temperatures are high. Still, Pascual says, scientists have lacked direct evidence that increasing temperatures would permit the parasite to encroach on new territory. Some scientists have also argued that socioeconomic change and associated control measures might outweigh the effects of climate change.

Malaria prevalence fluctuates over time in response to many factors, Pascual explains: changes in temperature, rainfall, land use, the parasite's resistance to antimalarial drugs, and mosquito-control measures all affect transmission of the disease. "When you're looking at long-term trends, it's difficult to attribute change to specific factors," she says.

So instead of analyzing long-term trends, Pascual and graduate students Mauricio Santos-Vega and Amir Siraj decided instead to compare malaria cases year-to-year, in two regions that had kept detailed records of the times and locations of infections. Working with colleagues at the University of London, the Oromia Regional Health Bureau in Ethiopia, and Columbia University in New York, they mapped the geographical distribution of malaria cases in the Antioquia region of western Colombia from 1990 to 2005 and the Debre Zeit area of central Ethiopia from 1993 to 2005 – before intense intervention efforts had begun. For each region, they analyzed how distribution along elevation gradients changed from one year to the next, then looked for statistical associations to fluctuations in annual average temperatures.

Their analysis showed that the distribution of malaria cases shifted to higher elevations in warmer years. When temperatures cooled, the distribution of cases retreated to lower elevations.

Pascual and her colleagues also used their analyses to consider the increasing prevalence of malaria in Antioquia and Debre Zeit during the 1980s and 1990s, and found that changing temperatures could account for those long-term trends. "In those decades, we are already observing an effect of climate change," says Pascual.

The impact of ongoing climate change will be large, she says. In Ethiopia alone, 37 million people live at altitudes of 1,600 to 2,400 meters (5,249 to 7,874 feet), the range the team analyzed in their study of Debre Zeit. In a previous analysis, Pascual and colleagues estimated that without control measures, an increase in 1 degree Celsius could cause an additional 3 million additional infections in children under 15 years old each year in that population.

"Now we have evidence that this shift to higher elevations does occur," she says. Furthermore, individuals living in regions where they have not previously been exposed to malaria are more vulnerable than those in endemic regions, who have often acquired immunity from prior infections to help fight off severe disease.

"This underscores the need to sustain interventions to mitigate the effects of climate change," Pascual says. Fortunately, she adds, malaria control strategies, such as insecticides, bed nets, environmental modification, and use of efficacious anti-malarial drugs, have a higher chance of success than at lower elevations, in these regions at the edge of the distribution of the disease, where transmission rates remain relatively low – even though those edges will shift as epidemic areas expand. Data from this study can be found at the Dryad Repository: doi:10.5061/dryad.dp78p.