Highland region in Ethiopia

Credit: Asnakew Yeshiwondim

Researchers say they have the first hard evidence that malaria creeps to higher elevations during warmer years and retreats to lower altitudes when temperatures cool.

The evidence comes from an analysis of highland regions in Ethiopia and Colombia.

It suggests that future climate warming will prompt a rise in malaria incidence in densely populated regions of Africa and

South America, unless efforts to monitor and control malaria are increased.

“We saw an upward expansion of malaria cases to higher altitudes in warmer years, which is a clear signal of a response by highland malaria to changes in climate,” said study author Mercedes Pascual, PhD, of the University of Michigan in Ann Arbor.

“This is indisputable evidence of a climate effect. The main implication is that, with warmer temperatures, we expect to see a higher number of people exposed to the risk of malaria in tropical highland areas like these.”

Dr Pascual and her colleagues reported these findings in Science.

It was more than 20 years ago that malaria was first identified as a disease that might be especially sensitive to climate change, because both the Plasmodium parasites that cause it and the Anopheles mosquitoes that spread it thrive as temperatures warm.

Some early studies concluded that climate change would lead to an increase in malaria cases as the disease expanded its range into higher elevations. But some of the assumptions behind those predictions were later criticized.

More recently, researchers have argued that improved socioeconomic conditions and more aggressive mosquito-control efforts will likely exert a far greater influence than climatic factors over the extent and intensity of malaria worldwide.

What’s been missing in this debate is an analysis of regional records with sufficient resolution to determine how the spatial distribution of malaria cases has changed in response to year-to-year temperature variations, especially in densely populated highlands that have historically provided havens from the disease.

So Dr Pascual and her colleagues looked for evidence of a changing spatial distribution of malaria with varying temperature in the highlands of Ethiopia and Colombia. They examined malaria case records from the Antioquia region of western Colombia from 1990 to 2005 and from the Debre Zeit area of central Ethiopia from 1993 to 2005.

By focusing solely on the altitudinal response to year-to-year temperature changes, the researchers were able to exclude other variables that can influence malaria case numbers, such as mosquito-control programs, resistance to antimalarial drugs, and fluctuations in rainfall amounts.

The team found that the median altitude of malaria cases shifted to higher elevations in warmer years and back to lower elevations in cooler years. This relatively simple analysis yielded a clear signal that can only be explained by temperature changes, the group said.

“Our latest research suggests that, with progressive global warming, malaria will creep up the mountains and spread to new high-altitude areas,” said study author Menno Bouma, MD, of the London School of Hygiene & Tropical Medicine in the UK.

“And because these populations lack protective immunity, they will be particularly vulnerable to severe morbidity and mortality.”

In addition, the study results suggest that climate change can explain malaria trends in both the highland regions in recent decades.

In the Debre Zeit region of Ethiopia, at an elevation range of between 5280 feet and 7920 feet, about 37 million people (roughly 43% of the country’s population) live in rural areas at risk of higher malaria exposure under a warming climate.

In a previous study, researchers estimated that a 1-degree temperature increase could result in an additional 3 million malaria cases annually in Ethiopia in the under-15 population, unless control efforts are strengthened.

“Our findings here underscore the size of the problem,” Dr Pascual said, “and emphasize the need for sustained intervention efforts in these regions, especially in Africa.”

Warmer temperatures push malaria to higher elevations