The worldwide incidence of malaria, which killed about 6,27,000 people in 2012 alone, could rise as temperatures warm due to climate change, a new study has warned.
Researchers have debated for more than two decades the likely impacts of global warming on the worldwide incidence of malaria, a mosquito-borne disease that infects more than 300 million people each year.
Now, University of Michigan ecologists and their colleagues are reporting the first hard evidence that malaria does - as had long been predicted - creep to higher elevations during warmer years and back down to lower altitudes when temperatures cool.
The study, based on an analysis of records from highland regions of Ethiopia and Colombia, suggests that future climate warming will result in a significant increase in malaria cases in densely populated regions of Africa and South America, unless disease monitoring and control efforts are boosted and sustained.
"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 Mercedes Pascual, senior author of the research paper.
"This is indisputable evidence of a climate effect," said Pascual.
"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," she said.
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, they were able to exclude other variables that can influence malaria case numbers, such as mosquito-control programmes, resistance to anti-malarial drugs and fluctuations in rainfall amounts.
They found that the median altitude of malaria cases shifted to higher elevations in warmer years and back to lower elevations in cooler years.
The relatively simple analysis yielded a clear, unambiguous signal that can only be explained by temperature changes, they said.
"Our latest research suggests that with progressive global warming, malaria will creep up the mountains and spread to new high-altitude areas. And because these populations lack protective immunity, they will be particularly vulnerable to severe morbidity and mortality," said co-author Menno Bouma, from the London School of Hygiene & Tropical Medicine.
The research was published in the journal Science.