March 7, 2014
Warmer Temperatures Help Malaria Reach Higher Altitudes
redOrbit Staff & Wire Reports - Your Universe Online
Scientists from the London School of Hygiene & Tropical Medicine and the University of Michigan have discovered the first concrete evidence that the mosquito-borne disease malaria travels to higher elevations during warmer years and returns to lower altitudes when temperatures become cooler.
In research appearing in the March 7 edition of Science, the study authors reviewed records from the highland regions of Colombia and Ethiopia to see what impact climate change had on the global incidence of the ailment, which infects a reported over 300 million people annually.
Their findings reveal that, without improved monitoring and control efforts, malaria cases in some of the most densely populated regions of Africa and South America will increase significantly as the planet’s temperatures increase in the years ahead – effectively bringing the disease to areas that have traditionally faced a low risk of infection.
“Traditionally, we think of malaria as a disease with limited prevalence in highland regions, but we are now seeing a shift due to climate change,” said Menno Bouma, Honorary Senior Clinical Lecturer at the London School of Hygiene & Tropical Medicine. “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 for severe morbidity and mortality.”
“The pattern is very clear, and the implication is that warmer temperatures cause an expansion in altitudes [where malaria infections occur],” added 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.”
Experts have long known there was a link between climate and the spread of malaria. The mosquitoes that transmit the malaria-causing parasites, Plasmodium falciparum and Plasmodium vivax, tends to thrive in warmer weather. In addition, higher temperatures cause the parasite itself to more rapidly mature into its infectious form. However, scientists lacked solid proof that higher temperatures actually caused the parasite to spread to new locations.
According to the researchers, early studies concluded that global warming would result in a significant increase in malaria cases because of the disease’s expansion into higher elevations. Some of those conclusions were later criticized, though, and some researchers have argued that improved socioeconomic conditions and better mosquito-control efforts would be more influential in the disease’s spread than climate change.
The debate had lacked an analysis of regional records describing how the spatial distribution of the disease changes in response to yearly variations in temperature, especially in East African and South American nations with densely populated highlands that have historically provided havens from the disease.
Pascual, Bouma and their colleagues combed 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. They looked for evidence of a changing spatial distribution of malaria with year-to-year temperature changes and found that the median altitude of malaria cases shifted to higher elevations in warmer years and back to lower elevations in cooler ones.
“Now we have evidence that this shift to higher elevations does occur,” Pascual said, noting that people living in regions that had not been exposed to the disease tend to be more vulnerable than those living in endemic regions who typically acquire immunity from previous infections.
She added that the study “underscores the need to sustain interventions to mitigate the effects of climate change.” On the plus side, Pascual said that malaria control strategies such as bed nets and anti-malarial drugs have a better chance of success at higher elevations due to the relatively low transmission rates in those regions.