Each year, more than 300 million individuals are infected with malaria, a life-threatening blood disease caused by a parasite transmitted to humans by mosquitos. Whether malaria cases could be affected by warming climates has been a topic of debate, but now, researchers present the first evidence that the disease climbs to higher elevations during warmer years.
The study, published in the journal Science, suggests future warming climate trends may prompt an increase in malaria cases, particularly in highly populated areas of Africa and South America that are at higher elevations.
Basing their study on an analysis of records from high altitudes in Ethiopa and Colombia, ecologists from the University of Michigan (UM) and the London School of Hygiene & Tropical Medicine say the mosquitos tend to migrate back down to lower altitudes when temperatures decrease.
They say that unless disease monitoring and control efforts are improved, the number of people at risk of malaria is going to significantly increase.
Over 20 years ago, scientists classified malaria as a disease likely to be sensitive to changes in climate, given that the Plasmodium parasites and the Anopheles mosquitos that spread them thrive in warm temperatures.
The team notes that although early studies suggested climate change would result in a rise in malaria cases as it climbed ever higher, some of the theories these predictions were based on were criticized.
Until now, however, there has not been a thorough analysis conducted of regional records to examine how the distribution of malaria cases has changed in response to climate changes. In particular, more information has been needed on East Africa and South America, which have heavily populated highlands - areas that have provided refuge from malaria in the past.
Malaria travels higher in warmer years
Pictured is an Anopheles gambiae mosquito, a source of malaria transmission in Ethiopia.
Image credit: Dan Salaman, London School of Hygiene & Tropical Medicine
To conduct their analysis, the researchers studied case records from the Antioquia region of western Colombia from 1990 to 2005 and the Debre Zeit area of central Ethiopa from 1993 to 2005.
They excluded variables that could influence case numbers - such as mosquito-control programs, changes in rainfall amounts and anti-malarial drug resistance. By doing so, they were able to focus on the response to year-to-year temperature changes in high altitudes.
Overall, they found that the median altitude for malaria cases changed to higher elevations in warmer years and to lower elevations in cooler years.
"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," says senior author and UM theoretical ecologist Mercedes Pascual. "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."
The team says their analysis produced a clear pattern indicating that these trends can only be explained by changes in temperature.
Higher altitude populations 'lack protective immunity'
Their findings also suggest that changes in climate explain malaria trends from recent decades in both regions studied.
- Worldwide, 3.3 billion people are at risk of malaria.
- In 2010, 90% of malaria deaths occurred in Africa, predominantly in children under 5.
- Malaria infects over 300 million people each year.
For example, in the region of Ethiopia the researchers assessed, they found that the 37 million people who live at an elevation between 5,280-7,920 feet are at increased risk of malaria exposure during warmer temperatures.
Previously, other findings indicated that a 1° C increase in temperature could result in an extra 3 million cases of malaria each year in Ethiopia in individuals under 15 years old.
Co-author Menno Bouma, senior clinical lecturer at the London School of Hygiene & Tropical Medicine, says:
"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."
Pascual adds that their findings "underscore the size of the problem and emphasize the need for sustained intervention efforts in these regions, especially in Africa."
In late 2013, Medical News Today reported on a study that suggested a "cocktail" approach could lead to a universal malaria vaccine.