As the climate changes, some parts of the world will become more hospitable to malaria, putting more people at risk, but large uncertainties still loom over the models making these forecasts, according to a new study.
Malaria is one of the most devastating vector-borne diseases in the world, surging at the intersection of temperature, rainfall, land use and human intervention (ClimateWire, Aug. 13, 2013).
"You can really tie epidemic [malaria] outbreaks to climate," said Adrian Tompkins, a research scientist at the International Center for Theoretical Physics in Italy. "You can also tie epidemic outbreaks to war."
According to the Centers for Disease Control and Prevention, half of the world's population lives in areas at risk for the disease. The World Health Organization reports about 219 million infections and 660,000 fatalities annually. Roughly 80 percent of these deaths occur in 14 countries in sub-Saharan Africa.
The infection stems from a single-celled parasite from the Plasmodium genus spread by Anopheles mosquitos. Symptoms of malaria often include fever, chills, joint pain and convulsions, surging on a two- or three-day cycle. Direct costs from combating the disease amount to roughly $12 billion per year, with a much larger economic footprint from lost productivity and diverted resources, according to the CDC.
Many aid and humanitarian groups have emerged to tackle malaria, with several reporting progress in slowing the disease. Since the 1950s, human intervention has eliminated malaria in 79 countries. Between 2000 and 2010, malaria incidence fell by 17 percent globally and shrank by 33 percent in Africa.
Risk increases for island nations
Malaria lingers in some of the most impoverished regions of the world, and with limited resources, putting what's available to good use is an important tactical decision in fighting malaria. That's where forecasting comes in.
In a report published yesterday in the Proceedings of the National Academy of Sciences, Tompkins and his collaborators compared different climate and malaria impact models, projecting malaria risk out to the year 2080.
Researchers studied models that relied on fundamentally different assumptions about rainfall, temperature, greenhouse gas emissions and malaria dynamics but used a scenario without human intervention. "This is kind of in a 'Mad Max' world where we don't interfere with our environment," said lead author Cyril Caminade, a research associate at the Institute of Infection and Global Health at the University of Liverpool.
Looking at global and regional scales, the scientists monitored climate suitability for malaria, the number of additional people at risk and the additional time at risk. They found a net increase in regions hospitable to the infection and an increase in the population at risk
"All the models show that the risk is going up on island regions," said Caminade. Other parts of the world, like West Africa, showed steady or declining vulnerability. "The risk is going down because it gets too warm for the mosquito," he added (ClimateWire, Sep. 20, 2013).
Better data needed from Africa
However, Caminade acknowledged that uncertainties abound when forecasting further out. "For the climate projections -- the really long-term scenarios -- that is really kind of blurry," he said. "In terms of predictive power, I think there's more usefulness in those models when you try to look at what's going to happen in the next season."
Andrew Morse, another co-author and professor of climate impacts at the University of Liverpool, said the findings show how much subtly different assumptions in a simulation can change the results. "I think the main take-home message is you shouldn't rely on any single climate model or any single impact model," he said.
To improve these models, scientists need better numbers to make their predictions more robust. "We're really not getting very good data sets back from African countries," he said. "They're improving, but those records are relatively short."
Still, health officials can glean useful information even from short records. "We're only just at the beginning stages of trying to utilize these health databases," Tompkins said. "What we're doing now, we couldn't do 10 years ago."
With this information, countries like Uganda and Rwanda could direct malaria interventions more effectively. "The underlying trend in variability associated with climate could still give you a lot of guidance in where you want to invest," said Tompkins. "Your investment in health services, for example, might need to go up where the risk is currently zero."
Researchers said the next step is to use more complicated models that can account for human migration, changes in agriculture and improved methods for fighting malaria.