| Temperature spikes key to malaria spread|
|Climate change is influencing the spread of malaria in ways far more complicated than previously thought, according to US researchers.|
The disease's ability to spread depends not just on how temperature changes from month to month and year to year, but also on how temperature fluctuates throughout the day, says entomologist Dr Matthew Thomas of Pennsylvania State University.
By looking at climate patterns in more detail, the new data suggest that scientists may need to reconsider their predictions of where malaria epidemics might strike next.
"We may be overestimating the risk in warmer environments, but underestimating the risk in colder environments," says Thomas, who presented his findings recently at the American Association for the Advancement of Science annual meeting in Chicago. "We can be wrong by 50 to 100% or more."
The implications are enormous, says Professor Chris Thomas, of Aberystwyth University in the UK, who does related work.
More than a million children die from malaria every year in Africa. "That's one child every 30 seconds," he says. "This is for a preventable and treatable disease."
And things could get worse. With global warming, malaria is spreading into new regions, though scientists don't yet know exactly where, when and how patterns of the disease are going to change.
One reason for the uncertainty, says Matthew Thomas, is that most predictive models consider only a rise in average temperatures over days, months or even years.
But climate projections also forecast a spike in extreme weather events as well as a rise in variability as the atmosphere heats up. Already, a typical day in a malaria-infested place like Tanzania might average 25°C, but nights can drop to 15°C with daytime highs of 32°C.
Those peaks and dips, Thomas suspects, might affect malaria-transmitting anopheles mosquitoes in a number of ways.
Parasites respond to temperature
When a mosquito drinks the blood an infected person, the insect also swallows the malaria parasite, which then must incubate and multiply before migrating into the insect's saliva.
The process can take weeks. And because mosquitoes are small-bodied and cold-blooded, outside temperatures make a big difference in how long it takes before they can transmit the disease. If it happens too slowly, the insects die before they can infect anyone.
In general, the malaria parasite does better at warmer temperatures, which is why the disease occurs most often in tropical environments.
But with mathematical models, Thomas has found that, even when conditions are warm, highly fluctuating temperatures over the course of a day slow down the rate of parasite incubation and larval development in the mosquito.
That, in turn, slows the spread of disease - and suggests that hot areas might be less at risk for malaria outbreaks than scientists have long thought.
At relatively cool temperatures, on the other hand, fluctuating temperatures speed up biological processes, which might help explain why malaria has already begun spreading into the traditionally cool highlands of East Africa.
These areas, which lie on the fringes of malaria's current distribution, are of extreme concern, because they are poor, rural, and have never been exposed to malaria. People there have no built-up immunity to the disease, so epidemics can be swift and deadly.
The research also underscores how little scientists know about the basic biology of mosquitoes. Getting to know the enemy, Thomas says, is the only way to figure out where to distribute bed nets, sprays and other limited resources.
"We have the whole genome of anopheles, but that tells us nothing about how temperature influences these mosquitoes," says Princeton University ecologist Professor Andrew Dobson.
"We need better ways to predict where malaria is going to occur."