Despite the success of prevention efforts, malaria killed 627,000 people in 2012, the majority of them African children, according to the World Health Organization. That averages out to at least one child per minute, according to WHO.
Researchers have known for some time that mosquitoes appeared to be more attracted to hosts carrying the Plasmodium parasite. A 2005 study showed that a species of Anopheles mosquitoes were more attracted to Kenyan children infected with the parasite.
The possible mechanisms behind the attraction, however, remained unknown.
Researchers from Penn State University and the Swiss Institute of Technology in Zurich collected and analyzed chemicals emitted by infected mice and found that the strongest attractors of mosquitoes kicked into higher production during a particularly infectious stage of the disease.
They wafted air over infected and noninfected mice, then watched the behavior of the mosquitoes. Sure enough, more of them traveled upwind toward the infected mice. And they seemed to prefer the mice in day 10-20 of infection, a period when acute symptoms — fever, vomiting and headache — subside. That’s also when Plasmodium has reached a critical phase of its reproductive cycle, when immature cells depend on the digestive system of Anopheles mosquitoes to mature.
The researchers repeated the experiment with the chemicals emitted by the mice during various stages of infection, and found the mosquitoes were more attracted to the mice’s chemical bouquet during a narrower time period of days 13-20. So they tested the individual components of the mice’s aroma, some of which have been associated with mosquito attraction to humans. They identified four particular chemicals associated with higher attraction during the crucial stage of Plasmodium reproduction.
A more detailed analysis of the mice’s changing chemistry showed that odor-related chemicals were somewhat suppressed in the early stages of the infection, when the body’s immune system summons its defenses, and dramatically elevated in a subsequent, less acute, phase of the disease.
More research into what chemicals from humans might attract mosquitoes will be necessary, the authors note. But the mouse results suggest that it might be possible to identify a “biomarker” for carriers of Plasmodium in its most infectious stage.