Malaria infection smells irresistible to mosquitoes
Why malaria-carrying mosquitoes throng to people already harbouring the disease appears to be, in part at least, because they smell attractive. Angus Bezzina reports.
While you might catch more flies with honey, you can lure almost every mosquito with HMBPP – a chemical produced by the parasite responsible for malaria.
This discovery, published in Science by Noushin Emami from Stockholm University in Sweden and her colleagues, helps explain why mosquitoes are more attracted to people already infected with malaria than to healthy alternatives.
With nearly half the world’s population at risk, according to the World Health Organisation, curbing malaria transmission is a potentially powerful tactic in the fight against it.
Researchers have long known that Anopheles gambiae mosquitoes, the species that hosts the disease-causing Plasmodium falciparum parasite, prefer to feed on infected humans, birds and mice. But the molecular mechanism behind this mysterious and sometimes fatal attraction has until now remained unknown.
So Emami and her team decided to drill into the various compounds produced by Plasmodium itself.
One compound in particular, (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate – or HMBPP, to its friends – turned out to influence the host mozzie’s blood-seeking behaviour.
To test the strength of HMBPP’s lure, Emami and colleagues gave A. gambiae mosquitoes a choice between two filled membranes: one containing regular human red blood cells, and another holding blood supplemented with HMBPP.
The researchers found that more than 95% of the mosquitoes opted to feed on HMBPP-spiked food. What’s more, they did so more intensively, and for longer periods.
In a closer examination, the team found that the compound indirectly triggered red blood cells to increase the release of carbon dioxide, aldehydes and monoterpenes.
Mosquitoes use these chemicals, particularly carbon dioxide, to track, probe and feed off hosts, says Tom Burkot, a medical entomologist from the Australian Institute of Tropical Health and Medicine and James Cook University.
“Mosquitoes will track the carbon dioxide plume to get close to a potential host. Other chemicals emitted from skin then act as close-range attractants to the mosquitoes, and these will vary by host and mosquito.
“Finally, after the mosquito lands, sensors on its legs will detect further chemicals to encourage probing and blood feeding.”
Not only did the insects drink more from the membrane containing the HMBPP-spiked blood, their meal activated Plasmodium-specific genes that protected their vital functions and overall fitness, further enhancing the parasite’s transmission.
Emami’s work will need to be replicated outside the lab, but it could have important implications for preventing disease transmission. Says Burkot: “Knowing what enhances mosquito feeding can then be used to design ways of blocking the signal to disrupt mosquito feeding.”