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Mosquitoes looking for a blood meal use a variety of clues to track humans, including the heat of our bodies and the carbon dioxide content in our breath. At present, research shows that a certain olfactory receptor in their antennae also serves as a human detector, reacting to the smelly chemicals contained in our sweat.
The targeting of this receptor could offer a new way to thwart mosquitoes in search of blood and prevent the transmission of diseases such as malaria, Zika virus and dengue, according to a study published Thursday in the newspaper Current biology.
"We found a receptor for human sweat, and we found that the acidic volatiles that emerge from us are essential for mosquitoes to find us," said Matthew DeGennaro, a neurogeneticist at Florida International University in Miami.
"I think what's exciting is that we finally have evidence that there is some kind of path, in the sense of smell, necessary for mosquitoes to appreciate us," he said. said Lindy McBride, a scientist at Princeton University, who studies mosquito behavior. and was not part of the research team.
It has long been known that mosquitoes rely on many clues to target humans. First, a mosquito will detect exhaled carbon dioxide at a distance of up to 30 feet. "After carbon dioxide," says DeGennaro, "then he begins to smell the human odor."
The mosquito follows this smell and, when it is very close, begins to detect body heat. Once the mosquitoes have settled on you, "they can actually taste your skin with their legs and then they look for a place to bite," says DeGennaro.
He and his colleagues genetically modified mosquitoes to block the activity of a specific olfactory receptor called Ir8a. The result was that female mosquitoes – those that suck the blood – were no longer attracted to lactic acid, an important component of human sweat.
In addition, the team conducted various laboratory tests to see if the disruption of this receptor would make mosquitoes less sensitive to humans. Scientists have asked people to put their hands in a device called "olfactometer" that allows mosquitoes to feel them at a distance. Captive mosquitoes can fly through the device to get closer, but not enough to bite. Tests have shown that genetic modification of mosquitoes to deactivate this olfactory receptor significantly reduces the likelihood that pests will fly to human skin.
And to show that it was an answer to an odor that was affected, the researchers also asked study participants to wear nylon sleeves for about 12 hours to collect sweat. Then they put these sleeves in the olfactometer. Once again, mutant mosquitoes were much less attracted to perfume than normal mosquitoes.
Now, it may seem obvious that the olfactory system paralyzing mosquitoes would prevent them from feeling people. "But the problem is that mosquitoes smell in different ways," says McBride. Previous work has shown that the deactivation of another part of the olfactory system "has no effect, and this is a big surprise," she noted. "So it's really exciting to see that this other kind of smell is actually essential."
"I thought it was a fantastic study," says Jeff Riffell, a biologist at the University of Washington, who studies chemical communication. "It's an open question about how mosquitoes, # 1, can locate people to bite, and then, how do they choose to bite – what are the mechanisms?"
Riffell says this study conclusively demonstrates that the behavior is mediated by a receptor that allows mosquitoes to "feel our kind of funk or body odor." You can really really demonstrate the importance of this receptor. "
It may be possible, he says, to create "a perfume or chemical that prevents this receptor from working, so you can imagine it being like a perfume or something you spray, and that the mosquitoes can no longer so detect our sweat ".
Laura Duvall of Rockefeller University in New York, who studies the behavior of mosquitoes in search of a host, says this research could also help explain how mosquitoes distinguish humans from other animals.
"Mosquitoes are very good at finding us because they are aware of many of the components of human odors, including the volatile acids we produce," she said.
Duvall says disturbing a single path would probably not prevent mosquitoes from biting us. But understanding all the signals that insects use could help "create better human imitations, which could be used to lure mosquitoes into a trap – and keep them away from humans".
Larry Zwiebel, a biologist who studies the olfaction and behavior of insects at Vanderbilt University, explains that evolution has created a sensory system for mosquitoes that has many overlapping and interacting parts to produce the most efficient machine in search of human.
According to him, some of the lab tests in this study, for example, clearly showed that carbon dioxide was needed to sensitize mosquitoes to lactic acid in human sweat.
"So this shows that for this process to actually work, and at its highest level, the mosquito wants to detect carbon dioxide and that this information makes information on lactic acid more valuable to the mosquito. – more usable, "says Zwiebel.
"The mosquito is looking for what is often called a coincidence detector," he says. "It's not just a signal, but many signals – which, when they coincide, give the strongest impetus that can lead to that behavior."
According to Zwiebel, finding ways to over-stimulate parts of the mosquito's human detection system could help scientists create a particularly powerful repellent. For the mosquito, he says, the effect would be "as if you were taking the elevator with someone who put in too much cologne."
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