How to stop mosquitoes from smelling human sweat

Female mosquitoes are known to rely on an array of sensory information to find people to bite, picking up on carbon dioxide, body odour, heat, moisture and visual cues. Now, US researchers have discovered how mosquitoes pick up on acidic volatiles found in human sweat.

The key is an olfactory co-receptor known as Ir8a, with the researchers finding that mosquitoes lacking a functional version of the Ir8a gene were much less attracted to people. The findings, published in the journal Current Biology, potentially suggest new approaches for designing new and improved mosquito repellents.

The inspiration came from work that Matthew DeGennaro, senior author on the current study, conducted as a postdoctoral student at The Rockefeller University. There, he and his team disrupted another olfactory co-receptor, called Orco, and watched to see how it changed mosquitoes’ behaviour.

They found that those mosquitoes had more trouble telling the difference between people and other animals. The mosquitoes also lost their interest in nectar and their aversion to insect repellent DEET, but they still were attracted to vertebrate animals, including people. It meant that there were more receptors still to find.

In the new study, DeGennaro and colleagues at Florida International University looked to another group of receptors broadly known as ionotropic receptors and specifically Ir8a, which is expressed in the antenna. They used the CRISPR/Cas9 gene-editing system to disrupt Ir8a in Aedes aegypti mosquitoes. Then, they tested the co-receptor’s relative contribution in human odour detection and its genetic interaction with other olfactory receptor pathways that had been implicated previously in Ae. aegypti host-seeking behaviour.

The studies show that mosquitoes carrying a mutant version of Ir8a weren’t attracted to lactic acid and couldn’t detect other acidic components of human odour. In comparison to wild-type controls in membrane blood-feeding assays, Ir8a mutants showed reduced responses to human odour, but not heat or CO₂. Ir8a mutants also were less responsive to humans and human odour than were wild-type controls in another set of experiments.

“Removing the function of Ir8a removes approximately 50% of host-seeking activity,” DeGennaro said.

Their findings further suggest that genetic interactions among various receptors are important, with CO₂ sensitising mosquitoes to human odours. They also highlight the importance of detecting human acidic volatiles in the insects’ ability to hunt and feed on humans.

“The Ir8a phenotype was not modulated by carbon dioxide, but required the function of the carbon dioxide receptor,” DeGennaro said. “This suggests that carbon dioxide is necessary to activate the IR8a response to acidic volatiles in human odour, but not sufficient to rescue the mutant phenotype. Our results strongly suggest that host odour detection by IR8a is an indispensable component of the mosquito’s host detection system.”

DeGennaro said the team’s ultimate goal is to develop a life-saving perfume to protect people from mosquito bites, stating, “Odours that mask the IR8a pathway could be found that could enhance the efficacy of current repellents like DEET or picaridin.” On the flip side, the Ir8a pathway also could be used to design new mosquito attractants, luring mosquitoes away from people and into traps.

“The transmission of diseases like dengue, yellow fever, Zika and malaria can be blocked if we stop these mosquitoes from biting us,” DeGennaro noted.

The researchers now hope to gain an even more detailed view of the IR8a pathway. Next, they’ll begin chemical screens, using the identified genes to lead them to potentially new mosquito attractants and repellents.

Image credit: ©stock.adobe.com/au/Ravio_Brazil

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