Mosquitoes Can Smell Your Sweat

When a mosquito lands and your arm and starts taking a drink, it’s not just an unhappy accident. Mosquitoes use an array of chemical neuroreceptors to track down their next blood meal. Now, researchers have identified a key receptor that detects the lactic acid in human sweat, a finding that could eventually help people avoid becoming fast food for the insects.

In particular, researchers looked at Aedes aegypti, one mosquito species that has adapted to dining on human blood and also happens to be a transmitter of many tropical diseases, according to the new study in the journal Current Biology. When mosquitoes hunt down human blood to complete their breeding cycle, they do it pretty methodically.

First, reports Deborah Netburn at The Los Angeles Times, carbon dioxide receptors alert them to the presences of a mammal from up to 30 feet away. Coming in for a closer look, it’s believed another group of receptors let the mosquito know that the animal is human. A closer inspection of body heat confirms that we are living, breathing animals full of tasty blood. Once the mosquito lands, receptors on her legs confirm that her prey is indeed human telling her it’s alright to plunge her syringe-like proboscis into your flesh.

Matt DeGennaro, a study co-author and mosquito neurobiologist at Florida International University, tells Netburn all those neuroreceptors are a cacophony of signals telling the mosquito to feast.

“At this moment they are experiencing all the cues at once, and it must be very intoxicating,” he says. “The mosquito is thinking, ‘I don’t care if you are going to swat me, I’m going to bite you.’”

Researchers have long hypothesized that there must be a receptor that helps the mosquitoes home in on the scent of humans in particular. In previous research, DeGennaro and his colleagues used CRISPR/Cas-9 gene-editing technology to remove one suspected olfactory receptor, called Orco, from a population of mosquitoes and then the team watched how they behaved.

While the insects had trouble differentiating between humans and other animals, they were still attracted to vertebrates. Also, the loss of Orco meant the bugs lost their aversion to DEET, the most commonly used and effect mosquito repellent on the market.

That meant the key receptor was still to be identified. For this new study, they focused on a receptor called Ir8a, found in the antenna of the insect. Removing that receptor from the mosquitoes genes led to insects that didn’t respond to the scent of lactic acid, a main component unique to human sweat as well as other chemicals that make up human odor. Their ability to sense carbon dioxide and heat, however, remained intact.

The study provides sound evidence that mosquitoes cue in on humans by using a suite of neuroreceptors, confirming the long-held hypothesis. “People have been looking for more than 40 years,” DeGennaro says in Cell Press statement. “Even in the 1960s, scientists knew it was sweat and lactic acid, but no one knew how those were sensed. Back then, mosquito scientists didn’t have genetics.”

Knocking out Ir8a isn’t perfect, but it does have a pretty major impact on mosquito behavior. “Removing the function of Ir8a removes approximately 50 percent of host-seeking activity,” DeGennaro says in a different statement. “Odors that mask the Ir8a pathway could be found that could enhance the efficacy of current repellents like DEET or picaridin. In this way, our discovery may help make people disappear as potential hosts for mosquitoes.”

It’s likely that Ir8a isn’t the only receptor that helps mosquitoes find us, Laura Duvall of The Rockefeller University in New York tells Nell Greenfieldboyce at NPR. “Mosquitoes are so good at finding us because they’re paying attention to many different components of human odor — including the acidic volatiles that we produce,” she says.

But the more we understand what chemicals the insects are paying attention to, the better we can become at thwarting them. For instance, we could make better traps that lure the blood-suckers away from our backyards or create a spray that masks the smell of human sweat.

Keeping mosquitoes away isn’t just a matter of keeping our backyards tolerable. In many parts of the world, mosquitoes are vectors of diseases like malaria, dengue and yellow fever, leading to about 725,000 human deaths each year.

source: Smithsonian.com by Jason Daley

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