It’s impossible to hide from a female mosquito: she will stalk any member of the human species by tracking our CO2 exhales, body heat and body odor. But some of us are different “mosquito magnets” who get more than our fair share of bites. Blood type, blood sugar level, consumption of garlic or banana, being a woman and being a boy are popular theories as to why someone might be a favorite snack. However, for most of them, there is little credible data, says Leslie Vosshall, head of the Rockefeller Laboratory of Neurogenetics and Behavior.
That’s why Vosshall and Maria Elena De Obaldia, a former postdoctoral fellow in his lab, set out to explore the leading theory to explain the attractiveness of variegated mosquitoes: individual odor variations related to skin microbiota. They recently showed in a study that fatty acids emanating from the skin can create an intoxicating perfume that mosquitoes cannot resist. They published their results in Cell.
“There’s a very, very strong association between having large amounts of these fatty acids in your skin and being a mosquito magnet,” says Vosshall, Robin Chemers Neustein Professor at Rockefeller University and chief scientific officer at the Howard Hughes Medical Institute. .
A tournament that nobody wants to win
In the three-year study, eight participants were asked to wear nylon stockings over their forearms for six hours a day. They repeated this process for several days. Over the next few years, the researchers tested the nylons against each other in every possible combination using a round-robin style “tournament.” They used a two-choice olfactometer assay constructed by De Obaldia, which consisted of a Plexiglas chamber divided into two tubes, each ending in a box containing a stocking. They placed Aedes Aegypti mosquitoes, the main vector species for Zika, dengue, yellow fever and chikungunya, in the main chamber and watched as the insects moved down the tubes into one nylon or another.
By far the most compelling target for Aedes aegypti was subject 33, which was four times more attractive to mosquitoes than the next most attractive study participant, and an astonishing 100 times more attractive than the least attractive, subject 19.
Trial samples were deidentified, so the experimenters did not know which participant had used which nylon. Still, they would notice that there was something unusual about any trial that included subject 33, because the bugs swarmed to that sample. “It would be obvious within seconds of starting the test,” says De Obaldia. “It’s the kind of thing that really excites me as a scientist. This is a real thing. This is not splitting hairs. This is a huge effect.”
The researchers categorized the participants into high and low attractiveness and then asked what set them apart. They used chemical analysis techniques to identify 50 molecular compounds that were elevated in the sebum (a moisture barrier in the skin) of highly attracted participants. From there, they discovered that the mosquito magnets produced carboxylic acids at much higher levels than the less attractive volunteers. These substances are found in sebum and are used by the bacteria on our skin to produce our unique human body odor.
To confirm their findings, Vosshall’s team enrolled 56 more people in a validation study. Again, Topic 33 was the most attractive and held over time.
“Some subjects were in the study for several years and we saw that if they were a mosquito magnet, they remained a mosquito magnet,” says De Obaldia. “A lot of things might have changed about the subject or their behaviors during that time, but this was a very stable property of the person.”
Even the knockouts find us
Humans primarily produce two classes of odors that mosquitoes detect with two different sets of odor receptors: Orco and IR receptors. To see if they could engineer mosquitoes that couldn’t detect humans, the researchers created mutants missing one or both receptors. The Orco mutants remained attracted to humans and could distinguish between mosquito magnets and low attractors, while the IR mutants lost their attraction to humans to varying degrees but still retained the ability to find us.
These were not the results the scientists expected. “The goal was a mosquito that would lose all attraction to people, or a mosquito that would have a weakened attraction to everyone and couldn’t discriminate subject 19 from subject 33. That would be tremendous,” says Vosshall, because it could lead to the development of more effective mosquito repellents. “And yet that’s not what we saw. It was frustrating.”
These results complement one of Vosshall’s recent studies, also published in Cell, which revealed the redundancy of Aedes aegypti’s exquisitely complex olfactory system. The female mosquito depends on security to live and reproduce. Without blood, she can’t either. That’s why she “has a backup plan and a backup plan and a backup plan and is tailored to those differences in the skin chemistry of the people she’s going after,” Vosshall says. .
The seeming unbreakability of the mosquito scent tracker makes it hard to imagine a future where we’re not the number one meal on the menu. But one potential avenue is to manipulate our skin microbiomes. It is possible that smearing the skin of a highly attractive person such as subject 33 with sebum and skin bacteria from the skin of an unattractive person such as subject 19 may produce a mosquito-masking effect.
“We haven’t done that experiment,” notes Vosshall. “This is a tough experiment. But if this worked, you could imagine that by having a dietary or microbiome intervention where you put bacteria in the skin that are able to somehow change how they interact with sebum, then you could turn someone. like issue 33 in an issue 19. But this is all very speculative.”
She and her colleagues hope that this paper will inspire researchers to test other mosquito species, including the genus Anopheles, which spreads malaria, Vosshall adds: “I think it would be really, really cool to find out if this is an effect universal”.