It’s impossible
to hide from a female mosquito—she will hunt down any member of the human
species by tracking our CO2 exhalations, body heat, and body odor. But some of
us are distinct “mosquito magnets” who get more than our fair share of bites.
Blood type, blood sugar level, consuming garlic or bananas, being a woman, and
being a child are all popular theories for why someone might be a preferred
snack. Yet for most of them, there is little credible data, says Leslie Vosshall, head of Rockefeller’s Laboratory of Neurogenetics
and Behavior.
This is why Vosshall and Maria Elena De
Obaldia, a former postdoc in her lab, set out to explore the leading theory to
explain varying mosquito appeal: individual odor variations connected to skin
microbiota. They recently demonstrated through a study that fatty acids
emanating from the skin may create a heady perfume that mosquitoes can’t
resist. They published their
results in Cell.
“There’s a very, very strong association
between having large quantities of these fatty acids on your skin and being a
mosquito magnet,” says Vosshall, the Robin Chemers Neustein Professor at The Rockefeller
University and Chief Scientific Officer of the Howard Hughes Medical Institute.
A
tournament no one 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 on multiple days. Over the next few
years, the researchers tested the nylons against each other in all possible
pairings through a round-robin style “tournament.” They used a two-choice
olfactometer assay that De Obaldia built, consisting of a plexiglass chamber
divided into two tubes, each ending in a box that held a stocking. They placed Aedes
Aegypti mosquitoes—the primary vector
species for Zika, dengue, yellow fever, and chikungunya—in the main chamber and
observed as the insects flew down the tubes towards one nylon or the other.
By far the most compelling target for Aedes
aegypti was Subject 33, who was four times
more attractive to the mosquitoes than the next most-attractive study
participant, and an astonishing 100 times more appealing than the least
attractive, Subject 19.
The samples in the trials were
de-identified, so the experimenters didn’t know which participant had worn
which nylon. Still, they would notice that something unusual was afoot in any
trial involving Subject 33, because insects would swarm towards that sample.
“It would be obvious within a few seconds of starting the assay,” says De
Obaldia. “It’s the type of thing that gets me really excited as a scientist.
This is something real. This is not splitting hairs. This is a huge effect.”
The researchers sorted the participants
into high and low attractors, and then asked what differentiated them. They
used chemical analysis techniques to identify 50 molecular compounds that were
elevated in the sebum (a moisturizing barrier on the skin) of the
high-attracting participants. From there, they discovered that mosquito magnets
produced carboxylic acids at much higher levels than the less-attractive
volunteers. These substances are in the sebum and are used by bacteria on our
skin to produce our unique human body odor.
To confirm their findings, Vosshall’s
team enrolled another 56 people for a validation study. Once again, Subject 33
was the most alluring, and stayed so 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. “Many things could have changed about the
subject or their behaviors over that time, but this was a very stable property
of the person.”
Even
knockouts find us
Humans produce mainly 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 unable to spot
humans, the researchers created mutants that were missing one or both of the
receptors. Orco mutants remained attracted to humans and able to distinguish between
mosquito magnets and low attractors, while IR mutants lost their attraction to
humans to a varying degree, but still retained the ability to find us.
These were not the results the
scientists were hoping for. “The goal was a mosquito that would lose all
attraction to people, or a mosquito that had a weakened attraction to everybody
and couldn’t discriminate Subject 19 from Subject 33. That would be
tremendous,” Vosshall says, because it could lead to the development of more
effective mosquito repellents. “And yet that was 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. It’s a failsafe that the female mosquito relies on to live and reproduce.
Without blood, she can’t do either. That’s why “she has a backup plan and a
backup plan and a backup plan and is tuned to these differences in the skin
chemistry of the people she goes after,” Vosshall says.
The apparent unbreakability of the
mosquito scent tracker makes it difficult to envision 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 slathering the skin of a high-appeal
person like Subject 33 with sebum and skin bacteria from the skin of a
low-appeal person like Subject 19 could provide a mosquito-masking effect.
“We haven’t done that experiment,”
Vosshall notes. “That’s a hard experiment. But if that were to work, then you
could imagine that by having a dietary or microbiome intervention where you put
bacteria on the skin that are able to somehow change how they interact with the
sebum, then you could convert someone like Subject 33 into a Subject 19. But
that’s all very speculative.”
She and her colleagues hope this paper will inspire researchers to test other mosquito species, including in the genus Anopheles, which spreads malaria, adds Vosshall: “I think it would be really, really cool to figure out if this is a universal effect.”
Source: https://www.rockefeller.edu/news/33019-why-mosquitoes-bite-some-people-more/
Source: Why some people are mosquito magnets – Scents of Science (myfusimotors.com)
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