t'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.
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."
