Are you one of the people considered to be real mosquito “magnets”? For a long time, this specificity—admittedly, unenviable—was misunderstood. Researchers from the Laboratory of Neurogenetics and Behavior at Rockefeller University have finally identified what makes a person irresistible to these insects: the presence of certain fatty acids on the skin. This discovery could lead to the development of more effective repellent products.
Mosquitoes are irremediably attracted to the CO2 we exhale, our heat and our body odor. But some people are, unfortunately for them, even more targeted than others. According to some popular beliefs, blood group, blood sugar or certain foods consumed could more or less attract these pests. However, scientific evidence is lacking to support either of these hypotheses. Researchers have set out to explore one of the most plausible theories that could explain the greater or lesser attraction of mosquitoes to a human body: variations in odors linked to skin microbiota.
First, they tested the effect of different human skin odors on mosquitoes, in order to identify which people were the most attractive and the least attractive to these insects. A chemical analysis of body odor revealed that the most attractive people produce significantly more carboxylic acids in their skin fumes. ” There is a very, very strong association between having high amounts of these fatty acids on the skin and being a mosquito magnet. says Leslie Vosshall, who led the study. It appears that even mosquitoes with olfactory deficits manage to distinguish these prime targets from the others.
An attractive power due to carboxylic acids
The researchers asked 64 volunteers to wear nylon sleeves on their forearms for six hours a day, on consecutive days, to collect human skin odor samples. Over the next three years of study, the team tested the attractiveness of the nylon sleeves, against each other, via all possible pairings. To do this, she used a two-choice olfactometer: a Plexiglas chamber divided into two tubes, each ending in a box containing a sleeve.
Mosquitoes of the species Aedes aegypti — the main vectors of dengue fever, Zika virus infection, chikungunya and yellow fever — were placed in the main chamber, and then the researchers observed the behavior of the insects. In total, they performed more than 2330 behavior tests. Trial samples were de-identified, so experimenters did not know which participant wore which nylon.
One of the participants, here “subject 33”, stood out in particular: he was found to be four times more attractive to mosquitoes than the second most attractive participant, and 100 times more attractive than the least attractive ( identified as “Subject 19”). It’s quite simple: in any test involving a sleeve worn by subject 33, mosquitoes gathered around him. The researchers then sorted the participants from most attractive to least attractive, then analyzed their olfactory profiles to determine what could explain this large difference.
This is how they identified about fifty molecular compounds present in greater numbers in the sebum of participants with a strong power of attraction. In particular, they found that the “mosquito magnets” produced carboxylic acids at much higher levels than the others.
A property that lasts throughout life
These acids are naturally present in sebum; they help protect and moisturize the skin and the amount of acids produced varies from person to person. However, the skin maintains a constant level of carboxylic acids over time, so body odor also remains constant. Indeed, subject 33 remained the most attractive, even after several months.
” Some subjects participated in the study for several years, and we found that if they were a mosquito magnet, they remained one. Many things could have changed in the subject or in his behaviors during this period, but it was a very stable property of the person. », emphasizes Maria Elena De Obaldia, co-author of the study. In other words, no matter what changes are made — in food or skin care products — a mosquito magnet remains a mosquito magnet for life.
Namely, mosquitoes detect human odors with two sets of olfactory receptors: Orco and IR receptors. Following their discovery, the researchers therefore created mutant mosquitoes, devoid of one or both receptors, to assess their ability to detect human odors. Mosquitoes without Orco receptors continued to be strongly attracted to humans and remained able to distinguish between highly attractive and less attractive individuals. Mosquitoes without IR receptors lost their attraction to humans to varying degrees, but still retained the ability to distinguish individuals.
These results disappointed the team, who hoped to find a way to discriminate the most attractive subjects from the others in order to develop a more effective repellent. They are however consistent with another study previously conducted by Vosshall and colleagues, which demonstrated the remarkable robustness of the mosquito olfactory system; their neurons co-express several chemosensory receptors, which gives the system an almost unstoppable redundancy.
According to Vosshall, one potential lead is to manipulate the microbiomes of the skin, for example by coating the skin of a very attractive person with the skin bacteria of an unattractive person, so as to modify its olfactory profile. However, the team has yet to put this experience into practice.
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