The female mosquito may be impossible to hide from: she monitors our carbon dioxide exhalations, body heat, and body scent. Some of us, however, are distinctly “mosquito magnets” who receive more than our fair share of bites. Many theories exist about why someone may be a favorite muncher, such as blood type, blood sugar levels, eating garlic or bananas, or being female. However, Leslie Vosshall, director of the Neurogenetics and Behaviour Lab at Rockefeller University, says most of these theories lack reliable data.
To explain mosquito attraction variation, Vosshall and Maria Elena De Obaldia decided to investigate the leading theory: that variation in personal scent is linked to skin microbiota. Researchers recently demonstrated that skin fatty acids may produce an intense scent mosquitoes cannot resist. Their findings were published in the Cell on Oct. 18. Having high amounts of those fatty acids on the skin makes you a mosquito magnet, said Vosshall, Robin Chemers Neustein professor at Rockefeller University and chief scientific officer at the Howard Hughes Medical Institute.
The study involved eight participants wearing nylon stockings around their forearms for six hours per day for three years. Over several days, the procedure was repeated. Researchers tested the nylons in round-robin style tournaments over the following years, pairing them in every possible way.
A Plexiglas chamber, divided into two tubes, with each tube ending in a box holding a stocking, was used for the olfactometer-based, double-choice assay De Obaldia had built. In the primary chamber, they placed Aedes Aegypti mosquitoes-the main vector species of Zika, dengue, yellow fever, and chikungunya-and watched them fly down the tubes toward nylon stockings.
The most alluring Aedes target was Subject 33, which was four times as attractive to mosquitoes as the next most attractive study subject, and a staggering 100 times as attractive as the least attractive study subject.
Trial samples were deidentified, so experimenters had no idea what type of nylon participants wore. However, in any trial with Subject 33, they would have noticed something unusual, since insects would have been flocking to this sample.
De Obaldia says it would be evident immediately after starting an analysis. As a scientist, that kind of stuff really excites me. We are not splitting hairs here. This has a huge impact.”
Scientists divided the participants into higher-attractive and lower-attractive ones and then identified what made them different. They used chemical-analysis techniques to identify 50 molecular compounds that were elevated in the sebum–the moisturizing barrier on skin–of high-attract participants. They discovered that mosquito magnets produced carboxylic acids at much higher levels than less-attractive volunteers. We produce our own, distinctive scent with these substances in our sebum, which bacteria use to make sebum. Another 56 individuals were recruited to participate in the validation study in order to verify Vosshall’s findings.
Subject 33 was once again most enticing, and it has remained so for some time. “Some subjects were in the study for a number of years, and we saw if they were mosquito magnets, they would remain mosquito magnets,” says De Obaldia. “Many things might have changed in a subjects or behavior during that period, but it is an extremely stable characteristic of a human being.”
There are two types of smells produced by humans, which are detected by mosquitoes using two distinct receptors: ORCO and IR. Using mutant mosquitoes lacking one or both of these receptors, researchers tested whether they could engineer mosquitoes that cannot detect humans. The ORCO mutants remained attracted to humans, and could distinguish mosquito magnets from less attractive ones, while the IR mutants lost the human-attraction to varying degrees, but retained their ability to detect us. Scientists had not expected these results. “The aim was either to get the mosquito to lose all its attraction to humans, or a mosquito with reduced attraction to everyone, that would be unable to distinguish between subject 19 and subject 33.”
This would have been huge, says Wosshall, because it could have led to more effective mosquito repellents. According to these findings, Wosshall’s more recent study, also published in Cell, showed that the exquisitely sophisticated olfactory system of Aedes aegyptis is redundant.
Female mosquitoes rely on it for life and reproduction. It cannot do any of those things without blood. Vosshall says she is attuned to the differences in skin chemistry between people she pursues therefore she has a back-up plan, a back-up plan, a back-up plan.
The seemingly invulnerability of mosquito tracers makes it hard to imagine a future in which we are not the number one food item. Manipulating our skin’s microbiome is one possible path. The mosquito-mask effect may be produced by coating the skin of high-attractive individuals, such as Subject 33, with the sebum and bacteria of low-attractive individuals, such as Subject 19.
“We did not run an experiment on this,” Wosshall noted. “That is the kind of hard experiment to do. But if it works, you can imagine, having either dietary intervention, or a microbiome intervention, in which you are going to be putting bacteria on your skin, which are somehow capable of changing the way that they interact with sebum, that you can turn somebody like Subject 33 into a Subject 19. But that is all highly speculative. She and her colleagues are hoping that the paper inspires researchers to check on other species of mosquitoes, including those from the family Anopheles, that transmit malaria, adding: I think that it would be really, really interesting to see whether that is an overall effect.