It’s not just popular imagination: mosquitoes bite some people more than others. We don't really understand why, but a recent paper in PLOS One suggests that genes could play a role in the attraction.

We’ve known for a while that smell is at least a partial explanation for why some people are mosquito fodder while others return from the outdoors unscathed. A number of different studies have found that differences in body odor are related to interest from mosquitoes. What we don’t properly understand is what causes those differences in smell.

A widespread myth is that certain foods can repel or attract mosquitoes, but there’s no clear explanation for how diet could change attraction levels, write the authors of the new study. What evidence we have seems to lean away from food as a factor. This makes sense: if mosquitoes use smell to find a suitable meal, they’d evolve to sniff out stable smells, not smells that change with every meal.

The way a person smells seems to be determined at least partly by genetics. A 2005 study found that people could match identical twins by smell at rates better than chance but couldn’t do the same for non-identical twins. This suggests that twins who share more of their DNA also have a more similar smell. So it's possible that the smells associated with genetics could overlap with the smells used by mosquitoes.

To test the hypothesis, the researchers used a group of brave volunteers drawn from the TwinsUK database. To avoid effects of either gender or menstrual cycle, they focused only on women aged between 50 and 90 years old. Altogether, there were 18 pairs of identical twins and 19 pairs of non-identical twins.

After washing their hands with fragrance-free soap and air-drying them, the twins were ready to become mosquito bait. The researchers used a Y-shaped tube, with mosquitoes held in an enclosure at the bottom end, to see which arm of the Y the mosquitoes would choose to fly into. The twins each placed a hand into one arm of the Y. Air was then blown up the two arms of the tube, carrying the smell toward the 20 female mosquitoes, which were held there for 30 seconds to ensure that they had smelled whatever was blowing their way. They were then released and given 90 seconds to fly into one of the arms of the Y shape.

For each set of twins, the researchers checked which person the mosquitoes preferred, and also compared the appeal of each twin to an empty, clean tube. Finally, they tested the mozzies in an entirely empty tube to check that there wasn’t something about the apparatus itself that was influencing their behavior.

They found that the mosquitoes had roughly similar levels of attraction to the identical twins. For non-identical twins, there was a distinct preference for one twin compared to the other. If the twins were tested separately (a twin’s arm in one tube and clean air in the other), about 67 percent of the difference in attractiveness to mosquitoes could be explained by genes. But when comparing one twin to another, with both tested at the same time, 83 percent could be explained by genes.

Tests using just the empty apparatus didn’t indicate that the insects had a preference for one arm of the Y over the other, and controls for body temperature and which arm was used (right or left) also didn’t seem to influence the results.

The sample size was very small, the researchers write, but the effect they found was strong. Their results indicate that attractiveness to mosquitoes could be as dependent on genes as height is—height has a very strong genetic component moderated by environmental differences like nutrition. It’s enough, they say, to motivate a much larger study.

At this stage, we don’t know what could cause the difference in aroma between people. It might be that some people smell repellent to mosquitoes or that some people smell especially attractive. The genetic influence on smell is itself still something that needs to be worked out. Whether it’s caused by chemical processes in the skin or a tendency toward different cultures of natural skin bacteria is still unknown.

For some people, mosquito bites are just an annoyance. But because mosquitoes are responsible for the spread of pathogens like malaria and dengue fever, studying their behavior could help with disease control. If we understand why they prefer some people to others, it could lead to the development of methods that help to reduce the transmission of deadly diseases.

PLOS One, 2015. DOI: 10.1371/journal.pone.0122716 (About DOIs).