Mosquito magnets: why are some people so irresistible to those itch-causing bloodsuckers?

We’ve all got that friend (or maybe it’s you).

They can go all night sitting next to a stagnant pond and not suffer a single bite, while everyone else looks like they’ve been test subjects for the newest cohort of microneedling students.

Why do mosquitoes find some people so incredibly delicious but eschew others? No, it’s not your blood type or whether you ate garlic or B vitamins the night before – despite the prevalence of this particular misconception.

According to a research collaboration led by Rockefeller University in the US, the answer seems to lie in the higher amounts of carboxylic acids secreted from the skin of particularly appetising individuals.

Illustration of the structural formula of acetic acid
Illustration of the structural formula of acetic acid CH3COOH, which is the second simplest organic carboxylic acid. Credit: Petrroudny/Getty Images

The study demonstrated that not only were some individuals more attractive to mosquitoes than others, but carboxylic acids (of which acetic and formic acids found in vinegar and ants, respectively, are common examples) were directly linked to receptors in mosquitoes.

Skin odour samples were collected from participants via nylon stockings worn on the forearms. Mosquitoes were then allowed to choose between two presented samples, with researchers noting the more attractive sample of the two and the proportion of insects attracted.

These skin odour samples were then presented to mosquitoes with different genetic alterations designed to interfere with the mosquitoes’ ability to detect certain odours.

The researchers found that, generally, highly attractive humans had higher levels of several carboxylic acids on their skin compared to their less-delicious counterparts.

In a round robin of eight samples, researchers found one sample was four times more attractive to mosquitoes than the next most attractive sample and 100 times greater than the least attractive sample. What’s more, this behaviour was stable over several months of testing.

There were no specific factors in common amongst the less enticing human samples, suggesting that there was no compound which rendered individuals less tasty to the mosquitoes.

Aedes aegypti mosquito head close-up
Extreme magnification on the head of an Aedes aegypti mosquito, known vector of zika virus, chikungunya, yellow fever and dengue. Credit: Joao Paulo Burini/Getty Images

It is not yet fully clear exactly which carboxylic compounds are involved, nor why the microbiota of some individual’s skin produce more of these compounds.

Previous studies have demonstrated other cues for mosquitoes hunting humans, such as exhaled carbon dioxide and body heat, but these simply indicated a warm-blooded animal. Some mosquitoes, such as the disease-carrying Aedes aegypti, specialise in humans, and so use odour to determine whether the animal is human or not.

As the authors note in the paper, “understanding why some humans are more attractive than others provides insights into what skin odourants are most important to the mosquito and could inform the development of more effective repellents”.

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