It's Not Your Imagination: Mosquitoes Love Some People More

Here's why mosquitos ravage you but leave your friend untouched.Here's why mosquitos ravage you but leave your friend untouched.When you sit around evening barbecues or lakeside campfires this summer, you may notice a few friends bemoaning mosquito bites more than others. This might not be because they're the complaining type: The bloodsuckers are more attracted to certain body chemistries.

Female mosquitoes need blood to reproduce (males don't lay eggs and thus don't bite). In order to eat blood, these ravenous female mosquitoes first need to find it, which means sniffing out a host to bite. The insects do this, in part, via smell receptors on their antennae and mouths. Just as we use smell receptors in our nose to detect the aroma wafting from our favorite food, a mosquito's smell receptors uniquely respond to chemical signals emanating from a host's body.

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So far, scientists have found that the human body gives off several hundred such chemicals. You, your friends, and everyone else each has a unique chemical signature with different blends and concentrations; the unlucky ones with more of the ingredients that attract mosquitoes are more likely to be bitten.

This doesn't mean that certain people are more attractive to all mosquitoes, says Ulirich Bernier, Ph.D., a research chemist and mosquito attractant expert at the Agricultural Research Service at the United States Department of Agriculture. There are around 3,000 mosquito species worldwide, each with a unique set of receptors that make them more or less drawn to certain body chemistries. This can make the same person more appealing to one species of mosquito than another.

Bernier says it may also explain why certain species are more attracted to humans than to other animals, such as Anopheles gambiae , a major spreader of malaria in sub-Saharan Africa that feeds almost solely on people.

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Ingredients For a Bite
The first chemical that mosquitoes typically detect is carbon dioxide, which you exhale each time you breathe. This is true for all blood-feeding insects-the CO2 alerts that a potential meal is nearby, prompting further investigation.

This may help draw a mosquito to any animal that gives off CO2. Since not all mosquito species will bite just any animal, the most important signals for seeking a specific kind of host-say, differentiating between a person and a cow-are the volatile chemicals that the body excretes as waste through the skin, including lactic acid, acetone and various fatty acids. These and many of the other chemicals that emanate from the human body are sometimes called kairomones, which one species may inadvertently communicate to another; predators, for example sometimes use kairomones to find prey (you may be more familiar with the related chemicals called "pheromones," which individuals of the same species use to attract mates or to communicate socially linked behaviors).

Pregnant women and people who have recently downed a few beers also appear to be especially attractive to mosquitoes, although it isn't clear if this is due to a change in body chemistry, differences in CO2 production, or something else entirely.

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Bacterial Boost
Recent work suggests harmless bacteria that thrive on our skin may also impact how attractive we are to mosquitoes. According to Niels Verhulst, a post-doctoral entomology student at Wageningen University in the Netherlands, and colleagues, bacteria such as Corynebacterium essentially chomp up the fatty acid our bodies exude through our skin or in our sweat, metabolize them, and spit out smaller compounds that produce a smelly scent as they evaporate.

In 2011, Verhulst and his team explored this idea by collecting these scents from 48 test subjects, running genetic analyses to see which types of bacteria lived in the samples, and performing a standard test to see which attracted the malaria mosquito An. gambiae . They found that people with a higher diversity of bacteria were less attractive to the mosquitoes and certain types of bacteria were more attractive than others.

The relationship between body chemistry, bacteria and body odor may be genetic. In a paper published this summer, Verhulst's team showed a correlation between HLA genes-thought to relate to body odor-and attractiveness to mosquitoes. The genes may be responsible for producing certain compounds that the bacteria like to eat.

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"The story is quite complicated and I think not all the links are proven yet," explains Verhulst, "However, in this paper we show that there seems to be a link between the HLA profile and attractiveness to mosquitoes, and we know it is also correlated with our odors and related to our bacteria. So, it almost seems obvious that this is the way it should be, however further experiments should prove this."

New Baits and Repellants?
Understanding which compounds mosquitoes like best could lead to new products, which will be especially important for people who live in areas rife with malaria, dengue and other illnesses spread by the pests (although your backyard barbeque attendees may benefit, too). Most common are lures, which trick mosquitoes away from a person and into a trap, and insect repellents, which may either mask attractive body chemistry or use a compound that wards off the insects.

Bernier and colleagues are working on lures that combine lactic acid, acetone and dimethyl disulfide (another body waste component). They are also trying to find compounds that jam mosquitoes' odor-sniffing receptors to "make us invisible to mosquito detection," he says.

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Meanwhile, Verhulst and colleagues are deploying 4,000 traps in Kenya baited with odor blends based on their bacteria research to see if they can lure malaria-spreading mosquitoes away from people.

A few similar lures are already available for sale, including the pricey BG-Sentinel Trap. Body-odor-based repellents, however, have a way to go before they come to market. Until then, the recommended way to avoid bites-whether mosquitoes love you or not-is to check for available insect repellents that have been scientifically proven to work.

- by Brooke Borel