Have you ever wondered why you seem to attract every mosquito within a mile radius while the person standing next to you appears completely immune? Or maybe you’re the lucky one that they tend to ignore in their quest for a tastier meal. Either way, there are a number of factors, from your skin microbiome to what you drank with dinner, that can affect your relative attractiveness to mosquitoes.
There are about 2,700 species of mosquitos in the world, but not all of them bite humans. Most prefer birds or other mammals to us. Mosquitoes get their daily nutrition from sipping flower nectar, but when it comes time for the female to reproduce, she needs the protein of a blood meal to develop her eggs. Male mosquitoes don’t drink blood at all.
A mosquito sips nectar from a flower
When a female mosquito lands on your skin, she inserts two tiny tubes. The first one injects an anti-clotting agent, while the second draws out the blood that pools as a result. Most of the time, the bitten human is unharmed, except for the itch caused by our body’s histamine response. But mosquitoes are also vectors for several human diseases, such as Yellow Fever, West Nile Virus, and Malaria. Because of this, we would expect that over time some pretty complex evolutionary interactions have built up as the human, the mosquito, and the parasite or virus all try to come out on top and survive.
And they have…
One major draw to a hungry, human-biting female mosquito is blood type. About 85% of all people are ‘secretors’. They secrete chemical signals through their skin that allow mosquitoes to identify their blood type, and Type O is preferred over all others. Having Type O blood and being a secretor makes you 83% more likely to be bitten. People with Type A are least likely to be bitten, but are also the most likely to suffer severe symptoms or die from untreated malaria. Those with Type O blood are likely to have less serious cases of the disease, due to physiological differences in their blood.
So why might mosquitoes flock to Type O? Humans, mosquitoes and malaria all co-evolved in Africa beginning around 200,000 years ago, so there is a long history between the three of us. If people with Type O blood were always better able to survive long enough to pass the malarial parasite (Plasmodium falciparum) on through another mosquito bite, then they were the ones the parasite would have favored. We already know that Plasmodium can affect the behavior of its mosquito host, because infected mosquitoes have been shown to be more strongly drawn to human odors.
And speaking of human odors…
Body odor is another strong draw for mosquitoes. Human sweat doesn’t smell until the bacteria on our skin begin to break it down, and different bacteria produce different odors, giving us each a unique scent. Smelly feet are especially attractive to mosquitoes, and they will land on them over any other body part. Oddly, they are also attracted to Limburger cheese. Or maybe not so oddly, since Limburger is made by one of the same bacteria that causes foot odor.
Even though we all have about the same total number of bacteria on our skin, some people have a wider variety of species, while others are dominated by just a few types. Studies have shown that people with only a few types of bacteria are more likely to be bitten, even when the total number of bacterial cells is the same. Having a wide variety of bacteria seems to protect us, and since you acquire you skin flora from your mother at birth, they tend to be passed down through families (or at least they were until the age of antibiotics and disinfectants).
Interestingly, people with ancestry from malarial areas of the world are less attractive to mosquitoes than those descended from populations that haven’t had exposure to malaria, despite their blood type. This suggests some level of co-evolution between humans and their skin flora to avoid a deadly disease.
And a few other attractive things…
-Mosquitoes are drawn to the carbon dioxide we exhale. If you are a larger person or have been exercising, you are more likely to be bitten.
-Exercise causes lactic acid to be secreted through your skin, also a draw for mosquitoes.
-Mosquitoes have also been shown to prefer the scent of a beer drinker’s skin.
-One study showed that mosquitoes are 500 times more likely to bite when the moon is full.
So… if you are a person with type O blood and low microbial diversity on your skin that has just gone for a run, taken off your shoes while sipping a beer and noticing that the moon is full… prepare to be bitten.
Smallegenge, R.C. et.al. 2013. Malaria infected mosquitoes express enhanced attraction to human odor. PLoS ONE 8(5)
Verhulst, N.O. et.al. 2009. Cultured skin microbiota attracts malaria mosquitoes. Malaria Journal 8:302
Verhulst, N.O. et.al. 2011. Composition of human skin microbiota affects attractiveness to malaria mosquitoes. PLoS ONE 6(12)