Words by Jess Fogarty.
Sitting under an African night sky, hearing the cackle of a distant hyena and the crackling of the fire, and enjoying some quiet chatter with your mates, is a distinct pleasure. But once it’s time to hit the sleeping bag, you might find yourself cursing the night as you swat at pterodactyl-size mosquitoes intent on sucking you dry. It’s enough to ruin anyone’s camping experience!
And a mosquito bite can result in more than an itchy bump, as some might have a more adverse reaction; and there are quite a few mosquito-transmitted illnesses to be leery of. Things like West Nile virus, dengue fever, elephantiasis, chikungunya virus, Rift Valley fever, and the more-commonly feared yellow fever and malaria.
The good news is that scientists are beginning to understand why some of us get bitten more than others. An estimated 20 percent of people are especially delicious to mosquitoes, and are bitten more often on a consistent basis. And while scientists don’t yet have the complete picture, they do have a number of ideas about why some of us attract mozzies more than others.
It has been found that human genetic factors account for about 85 percent of the reasons that mosquitoes find some of us more appetising than others.
One of the key ways mosquitoes locate their targets is by homing in via the carbon dioxide (CO₂) in your breath. They use an organ called a maxillary palp to do this, which can detect CO₂ from as far as 50 metres away. The scent and the amount of CO₂ you exhale is unique to you and your genetic makeup.
People who exhale more of the gas over time are bitten more often. On average, larger people exhale more CO₂, which is why adults are more likely to be bitten than children.
There are many sources of CO₂ in nature, so it isn’t just the carbon dioxide that attracts mosquitoes. Every time we exhale, we release chemicals such as lactic acid, octenol, uric acid and fatty acids which combine with the CO₂ to form our own unique carbon dioxide cocktail. This combination of scents is what tells mosquitoes that there’s a human target nearby. And some of these particular combinations are more attractive to mosquitoes than others.
A recent study found that mosquitoes landed nearly twice as often on people with Type O blood as those with Type A. People with Type B blood fell somewhere in the middle of this itchy spectrum.
Looking at other genes, they note that about 85 percent of people secrete a chemical signal that indicates which blood type they have – which is like a ‘free dinner’ advertisement. Mosquitoes also appear to be more attracted to these secretors more than non-secretors regardless of which type they are – and unfortunately there isn’t much you can do to change your attractiveness other than mask your scent. (Sigh)
Exercise and Metabolism
In addition to carbon dioxide, mosquitoes find victims at closer range by smelling the lactic acid, uric acid, ammonia and other substances expelled in their sweat, and are also attracted to people with higher body temperatures.
Because strenuous exercise increases the build-up of lactic acid and heat in your body, it makes you stand out. Meanwhile, genetic factors influence the amount of uric acid and other substances naturally emitted by each person, making some people more easily found by mosquitoes than others.
Having skin bacteria doesn’t mean that you are unclean. Research has suggested that the particular types and volume of bacteria that occur naturally on human skin affect our attractiveness to mosquitoes. In a 2011 study, scientists found that having a large quantity of only a few types of bacteria made skin more appealing to mosquitoes. This could explain why mosquitoes are especially prone to biting our ankles and feet, as they naturally have more robust bacteria colonies.
Surprisingly, it seems that having large quantities of bacteria of a greater diversity of species seemed to make skin less attractive.
In multiple studies, pregnant women have been found to attract approximately twice the number of mosquito bites that non-pregnant women do. This is probably because pregnant women exhale about 21 percent more carbon dioxide, and also because their body temperature is higher, on average, by 0.7°.