Why Do Mosquitoes Bite Us, but Not Everyone Equally?

With the arrival of good weather comes the return of a universal summer nuisance that knows no borders and almost no latitudes. But while mosquito bites are only a minor annoyance for the vast majority of people in the developed world, for much of the tropical belt they pose a risk of death or serious illness. We know what mosquitoes are looking for when they bite us, but we also know that not everyone is attacked equally: while some people only get a few bites, others are treated like an open buffet. Why do mosquitoes discriminate between their victims?

What is commonly known as a “mosquito”, a term that means “little fly” in Spanish, is actually a family of dipteran insects, the Culicidae, of which more than 3,700 species are known to date, distributed in 112 genera and found on all continents except Antarctica. Scientists estimate that these animals arose in the Jurassic period, more than 145 million years ago, although the oldest fossils found so far—encapsulated in amber, as in the movie Jurassic Park—date from the Cretaceous period, 130 million years ago.

Animals that cause the most human deaths

Of the thousands of species that exist today, only a minority bite humans, and even fewer are those that prefer us to other targets. But that is still enough to put them at the top of the list of the animals that cause the most human deaths, up to a million a year. A total of 88 mosquito species transmit 78 diseases, but a further 243 species are potential vectors of human pathogens. Of all these diseases, malaria is the deadliest, killing around 600,000 people per year, mostly children.

So the first thing to know about mosquito bites is that only a few species attack us, and only the fertilised females: all mosquitoes feed on plant nectar and juices, but the females extract proteins from the blood of the animals they parasitise to obtain amino acids that allow them to hatch eggs or produce them in greater quantities. Curiously, in the Cretaceous mosquitoes preserved in amber, the males also had these blood/sucking mouthparts; scientists suggest that these mouthparts were originally used to pierce plant tissues, that they evolved into a blood-sucking organ, and that the males eventually dispensed with it when flowering plants appeared.

But how do mosquitoes manage to find their long-suffering victims? In recent years, researchers have been able to detail the sophisticated battery of sensors based on smell, taste, heat and vision that enable them to detect our physical and chemical cues. It all starts when the mosquito smells the gas that all vertebrates exhale: carbon dioxide (CO₂). Although it is present in the atmosphere, mosquitoes can detect changes in CO₂ concentration of up to 0.01% and from a distance of more than 10 metres, and even up to 50 metres.

Once a potential prey has been located by the CO₂ it emits, the mosquito switches to visual mode at a distance of five to ten meters to follow the trail that brings her closer to us. At this point we are still just living lumps. When she gets closer than 20 centimetres, she can already detect our heat, skin moisture and the volatile compounds we exude. At three centimetres, all the cues confirm to her that we are her target, and when she lands, the receptors on her legs taste us.

So it is the volatile compounds on our skin—our body odour—that set us apart from other potential prey. We produce more than 500 of these compounds, and studies have identified about 20 of them that may be more attractive to mosquitoes and that are produced in varying amounts in different regions of the skin, which would explain why they sometimes tend to bite us more in certain places on the body.

Preferred target

All this may explain why some people are rewarded with more bites than others. For example, women in the later stages of pregnancy exhale up to 21% more CO₂ with each breath, making them a preferred target, as are larger people who exhale a greater volume of gas. If we have just exercised, our temperature and sweat can make us prime targets. 

Body odour also varies from person to person. It is thought that 85% of this variability is genetic—meaning that our attractiveness to mosquitoes is largely inherited—but the microbiota on our skin can also play a role. And our odour is not always the same either, but can change with age, medication, diet, illness and other factors. The influence of other factors is still unclear. For example, studies have found that people with blood type 0 are more attractive to mosquitoes than those with blood types B and A, in that order, which could be related to a difference in skin secretions.

Despite what we already know, there are still questions to be answered and gaps to be filled in our knowledge about what makes us more or less appetising to mosquitoes, or what the relative contributions of the different factors are that make us unequally susceptible to being targeted by mosquitoes.

Javier Yanes