Picture this: you’re sitting outside on a warm summer evening enjoying a barbecue with friends when your garden is invaded by buzzing critters with dreadful needle-like mouthparts. Yes, we’re talking mosquitoes. But why is it that some people can continue to enjoy their evening without any complaints, while others are seemingly irresistible to the biting bugs?
Surprisingly, out of the 3,000 or so species of mosquitoes, only a few have a preference for human blood. Most opt for a wider audience and can feed from different animals. But some – like Aedes aegypti and Anopheles gambiae – are known for their appetite, specifically for human blood. Unfortunately, these insects can also transmit various diseases to humans, including zika, dengue and malaria.
Many old wives’ tales explain how mozzies pick their next meal, each as far-fetched as the next. Avoid bananas, eat garlic, go easy on the crisps and sweets; none of these survive scientific scrutiny. But not all is lost; there is a great deal of research going on to understand mosquitoes’ feeding choices, mainly to control the spread of disease in humans.
All mosquitoes use carbon dioxide to spot a target nearby. In fact, these annoying bugs can detect a person exhaling CO2 up to 15 metres away. Everybody is at risk, but if you’re exercising or drinking alcohol, suddenly you become particularly attractive to these insects as you’re exhaling more CO2. Carrying extra weight, including being pregnant, has a similar effect as it makes you breathe more heavily.
“Once they’ve set their sights on possible human targets, how do mosquitoes choose a specific prey?”
However, humans are not alone in exhaling CO2. This gas is everywhere, providing little information to help a wandering mosquito find its human target. It turns out mosquitoes can ‘smell’ beyond carbon dioxide, tracking down human victims by detecting a variety of other chemicals, including lactic acid, ammonia, and even some carboxylic acids. This explains how mosquitoes can find humans instead of a dog or a cow – but once they’ve set their sights on human targets, how do mosquitoes choose a specific prey?
There is some anecdotal evidence that certain blood types may be more desirable than others – people with Type O seem to attract more mosquitoes than those with Type A blood – but the most substantial evidence to explain why a mosquito bites a specific person is their skin microbiota.
We have hundreds of different species of microorganisms living on our skin, and each person is slightly different. Composition depends partly on what we eat and where we live, but there’s some evidence that genetics can also shape skin microbiota. It can affect the production of certain proteins and more ordinary aspects like oiliness and tendency to sweat. So if mosquitoes are attracted to you more than your mates, it’s because the composition of microorganisms in your skin suits their purposes. Unfortunately for their victims, finding out precisely what this composition is and how to use that knowledge to develop better insect repellent is still work in progress.
There’s one last factor, which beer lovers may not appreciate. It seems that drinking one can of beer is enough to make you more attractive to these mini beasts. The reason is still a mystery, as neither the resulting increase in body temperature nor ethanol content in sweat are seemingly enough to explain the effect. Maybe the sneaky pests think drunk people make easier targets?
Alex Reis is a freelance writer based in the UK www.alexreis.science
