As an avid surfer, I occasionally hear concerns about shark attacks at the beach. Diving into the statistics of unprovoked shark attacks, I learned that they are astronomically unlikely. Deaths are even rarer, with only around one fatal shark attack per year in the U.S. So then what is the deadliest animal? Worldwide, scorpions kill a few thousand people annually, dogs around ten thousand annually, and snakes kill some 75,000 people a year! That’s a drop in the bucket compared to other humans, who kill around half a million people per year. But then there are mosquitoes. Mosquitoes kill more people than every other animal combined – including humans; something like 750,000 to 1 million people per year. Let’s get the buzz on why.

Mosquitoes don’t kill us directly. At least not normally. Sometimes blood loss from mosquito bites can kill animals as big as cows, but this is an exception rather than the rule. Normally, mosquitoes kill people by acting as vectors, which transmit disease. The deadliest known disease in the history of the planet is malaria, and it is responsible for at least several billion deaths throughout history (the exact number is quite controversial, with estimates ranging from 5% – 50% of all people ever to live). Mosquitos also transmit dengue, yellow fever, chikungunya, zika, and more. The question then is… why? Why us in particular?

Actually, only some mosquitoes like humans in particular. There are around 3,600 types of mosquito. Some species, like Aedes aegypti (a-ee-dees a-gyp-thai), hunt humans specifically for blood. Others target snakes, frogs, or birds. Many are generalists and hunt anything with blood. However, consuming blood is actually a rare occurrence in the life of a mosquito. For most of their lives mosquitoes are vegetarians. They eat plant nectar, fruits, and the sugary waste of aphids called honeydew. Mosquitoes pollinate flowers, like to eat apples and bananas, and wanna hang out for a nice long walk on the beach. Some mosquito species actually stay vegetarian their whole lives. In fact, male mosquitoes don’t consume our blood, it’s only females when they need to lay eggs. Fruit juice is nice, but – as every good vegan knows – you need to get your protein somehow. For mosquitoes, some species need the extra protein found in blood to help their young thrive. How mosquitoes actually locate a host is pretty complex.

It’s easy to guess how a mosquito might find us by looking at what signals we give off. We breathe, we smell, we’re warm, we look like people, and we taste like humans. Each of these features attract mosquitoes from progressively shorter distances. Let’s move through how.

• Breath
  • When we exhale, CO2 comes out. These puffs of carbon dioxide travel through the air, dispersing into relatively big clouds. Mosquitoes have a special nerve cell called a cpA neuron that can detect CO2. Mosquitoes follow the trail of CO2 upwind until they smell us.
• Odor
  • Mosquitoes can detect the specific scent profile animals emit using those same cpA neurons. They then determine if the smell matches the creatures they prefer to hunt using their antennae and other nose-like organs. Humans emit a lot of scents. Key among these are acetoin, made by skin bacteria, and volatile carboxylic acids, like lactic acid. The amount and composition of these chemicals change based on genetics and environmental changes. Having malaria, for instance, makes you smell more attractive to mosquitoes.
• Temperature
  • When a mosquito gets close enough, it can start detecting body heat, which draws them in.
• Shape and Color
  • Mosquitoes use vision to detect us from a few inches away. Their eyes are specialized to detect redder wavelengths of light, similar to many skin tones, and they preferentially fly towards high contrast objects: think a dark arm against a bright blue sky.
• Taste
  • The last step before ruining our outdoor fun is to make sure we taste good. Rubbing disgusting-tasting bug spray all over our bodies helps keep mosquitoes from wanting to eat us, but normally they’re way into the taste of old skin and sweat.

People exhibit variations on all these areas (except for breathing). Our smells change, some of us wear insulating clothes, skin tones vary, and according to Dr. Hannibal Lecter, we taste different. Scientists have studied the variation between people and how many mosquitoes bite them in an effort to seek relief from mosquito bites. Mosquitoes tend to bite pregnant individuals more frequently and genetics play a role, but these factors are difficult to alter in many people. Instead, researchers tend to target our most modifiable attractant, smell. Our skin microbiome and genes affect our scent, but diet seems to as well – though not as much as many people claim. Randomized clinical trials have found no evidence that vitamin B, garlic, and green grapes affect mosquito bites. There is some preliminary evidence pointing to caffeine as a possible attractant. Studies have found evidence that eating bananas and drinking beer both increase mosquito interest. As stated before, having malaria makes you more attractive to mosquitoes. Unfortunately, you may have noticed that none of these reduce our attractiveness to mosquitoes. 

Bug spray containing DEET makes it more difficult for bugs to smell you and is recommended, but can be sticky, stinky, and unpleasant to use. Next-generation bug repellents may block multiple scents or even inhibit the cpA neurons directly! Physical barriers like long sleeves can help as long as they don’t overheat you. Really, the problem is best summarized in a paper by Van Breygel et al. (2015):

For a human hoping to avoid being bitten by a mosquito, our results underscore a number of unfortunate realities. Even if it were possible to hold one’s breath indefinitely, another human breathing nearby, or several meters upwind, would create a CO2 plume that could lead mosquitoes close enough to you that they may lock on to your visual signature. The strongest defense is therefore to become invisible, or at least visually camouflaged. Even in this case, however, mosquitoes could still locate you by tracking the heat signature of your body provided they get close enough. The independent and iterative nature of the sensory-motor reflexes renders mosquitoes’ host seeking strategy annoyingly robust.

The obvious reaction to this is to think “kill ‘em all!” Unfortunately, even this method fails. Insecticides have a nasty habit of prompting natural selection to favor bugs immune to them – and they manage to kill many innocent bugs in the process. Traps have limited effectiveness, can be expensive, and also manage to murder countless other ecologically important bugs. With this in mind, perhaps the solution to saving lives from the world’s deadliest animal isn’t in reducing our attractiveness (my mom tells me I’m very attractive), but in reducing our susceptibility to the diseases they carry. Across the globe, scientists are in various stages of research seeking vaccines for malaria, dengue, and other mosquito-borne diseases. If these manage to be successfully tested and distributed, maybe we won’t have anything to fear from mosquitoes after all! Except for the itching. And the annoyance. And the constant ankle biting. And that they like to fly at our eyeballs. And that they might literally take more blood out of us than those sharks everyone tells me to watch out for.

Staff Writer / Editor Benton Lowey-Ball, BS, BFA

References:

Associated Press. (September 9, 2020).  Thick clouds of mosquitoes kill livestock after hurricane. https://apnews.com/article/horses-animals-insects-storms-hurricane-laura-fa0d05b046357864ad2f4bb952ff2e3e

CDC Global Health Center. (April 8, 2024). Fighting the world’s deadliest animal. Centers for Disease and Control. https://www.cdc.gov/global-health/impact/fighting-the-worlds-deadliest-animal.html

Brown, J. E., Evans, B. R., Zheng, W., Obas, V., Barrera-Martinez, L., Egizi, A., … & Powell, J. R. (2014). Human impacts have shaped historical and recent evolution in Aedes aegypti, the dengue and yellow fever mosquito. Evolution, 68(2), 514-525. https://academic.oup.com/evolut/article/68/2/514/6852391

Ellwanger, J. H., da Cruz Cardoso, J., & Chies, J. A. B. (2021). Variability in human attractiveness to mosquitoes. Current Research in Parasitology & Vector-borne Diseases, 1, 100058. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8906108/

U.S. Environmental Protection Agency. (September 25, 2023). Insect repellents: DEET. https://www.epa.gov/insect-repellents/deet

Giraldo, D., Rankin-Turner, S., Corver, A., Tauxe, G. M., Gao, A. L., Jackson, D. M., … & McMeniman, C. J. (2023). Human scent guides mosquito thermotaxis and host selection under naturalistic conditions. Current Biology, 33(12), 2367-2382. https://www.cell.com/current-biology/abstract/S0960-9822(23)00532-8

Peach, D. A., & Gries, G. (2020). Mosquito phytophagy–sources exploited, ecological function, and evolutionary transition to haematophagy. Entomologia Experimentalis et Applicata, 168(2), 120-136. https://doi.org/10.1111/eea.12852

Potter, C. J. (2014). Stop the biting: targeting a mosquito’s sense of smell. Cell, 156(5), 878-881.https://www.sciencedirect.com/science/article/pii/S0092867414001585

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Tauxe, G. M., MacWilliam, D., Boyle, S. M., Guda, T., & Ray, A. (2013). Targeting a dual detector of skin and CO2 to modify mosquito host seeking. Cell, 155(6), 1365-1379. https://www.cell.com/cell/fulltext/S0092-8674(13)01426-8

Van Breugel, F., Riffell, J., Fairhall, A., & Dickinson, M. H. (2015). Mosquitoes use vision to associate odor plumes with thermal targets. Current Biology, 25(16), 2123-2129.https://www.sciencedirect.com/science/article/pii/S096098221500740X