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Human Mosquito Magnets

Donate Is it true that mosquitoes like some people better than others?  

by Brian Dunning

Filed under General Science, Health

Skeptoid Podcast #862
December 13, 2022
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Human Mosquito Magnets

If you're like me, you're the one person at the campfire that all the mosquitoes come directly to. Having me around is like having the world's most effective mosquito repellent for yourself, because I draw them all away from everyone else around. But then I have friends and family who are just the opposite. While others like me are getting sucked dry and injected with itching juice, they'll sit around completely unaware that the mosquitoes even exist. They just don't get landed on. The mosquitoes take one look, or sniff, or whatever it is they do, then fly away to find me. And so the conversation is inevitable: Are some people truly more attractive to the little pests than others? Might there be some kind of natural repellent or attractant that some of us have, and others lack? Today we're going to turn to science to find out what we know.

It's not really a spoiler to state up front that scientists have known for some time that certain people are more attractive to mosquitoes. There are widely cited papers making this finding as far back as the 1960s. It's a very important topic to study, and not just because of the suffering of people like me who are always the first ones at the campfire to become encrusted with hundreds of mosquitoes while other people just sit there laughing at me and enjoying their lack of torment. The real reason such studies are important, as you can probably surmise, is that there are many deadly diseases that are mosquito borne throughout the world. In many countries, whole populations live under a very real risk of potentially serious or fatal illnesses contracted through mosquito bites — diseases like malaria, dengue, yellow fever, the West Nile and Zika viruses. Therefore it's important to develop increasingly effective repellents, and to do that we need as deep an understanding as possible into what mosquitoes like and don't like.

The basic finding is that people with high levels of carboxylic acids on their skin are the ones the mosquitoes like the most. Carboxylic acid comes from your sebum, which is the oily substance produced by the sebaceous glands in your skin. These are connected to hair follicles, and as the little hairs all over your body grow up out of your skin, they pull the sebum with them, and that's how it gets onto the outside of your skin. That's one reason we all have those tiny little hairs, called vellus hairs, all over our bodies. The sebum moisturizes and protects the skin, and also provides a yummy food source for the bacteria that live on us. They love those carboxylic acids. All of this chemical complexity going on is a large part of the smell that humans have, and that probably plays a major role in attracting mosquitoes.

In 2022, an article was published in the journal Cell that sought to find out just how much of an effect the carboxylic acids have. The researchers took 64 volunteers and had them all wear nylon sleeves for six hours a day, then those sleeves were used in the experiment, so nobody actually had to get bitten by mosquitoes. The sleeves were placed into a clear acrylic chamber, two at a time, illustriously called a "two-choice olfactometer assay" — basically two chambers, equal access for the mosquitoes; and each chamber already controlled for temperature, humidity, other odors, whatever, so that mosquitoes did not have a preference for one chamber or the other. Then they simply ran thousands of head-to-head contests by placing sleeves from two different people into the two chambers. They noted how many mosquitoes went to each, and eventually had all the human subjects arranged by attractiveness. Or, I guess I should restate that: they had all the human-worn sleeves arranged by their attractiveness to mosquitoes.

We'll skip over the complexities of how they calculated the score for each person, but you're probably wondering how it came out. Did everyone fall into one of two groups, the "mosquitoes love them" and the "mosquitoes hate them" groups? Well, no. It's not quite that simple. The scores ranged from 0 to 144, 0 being the least interest to the mosquitoes, and 144 being the mosquitoes' favorite person on Earth. Most subjects clumped around the 18 to 34 scores; not super interesting to mosquitoes, but that seems to be about the average. Just a few people were way down at 0 or 1, meaning there was something about them the mosquitoes simply would not tolerate. Lucky devils. But then the study showed one person — just one — way up at 144, more than 100 points higher than the second most attractive. Somewhere, mosquitoes are probably carving a marble sculpture to this person. Something about their sebum was outrageously attractive to mosquitoes.

64 people is not an enormous sample size. With more subjects we'd have a better idea of the shape of the curve, but from the sample we do have, it appears that people's attractiveness to mosquitoes falls into a bell curve, with the tallest part of the curve down toward the lesser end of the scale. Lucky people like me make up that long upper tail of the curve. Other research over the years has even pinned a rough number on it: about 20% of people seem to be significantly more mosquito-friendly than average.

But that's just your natural genetics that you can't do much about. There are ways you can artificially make yourself even more attractive to the little beasties. One way is to get bitten by one that carries malaria, and catch malaria. Malaria is an infection with the Plasmodium malariae blood parasite. The exact mechanism isn't yet known, but having malaria alters your body's scent in some way. Other mosquitoes key onto that, and zoom in to bite you. This gives the parasite to those mosquitoes, who then go off and spread it to others. Those little Plasmodium guys have figured out the ultimate evolutionary advantage: they infect the mosquitoes and vertebrates upon whom their life cycle depends in such a way that it causes them to interact with each other more.

In 2019, a team of Chinese researchers published a study in which they developed a synthetic mosquito attractant that's approximately twice as effective as a commercial version called BG-Lure. They tested all kinds of mixtures and found that a blend of lactic acid, hexanoic acid, 3-methyl-1-butanol, cyclopentanone, 1-octen-3-ol, 6-methyl-5-hepten-2-one, and ammonia was best. They called it Mix-5 in their study. So if you want to impress your friends, get ahold of some Mix-5 and spread it on yourself. As you can probably surmise, the purpose of the study was to find better ways to bait mosquito traps and electric bug zappers in a region of China where dengue is especially problematic.

Your breathing also attracts mosquitoes — actually, all vertebrates' breathing attracts them. One of the ways mosquitoes know someone is there to bite is that they exhale carbon dioxide, and they can follow that to the source. They have an organ called a maxillary palp which detects the gas. If you are bigger and exhale more, you're an easier target for them to find.

If you want, you can enhance that a bit more by exercising. Not only will you exhale more carbon dioxide, you will sweat — and sweat contains most of those compounds used in the Chinese study, such as the lactic acid and the ammonia.

Exercising will also bring up your skin temperature as your body offloads the heat generated, and it turns out higher body temperatures are another thing that mosquitoes like. On the tips of their antennae are temperature sensing cells. More accurately, they are temperature-change sensing. In effect, they don't know if they're hot or cold, just if they're getting hotter or colder. They can sense changes in temperature as small as a hundredth of a degree. As long as they're on their way toward a warm body, they'll keep flying in that same direction.

Here's the way that works. Insects have a receptor called IR21a that helps neurons transmit signals. Normally it's quiet and doesn't do anything; but if the mosquito is flying in a direction that's getting colder, it activates. It's like a course-correction alarm. If you're not flying toward food, change direction until you are. In 2020, a team at Brandeis University published in the journal Science about how they disabled the gene for IR21a in some mosquitoes. They constructed a chamber with two metal plates on the wall: one which was room temperature at 73°F, and the other at 88°F (skin temperature). Mosquitoes were attracted into the chamber by carbon dioxide, not unlike a dinner gong. While the normal mosquitoes showed a very clear preference to land on the warm plate, the mosquitoes with IR21a deactivated didn't seem to find it. They just went flying around in the chamber, sniffing the carbon dioxide, and wondering where all the food went.

It's also been found that when women become pregnant, they become about twice as attractive to mosquitoes, on average. But this doesn't have to do with anything hormonal or anything like that; the change appears to be caused entirely by their increased body temperature (which is 1.26°F warmer than normal) and their increased carbon dioxide output (21% more than normal). So to all you moms out there, no you weren't imagining it. You did indeed attract more mosquitoes when you were pregnant.

So what about factors that we can control? Is there anything we can do to make ourselves less attractive? Certain dietary items are about the only ones I could find. Now it's important to point out that diet is only one factor among the others we've discussed, and it probably has a very small impact, so don't get too excited about it. I found no studies that identified any foods that made a person less attractive to mosquitoes. Garlic and vitamin B supplements are the popular ones that everyone talks about and believes, but the data is very clear that neither provides any benefit at all — this is something that's been tested and debunked many times. However, there are two foods that a few studies have found may make you slightly more attractive to mosquitoes, if that's what you're after: beer and bananas. The effect was small and has not been widely replicated, nor was any dosage established; but there is an element of plausibility to it. Foods can and do affect the rate and release of certain metabolic byproducts, and if you can exhale it, mosquitoes can smell it. Whatever the crucial elements that beer and bananas might add to your breath are, they're probably not helping you any.

And so that's about it. Yes, there are absolutely people who the mosquitoes like best, and no there probably isn't very much they can do about it except apply DEET — the only repellent that actually works decently, as we discussed in Skeptoid #195 in answer to a student question. If you, like me, are in that lucky 20%, then cheers, let's toast to each other's odors — just not with beer.


By Brian Dunning

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Cite this article:
Dunning, B. "Human Mosquito Magnets." Skeptoid Podcast. Skeptoid Media, 13 Dec 2022. Web. 31 Jan 2023. <https://skeptoid.com/episodes/4862>

 

References & Further Reading

Busula, A., Verhulst, N., Bousema, T., Takken, W., De Boer, J. "Mechanisms of Plasmodium-Enhanced Attraction of Mosquito Vectors." Trends in Parasitology. 1 Jan. 2017, Volume 33, Number 12: 961-973.

De Obaldia, M.E., Morita, T., Dedmon, L., Boehmler, D., Jiang, C., Zeledon, E., Cross, J., Vosshall, L. "Differential mosquito attraction to humans is associated with skin-derived carboxylic acid levels." Cell. 27 Oct. 2022, Volume 185, Issue 22: 4099-4116.

Ellwanger, J., Cardoso, J., Chies, J. "Variability in human attractiveness to mosquitoes." Current Research in Parasitology & Vector-Borne Diseases. 27 Oct. 2021, Volume 1.

Greppi, C., Laursen, W., Budelli, G., Chang, E., Daniels, A., Van Giesen, L., Smidler, A., Catteruccia, F., Garrity, P. "Mosquito heat seeking is driven by an ancestral cooling receptor." Science. 7 Feb. 2020, Volume 367, Number 6478: 681-684.

Winegard, T. The Mosquito: A Human History of Our Deadliest Predator. New York: Dutton, 2019.

Xie, L., et. al. "Enhancing attraction of the vector mosquito Aedes albopictus by using a novel synthetic odorant blend." Parasites & Vectors. 1 Jan. 2019, Volume 12, Number 1.

 

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