Why Mosquitoes Bite Some People More Than Others

Are you the person in the group who's always getting bitten by mosquitoes? Because I certainly am, and science has shown that this is a thing—that mosquitoes are more attracted to some people than others. And the reason for that is at least partially genetic, which is why this video is supported by 23andMe, a company that reads sections of your genetic code and then helps you understand what's in there.

Now, they've actually been involved in some research that has identified particular spots on your DNA that make you more or less likely to be attractive to mosquitoes, and in this video, I want to put that to the test. So I flew to New Mexico State University to meet mosquito man Professor Immo Hansen.

Everyone: Hello!

Hansen: We have a collection of lab strains.

Derek: Here, he maintains colonies of many different species of mosquitoes, and one exotic strain he actually feeds with his own blood(!)

Hansen: No, no, I'm serious. These ones are made from my blood.

Derek: Really!?

Hansen: Yes! Absolutely, yeah.

Derek: You feed these!?

Hansen: And I'm the only one feeding them—I can't ask my students, you know, that would be a nightmare getting permissions to this room here.

Derek: How do you blood-feed them?

Derek: With just, sticking your arm in there?

Hansen: Well, I just put my arm in there and wait.

Derek: Is it psychologically itchy or...?

Hansen: No, it is really itchy [laughs]. Yeah, and I'm getting, I mean, 25 bites or so right now.

Derek: Mosquitoes need our blood proteins to make their eggs, so only the female mosquitoes bite. The eggs hatch into these wriggly larvae, which develop into pupae before becoming flying, adult mosquitoes. Then they, once again, seek out vertebrate blood to make more eggs. The whole lifecycle takes just two weeks!

But what I wanted to know is: am I a desirable target?

Derek: How can we figure out if I'm attractive to mosquitoes?

Hansen: Okay, we have a Y-tube, basically, which has a holding chamber. We put the mosquitoes down here; there's a little fan inducing a draft, something like 4 m/s. We're gonna insert a bait into one of these chambers, in the green or the yellow one.

Derek: So, by bait, you're saying me?

Hansen: Yes, you, exactly.

Derek: [laughs]

Hansen: You put your hand in there. [It] would be good if you would rub your hand first, so...

Hansen: If you're sweaty...

Derek: Get some oils?

Hansen: Yes!

Derek: This is the right spot...

Hansen: That's where the good stuff is, exactly.

Hansen: Almost feeds exclusively on humans. They are really specialized on biting humans.

Derek: That's good to know...

Hansen: Yeah [both laugh].

Hansen: There's 20 mosquitoes in the holding chamber.

Derek: And they're gonna decide whether to come and find me, or go down the other side and... they find nothing.

Hansen: Right.

Derek: We're releasing the mosquitoes. I see them coming!

Derek: Oh, he's chosen wrong!

Hansen: Do you see that?

Hansen: Man, you are attractive!

Derek: Really?

Hansen: I mean, sorry I—

[both laugh].

Hansen: You are a strong attractant to mosquitoes. They all went in your direction.

Derek: None of them went the other way!

Derek: (laughing) This is amazing!

Hansen: Well, there's one stranded in here.

Derek: I did not expect to have such a strong... response, like...

Derek: You see all those mosquitoes...

Derek: This type of test was actually used to discover that the basis for our attractiveness to mosquitoes is at least partially genetic. Researchers recruited 18 pairs of identical female twins and 19 pairs of non-identical, or fraternal, female twins. Then they used the Y-tube Test to evaluate the mosquito attractiveness of each individual, measured by the fraction of mosquitoes who correctly flew to the arm of the tube where the twin was standing.

What scientists found is that the mosquito attractiveness of twins is correlated; that is, the more attractive you are to mosquitoes, the more likely your twin is, too. But that's... not all that surprising and could be caused by environmental factors or a common diet.

But, comparing identical twins to fraternal twins revealed the correlation was higher for identical twins. This strongly suggests genetics influence how attractive you are to mosquitoes. Since the identical twins share more of the same genes than fraternal twins, this explains why their mosquito attractiveness would be more closely correlated if it really is genetics that determines how much mosquitoes like you.

Derek: With that last test, I was clearly attractive to these mosquitoes.

Hansen: Yes.

Derek: Except we weren't comparing me to any other human; we were just comparing me to a control—no human.

Hansen: Exactly, yes.

Derek: So, what if we compare myself with my wife Raquel? [laughs]

Hansen: Uh-huh?

Derek: Normally, I get bitten and she doesn't.

Hansen: Okay.

Derek: So this should be a good test... maybe?

Derek: To see whether under lab conditions...

Hansen: We can reproduce your...

Derek: We'll reproduce our anecdotal finding. That I'm more attractive.

Hansen: Exactly, anecdotal evidence a little once again.

Hansen: They are a little...

Derek: Once again...

Derek: ...preparing.

Hansen: Okay!

Derek: Hands in position!

Raquel: Yeah.

Hansen: Yeah, see, they actually start to wake up. They... smelled something. Something's going on here.

Raquel: Here we go.

Hansen: Okay.

Derek: What are we seein'?

Derek: Whoa!

Derek: I'm getting a good amount over here, but so are you?

Derek: Let's see. I think I'm getting more. It looks to me like you got three.

Hansen: Okay, I'm gonna stop the experiment right now.

Derek: Alright.

Hansen: chuck! Okay, now let's count. I think it's seven on your side here?

Lab assistant: Five in the holding.

Hansen: Five in the holding.

Hansen: And quite a few on your side!

[Hansen and Derek laugh].

Hansen: So, who gets bitten?

Derek: I get bitten.

Hansen: You get bitten!

Hansen: In reality, we would repeat this experiment, maybe eight times or so.

Derek: Right.

Hansen: And switching you guys around, but I think this is a good way to show how this works.

Derek: Okay, so that result was not particularly scientifically rigorous, but it did reproduce our experience in the wild, which is that I am much more attractive...

Well at least when it comes to mosquitoes, than Raquel.

Derek: But the question is, I guess, does this come down to our genetics?

Raquel: Yeah, so, we spat into some tubes and sent them off to be tested.

Derek: Right, and we were interested in seven particular locations on our DNA, which were located in a 2017 study that involved 23andMe.

Derek: So the way 23andMe works is, when you sign up, you can opt in to be part of research, and 16,000 people agreed to be part of this study and rate their perceived attractiveness to mosquitoes.

Derek: So then what the scientists did was a genome-wide association study; that is, they looked at all the DNA of all of those participants and tried to see if there were commonalities amongst the people who said they were attractive to mosquitoes and that were different from the people who said they weren't attractive to mosquitoes.

Derek: And they identified seven particular locations on the DNA. Seven single letter changes which seemed to be associated with different levels of attractiveness to mosquitoes.

Derek: So we have our results back, and do you wanna see them?

Raquel: I do. I do, I'm very curious.

Derek: Alright, let's pull it up here.

Derek: Okay, so of the seven locations that are related to mosquito attractiveness, it turns out we have the identical DNA at four of them.

Derek: So you can rule those out.

Raquel: Okay.

Derek: Which leaves only three areas where we actually have differing DNA. So, at the first location, you have one copy of a letter change which actually makes you significantly protected from mosquitoes.

Raquel: Oh, no way!

Derek: Yeah, it's associated with decreased attractiveness to mosquitoes.

Raquel: Oh, that's cool!

Derek: I do not have any letter changes at that location. Now, at the second location where we differ, I have a letter change compared to you that makes me less attractive to mosquitoes, so more protected.

Raquel: Interesting.

Derek: If you look at the significance of those two letter changes, yours is about twice as significant as mine.

Raquel: Okay.

Derek: But still we both have a protective letter switch. So, in the last snip, we actually differ significantly.

Derek: And I have two copies of a variant that makes me more attractive to mosquitoes.

Derek: This was the only snip which was associated with being more attractive to mosquitoes, and I have two copies of that change, and you have no copies of that change.

Raquel: So that makes sense.

Derek: So overall, I would say our genetics really adds up here.

Derek: And of course, we can't say that this proves that it is right, but it is definitely consistent with these snips actually being associated with your attractiveness to mosquitoes, and that's sort of... borne out by our experience.

Raquel: That's so cool!

Derek: Now, it's unclear exactly how these genetic changes might make us more or less attractive to mosquitoes, but it's likely that it has something to do with the odor or the volatile chemicals that our bodies give off, and due to the microbiome—the bacteria on our skin.

One of the main signals that mosquitoes like to follow is carbon dioxide. So that means if you have a higher metabolism, or if you've just been exercising, or if you're a bigger person, or even if you're pregnant, you are more likely to attract mosquitoes.

But mosquitoes are also attracted to some other volatiles that we give off, things like lactic acid, acetone, and ammonia. But scientists have also found some chemicals that repel, or appear to impair mosquitoes' ability to find us. Those chemicals we naturally give off are octanal, nonanal, decanal, and 6-methyl-5-hepten-2-one.

So, why is this important? Well, the researchers estimate that our attractiveness to mosquitoes is about as hereditary as height or IQ; that is to say genetics play a significant role here, so understanding that relationship is really important, especially when you consider that, of all the animals, mosquitoes have the greatest impact on human health.

Derek: Are mosquitoes the worst animal of all time for humans?

Hansen: Absolutely. Absolutely, there's no question about that. Malaria has killed more people than people have killed people. They are the most dangerous animal in the world.

Derek: By some estimates, mosquitoes have killed more than half of the humans who have ever lived. Now, that estimate has been debated and is likely too high, but even so, this year, over a million people will die of mosquito-borne illnesses.

So if not half, certainly a significant number of humans have died due to mosquitoes, probably more than any other single cause, which led me to wonder:

Derek: Do you think humans may have evolved this trait to smell worse to mosquitoes as an adaptation to avoid the diseases? Or...

Derek: is it just by accident that some people are less attractive to mosquitoes than others?

Hansen: ....... That is a really good question!

[both laugh].

Derek: And while we're on the subject of evolution, consider this: if you ever contract malaria, it actually changes your body chemistry to make you produce an odor that makes you more attractive to mosquitoes.

Think about that! The malaria parasite has evolved so that, when it's in its host, it makes the host more attractive to mosquitoes... mosquitoes are the thing that transmit malaria!

It's phenomenal! I mean evolution is incredible!

[Veritasium logo]

This episode of Veritasium is supported by 23andMe, a company whose name comes from the fact that humans have 23 pairs of chromosomes, and the point of the company is to help people understand what's written in your chromosomes.

Now, at the minute, you can't get access to the mosquito attractiveness test, but maybe, one day in the future, you will be able to.

Right now, you can access tons of information about your physical traits, about aspects of your health, and about where your DNA and, I guess, you come from.

So I thought I would share my results with you. I come from all over the world as you can see from the map, but significantly more from Europe, about 95% European.

You can drill down into that and see that I'm largely British and Irish, and French and German, with a bit of Scandinavian.

What's interesting to me is, there's a 3.3% South Asian which is something I think my family suspected but didn't know for sure, so it's interesting the types of information you can find out which is stored in your DNA.

So, if you wanna find out about the information in your DNA, you can go to 23andme.com/veritasium.

So I want to thank 23andMe for supporting Veritasium, and I want to thank you for watching.