07/24/2020

Why Mosquitos Are Mingling More With Humans

16:41 minutes

a mosquito biting into human skin
Aedes aegypti. Credit: Shutterstock

There are a lot of things that come back during summertime: There’s the heat, cicadas—and those pesky mosquitoes. If you stay outside late enough you might become their dinner, for some mosquitoes at least. There are over 3,000 mosquitoes, but only a handful feast on blood, like the yellow fever mosquito, Aedes aegypti. Other mammals also have blood running through their veins, but are bit less frequently. So why do mosquitoes love humans so much?

New research on these bugs look into the cause, investigating mosquitoes’ preference for certain mammal odors and human population densities. Another paper examines a potential gene solution to decrease mosquito bites—thus lowering transmission of mosquito-borne diseases. Joining Ira to talk about the latest research and more mosquito science is “Lindy” McBride, biology assistant professor at Princeton University and Jake Tu, biochemistry professor at Virginia Tech.


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Segment Guests

Carolyn “Lindy” McBride

Carolyn “Lindy” McBride is an assistant professor in the department of Ecology and Evolutionary Biology and the Princeton Neuroscience Institute at Princeton University in Princeton, New Jersey.

Jake Tu

Jake Tu is a professor of Biochemistry at Virginia Tech in Blacksburg, Virginia.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. You know there are some things that just remind you of summertime– heat, cicadas, and unfortunately, mosquitoes. Stay outside late enough and you might become their dinner, for some mosquitoes at least. But why? Why do they love us so much? Last time I checked, there were more non-humans on the menu.

Well, new research on these bite-y bugs looks at two big things– why we have become the main course for some mosquitoes, and how a gender swap might fix the problem.

Joining us today is Lindy McBride, an assistant professor in the Department of Ecology and Evolutionary Biology at Princeton University, and Jake Tu, professor in the Department of Biochemistry at Virginia Tech. Lindy and Jake, welcome to Science Friday.

LINDY MCBRIDE: Thank you.

JAKE TU: Thank you. Lindy, before we talk about why mosquitoes seem to prefer we humans, let’s talk about why we humans showed up on the mosquito’s menu in the first place.

LINDY MCBRIDE: Excellent question. Most mosquitoes are fairly opportunistic, and they’ll bite many different types of organisms with which they come into contact. But a few species– just a few– 3,000 species– have evolved to really specialize in biting humans. And people have speculated for a long time as to why that might be true. And we set out to collect some systematic data to support some of those hypotheses.

IRA FLATOW: And where did you go to do your collection?

LINDY MCBRIDE: We collected mosquitoes in Africa. We were focused on the species Aedes aegypti– the main vector of dengue, Zika, chikungunya, and yellow fever. And this species originally evolved in Africa.

It was previously known that African populations of the species are likely ancestral, and not necessarily interested in biting humans. But at some point in the past 5,000-to-10,000 years, certain populations somewhere in Africa evolved to specialize in biting humans– and then escaped and spread around the world.

So outside of Africa, this species loves biting humans. Within Africa no one had previously found those sort of first human-preferring populations. And it was thought that most of these mosquitoes in Africa did not like humans.

IRA FLATOW: So you went over to Africa and brought back some mosquitoes?

LINDY MCBRIDE: Yeah, exactly. A really talented postdoc in my lab, Noah Rose, collaborated with several African researchers in various countries. And Noah went to six-or-seven different countries, and in collaboration with his other researchers collected mosquito eggs at 27 different locations spread across sub-Saharan Africa, from the east coast to the west coast.

IRA FLATOW: Just as a point of reference– how do you get mosquitoes through customs? Do you say, hey, I’ve got a whole bag full of mosquitoes here you ought to know about it?

LINDY MCBRIDE: [LAUGHS] Sometimes we do, just to avoid problems. And the great thing about this particular species is that the eggs are adapted to survive dry periods, by becoming dormant.

And so if we trick the eggs into thinking that it’s the dry season, we can get them to enter this resting phase. And we can basically just put them in our hand luggage and bring them back. And we show them at customs we have a special permit. And they wave us through.

IRA FLATOW: A pro tip for anybody wanting to bring back mosquito eggs. So you bring back the eggs to your lab. And after the eggs hatch, you give the mosquitoes a choice– human or guinea pigs. How does that choice come about?

LINDY MCBRIDE: Well, we needed some sort of non-human animal. And guinea pigs are small, friendly, cute. They fit in our own olfactometer, we call it. And previous work had shown that they were attractive to these animal-preferring mosquitoes.

We also sometimes use a quail, because quails are very different from guinea pigs. So the fact that these mosquitoes respond the same to guinea pig and quail gives us a good idea that the behavior isn’t guinea-pig specific. We’re talking about non-human animals, in general.

IRA FLATOW: And so that’s when you figure out that the mosquitoes have different preferences.

LINDY MCBRIDE: Yeah. So Noah would put 50-to-100 female mosquitoes in a box at one time, and turn on a fan that pulls air over a guinea pig and over a human. So the mosquitioes get to choose– do you want to fly towards the human-scented air or the guinea-pig-scented air? And they sort of a show us by numbers what they prefer.

IRA FLATOW: And did they have different preferences depending on where they were collected and their climate?

LINDY MCBRIDE: Yes. And that was the really surprising and interesting thing, is that we found that throughout most of sub-Saharan Africa these mosquitoes tended to fly towards the guinea pig. Sometimes they made a very strong choice for guinea pig– especially in places where there were few people, and in places that were relatively wet, with dependable rainfall. But in really seasonal environments, where there’s a long, hot, dry season, those mosquitoes loved humans.

IRA FLATOW: Does that mean maybe Everglades, or my swampy backyard in the heat in the summer, I’m safer, because it’s not hot and dry?

LINDY MCBRIDE: Unfortunately not. This only applies to this species, and it only applies to the species within Africa where it originally evolved. So if you live in Florida, for example, you’ll have the same species, probably– in southern Florida, at least– in your backyard. And that they will very likely be human-preferring despite the wet, humid conditions.

IRA FLATOW: From that handful of mosquitoes is it true that it’s the females, then, that need the blood, Lindy?

LINDY MCBRIDE: Yes. Females need blood to make eggs, essentially. Males do not bite.

IRA FLATOW: And Jake, that’s where you come in. Because you were looking at a way to turn the females into males.

JAKE TU: Yeah, exactly. Because we are pretty interesting in this particular topic. Because as Lindy was saying, only the females bite. And so only females are able to transmit disease-causing pathogens, like viruses and parasites. So there’s really a potential for us to control mosquito-borne diseases, if we can understand the sex-determination pathway better– so can manipulate the sex ratio.

IRA FLATOW: Were you able, then, to swap-out genetic material from a female and make it into a male?

JAKE TU: It’s really not swapping-out. It’s basically adding-in. Because in mosquitoes– at least this particular mosquito– the presence of a male-determining locus established a male sex, just like in humans. Let’s say, if you have Y in human, you become a male. So that extra male-determining locus determines male sex.

So what we did was we actually put a gene– inserted a gene that’s called nix, which we know is a male-determining factor– into the part of the chromosome that can be inherited by both sexes. So now the female had a chance to acquire this nix. And then they were to converted into fertile males.

IRA FLATOW: So then you really didn’t have to get rid of the whole chromosome. You just had to take out a little bit of a gene.

JAKE TU: Yeah, this was one gene. So only adding this nix into the female genome. So the females are still females, in terms of the genetics. But they don’t have to have locus, or the Y. But they acquire this just one gene. And they were able to be converted into a male, that are able to mate and produce progeny.

IRA FLATOW: And you were able to meet the males with the females and test this out?

JAKE TU: Right. So these males actually converted females. So they genetically speaking, they don’t have the M locus, Y. But they were able to mate was the wild-type females, producing converted males, as well as wild-type females.

IRA FLATOW: But I understood there was sort of a– I hate to use this pun– a fly in the ointment here, in getting these new these newly-genetically-modified males to mate.

JAKE TU: Right, exactly. As I said, there’s this male-determining locus that determines male sex. That locus has multiple genes– like 30 genes. So by just inserting this nix into the females, we did not bring up the other genes that’s in the m locus. So it turns out there’s a gene called myo-sex sex in the M locus, or male-determining locus. And that’s required for flight.

So we know that. Because when we knocked-out this myo-sex gene in a wild-type male, the wild-type male lost flight ability. So now because we only inserted nix into the female genome, not myo-sex, these females are not able to fly. And flight is actually required for mating. So what we have to do is we have to help this converted male a little bit, by chilling the female [INAUDIBLE].

IRA FLATOW: So you put the females in the freezer.

JAKE TU: The fridge.

IRA FLATOW: The fridge, excuse me. I didn’t want to go too far with that.

JAKE TU: Yeah, you don’t want to go there.

IRA FLATOW: You refrigerated the females, so that they would sort of slow down and could be caught by the males who can’t fly. Is that basically it?

JAKE TU: Not exactly. So we’d chill them down, so the females will not reject these males. So if they’re alert, this mating won’t happen. So the males and females really have to detect each other’s wing beat frequency– and a lot of things going on. That’s not my area of expertise. But lots of things are required for the mating to occur. So this was just to chill the female down so they don’t reject the male.

IRA FLATOW: Well, if you had that little problem with them flying, how do we know, then, that they will actually bite, or mate out in the wild?

JAKE TU: Right. So the males don’t bite anyway. So these converted males would not bite. But in order for them to be competitive in the wild. So one idea was to introduce post-genes– the nix and myo-sex into the female. So now you have a fertile and flying male. And that’s the hope. And we’re not there yet.

IRA FLATOW: So what is your next step, if you’re not there, then? You have to find a way to allow them to fly.

JAKE TU: Right. So basically we’re trying to do what I just said– try to insert post-genes into the female genome.

IRA FLATOW: Lindy, what do you think of all of this?

LINDY MCBRIDE: I think it’s fascinating. I just love how biology cooks-up these amazing things. I would have never guessed there would be a separate gene for male morphology and male flight.

IRA FLATOW: What could possibly go wrong with this, if we release these mosquitoes in the wild? Would it would it be better than pesticides, do you think?

JAKE TU: For one thing, these genetic approaches requires mating. We’re targeting this particular species that’s invaded the Americas a few years ago ended up causing problems to humans. And so that’s one of the advantage of this type of approach.

Of course, as I said earlier, we’re still far away from even testing this in the lab cages. So a lot needs to be done.

IRA FLATOW: And Lindy, think this is a good way of doing it?

LINDY MCBRIDE: Yeah. I think we need to try all the ideas that we have. And I think this is a promising one, that is exciting to be this far in developing this type of approach. It’s one that we certainly need to try.

IRA FLATOW: People will say, why do we need mosquitoes in the first place? They’re such nuisances. They carry around these diseases. They spread them around the world. How do you answer the question– why do we need mosquitoes at all? Why don’t we just wipe them all out, Lindy?

LINDY MCBRIDE: Mosquitoes are a really important part of the food chain. So if you like to fish, a lot of the fish that you catch probably mosquito larvae. Birds and bats eat a lot of mosquitoes. So I’m not a mosquito ecologist, but I would be afraid of what might happen if we got rid of all mosquitoes.

IRA FLATOW: Jake, you agree?

JAKE TU: I would just add, there are more than 3,000 species of mosquitoes. So I don’t think anybody is trying to wipe-out all 3,000 species of mosquitoes. And so what scientists are trying to do is to control the population of a few specific species that are causing lots of problems to humans.

IRA FLATOW: Just a quick note that I’m Ira Flatow. And this is Science Friday, from WNYC Studios.

OK, so let me give you the Science Friday blank-check question. I’ll give it to both of you. I’ll give it to you, Lindy, first. If you had a blank check, like I don’t have in my back pocket, and you could spend it on any kind of research or any technological whiz-bang thing you’d like to invent, what would you do with it?

LINDY MCBRIDE: I’m really interested in understanding how this mosquito finds us in the first place– how it recognizes the way we smell. So one of the first things I would do with that money is to dive into the mosquito brain and figure out exactly how it works.

IRA FLATOW: That is interesting. We don’t know that yet, of all these years of research?

LINDY MCBRIDE: It is surprising. We know remarkably little about what it is specifically about human odor that these mosquitoes love. I mean, we know some things. There is no one compound that will attract these human-preferring mosquitoes. You need combinations of compounds– odor blends– in order to attract the mosquitoes. But it’s still, I’d say, largely a mystery.

IRA FLATOW: Do we know what keeps them away? Like, citronella candles and things like that, do they really work? Yeah, citronella works. DEET, which most people would be familiar with, is extremely effective.

But that’s another surprising thing, is we’ve only really recently begun to understand how DEET works. DEET was discovered in just sort of a random screen of thousands of chemicals. Someone at the Department of Defense, actually, was looking for a chemical in the middle of 20th century that would repel mosquitoes.

And they stumbled across DEET. And it’s only recently that we understand how it works. So like I said, there’s a lot of mystery still in mosquito sense of smell and what pushes them away and pulls them towards humans.

IRA FLATOW: And Jake, what would you do with the nonexistent cash?

JAKE TU: Well, we talked about applications we could pursue– so the potential application. Because we still have a long way to go to have something that can be tested in the lab. But I’m also quite interested in the fundamental biology.

So as a scientist, I work towards mosquito control, but also I appreciate the biology of these organisms. And it’s really, really fascinating. So for example, we know nix is the master switch to put or set the embryo onto a path of male development. But how does it work? What does it control? What are the other genes that’s involved?

And then the other question is, how has this evolved within the mosquitoes? And so those are really fascinating questions to me.

IRA FLATOW: And I thank both of you for taking time to talk with me today. Lindy McBride, Assistant Professor in the Department of Ecology and Evolutionary Biology at Princeton University. And Jake Tu, Professor in the Department of Biochemistry at Virginia Tech. Thank you both for enlightening us about mosquitoes.

LINDY MCBRIDE: Thank you so much.

JAKE TU: Thank you.

IRA FLATOW: Lindy McBride leaves us with one pro tip for dealing with the pain of a mosquito bite. After being bitten 2,000 times by mosquitoes, Dr. McBride says that the best treatment you can do for that pain and itch is hot water.

Put hot water– as hot as you can take it– on the mosquito bite. The heat and pain sensors in your brain sort of crossover in the area. So the heat of the hot water cancels out the pain of the bite, so you forget about it. Hope you share that tip with your friends this weekend.

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