07/12/2019

Mosquitoes Have Evolved To Sniff Out Their Prey Of Choice

11:39 minutes

two photos cropped together side by side. on the left is a researcher cradles a guinea pig with brown and white fur. on the right is a mosquito biting human skin, its body filled with blood
(left) Researchers in Lindy McBride’s lab at Princeton University often waft air across guinea pigs to collect their odor for mosquito research. In experiments, mosquitoes are given the choice between the guinea pigs’ odor and human odor to understand how mosquitoes distinguish between humans and other mammals; neither the humans nor the guinea pigs are directly exposed to mosquito stings. (right) Aedes aegypti can spread several diseases as it travels from person to person. Credit: Danielle Alio, Princeton University/U.S. Department of Agriculture/flickr/CC BY 2.0

The human scent is made up of a combination of 100 odor compounds. Not only are the compounds produced by a person but some are from their microbiome. These molecules aren’t unique to humans; other mammals such as guinea pigs also emit the same odor compounds—just in different blends. And even though human odor can also differ from person to person, mosquitoes can still distinguish the scent of a human from other mammals. 

Dr. Carolyn “Lindy” McBride, assistant professor at Princeton University, joins Ira to discuss how mosquitoes have evolved to sniff out humans and how learning what chemicals they are attracted to could lead to new traps or repellents.


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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.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. A bit later in the hour, we’ll be talking with a world-class fermentation expert about experimenting in your home kitchen. What’s the weirdest thing you’ve ever eaten fermented? How did they taste? Call us and tell us. Our number is 844-724-8255, 844-SCI-TALK, or tweet us @scifri. 

But, first, did you know that the human scent is made up of a combination of 100 different odor molecules. Not only are they produced by a person, but some are from their microbiome. These molecules aren’t unique to humans. Other mammals such as guinea pigs also release the same ones, just in different blends. And even though human odor can also differ from person to person, mosquitoes can still distinguish the scent of one human or a human from other mammals. My next guests are going to tell us how mosquitoes have learned to sniff us out– Dr. Carolyn Lindy McBride, an assistant professor at Princeton University. Welcome to Science Friday. 

CAROLYN LINDY MCBRIDE: Thanks, Ira. It’s so fun to be here. 

IRA FLATOW: It’s nice to have you. We’re all familiar with the mosquitoes that bite us every summer. But how many types of mosquitoes are there out there? 

CAROLYN LINDY MCBRIDE: Yeah, there are at least over 3,000 species. There are a lot of different types of mosquitoes out there. 

IRA FLATOW: And how many of them actually drink the human blood? 

CAROLYN LINDY MCBRIDE: Well, a lot of them will bite humans. 

IRA FLATOW: Yeah. 

CAROLYN LINDY MCBRIDE: But what’s really interesting to me is there are few species that really specifically look for humans. They are really specialize in biting humans. That’s rare. 

IRA FLATOW: And your study looks at how mosquitoes differentiate between humans and guinea pigs. Why didn’t you choose– what a combination. 

CAROLYN LINDY MCBRIDE: That’s right. Yeah, a lot of people ask why guinea pigs. Honestly, they’re practically easy. They’re docile. They’re small. They fit in our olfactometer, it’s called– the box we use to test mosquito preference. And so we use them as a representative of many different non-human animals. We always check to make sure our results aren’t just about guinea pigs. They’re more about humans versus any non-human, but guinea pig is sort of our workhorse. 

IRA FLATOW: OK, so how are mosquitoes so good at finding humans? 

CAROLYN LINDY MCBRIDE: So mosquitoes use a lot of different cues to find humans, not just odor, but also our body heat. They respond to visual cues– really, importantly, the CO2 in our breath. But the way that they make choices between different species– I mean all warm-blooded animals at least give off body heat and all animals in general give off CO2– so it’s the specific body odors that help them really differentiate between different species of animals. 

IRA FLATOW: So it’s true we give off 100 different odors? 

CAROLYN LINDY MCBRIDE: Yeah, there are hundreds of compounds in our odor. Not all of them are highly volatile. Many of them are in very small quantities, but it’s extremely complex. 

IRA FLATOW: So mosquitoes sniff the odors and then decides which one it’s going to have a meal with. 

CAROLYN LINDY MCBRIDE: Yeah, that’s right. Most people think the first thing that they detect is CO2– that kind of sensitizes them to other cues. And then they catch a whiff of human– at least these mosquitoes that we study– and they’ll start flying in that direction. 

IRA FLATOW: And how much does human odor differ from, say, a guinea pig’s odor? 

CAROLYN LINDY MCBRIDE: Yeah, animals tend to give off the same types of compounds. Not every human odorant will be in guinea pig odor, but it’ll be found somewhere in the animal kingdom– the odor of some animal. And most of the compounds are shared. 

IRA FLATOW: And how different is the odor from one person to another person? 

CAROLYN LINDY MCBRIDE: Yeah, there’s variability there, too. But the variability between humans is less than the difference between a human and non-human. But there are definitely differences between humans, and humans are differently attractive to mosquitoes. 

IRA FLATOW: How do you collect the human odor when you do your research? 

CAROLYN LINDY MCBRIDE: Yeah, we take– followed a protocol developed by some colleagues in Sweden– but we do a full human body odor extraction– is what we call it. We recruit volunteers, and we ask them to undress and crawl into a big oven bag with their head sticking out. It’s sort of cinched around their neck. And they lie in this bag for two hours. They can watch a movie of their choice. And we basically introduce clean air into the bag. It sort of feels like a gentle breeze. The air is moving really slowly. It exits the bag through eight different holes, and it has to pass through a small filter that traps all the odor. 

IRA FLATOW: And so you trap the odor, and then you feed it to the– I guess to the mosquitoes. 

CAROLYN LINDY MCBRIDE: Yeah, well, exactly. We use it for all different types of things– for behavioral experiments. We use it in neural imaging experiments. We analyze it– do all sorts of things with it. 

IRA FLATOW: Is there one kind of odor that’s more prevalent in humans than any other? 

CAROLYN LINDY MCBRIDE: Yeah, definitely, I can think of the top of my head of two compounds that are particularly abundant in human odor relative to animal odor. One is a compound called lactic acid. And, in fact, lactic acid is the only compound I know of that is slightly attractive to mosquitoes on its own– to these mosquitoes. Another is a compound called sulcatone. It’s a ketone, and it’s not attractive on its own to mosquitoes. 

IRA FLATOW: And so I know that our skins, our bodies, are composed of trillions of microbes. We have a whole big microbiome. How much of our odor comes from our microbiome? 

CAROLYN LINDY MCBRIDE: Yeah, a lot of it– there have been some seminal experiments that show that sweat on its own is slightly attractive. But sweat that’s been incubated on human skin with human microbiome is very attractive. So I don’t think we know precisely how much of human odor is from the microbiome. But it’s very clear that they do something to our odor that makes it very attractive. 

IRA FLATOW: Do the people who make mosquito repellent know all about this stuff? 

CAROLYN LINDY MCBRIDE: Yeah, people who study this– many of those people are aimed at designing repellents or attractants that can pull mosquitoes into lethal traps. 

IRA FLATOW: What about DEET? Why does DEET work so well? 

CAROLYN LINDY MCBRIDE: Well, that is very controversial. And it’s not something that my group has directly studied. But there are a few different theories. One is that DEET scrambles the olfactory codes. So a mosquito is flying along– it’s like, oh, there’s a human. And all of a sudden, DEET is there. And it changes the way all of the neurons in its brain respond to human odor. And suddenly, the mosquito can’t smell the human anymore. 

IRA FLATOW: So you look at the brains of mosquitoes? 

CAROLYN LINDY MCBRIDE: Yeah, we do. 

IRA FLATOW: What did they tell you about scent perception? 

CAROLYN LINDY MCBRIDE: Yeah, well, we’d like to understand exactly how their brains have evolved to make them so good. Again, we’re just talking about Aedes aegypti– this one species of mosquito that really, specifically, targets humans. And we want to know exactly how its brain has changed that allows it to be so sensitive to the way we smell. So we make transgenic mosquitoes that express calcium sensors and neurons, and then we can cut a little window in the head and mount them under a big microscope. And we can basically see the neurons responding to human odor. 

IRA FLATOW: You always hear some people saying, you know, I get bitten by mosquitoes so much more than other people. Is that true or just that perception? 

CAROLYN LINDY MCBRIDE: Well, it’s definitely true that different people attract different numbers of mosquitoes. But what’s interesting is that most of people’s perception of how attractive they are is really driven by how strongly they react to bites. And if you think about it– in an extreme case– a postdoc in my lab, her husband absolutely does not react to mosquito bites. And so he never knows he’s getting bitten. He may actually be quite attractive. And someone who reacts strongly, remembers every single bite. 

IRA FLATOW: That is quite interesting. People may not even realize that– because they don’t react strongly, that they are even bitten by mosquitoes. 

CAROLYN LINDY MCBRIDE: That’s right. That’s right. And so that drives people’s perceptions, but it is true that some people are more attractive than others. 

IRA FLATOW: Would using, let’s say, perfume mask a smell so the mosquito stays away? 

CAROLYN LINDY MCBRIDE: Yeah. Finding a human and getting blood is so important to a mosquito. They are very good at this. Whether use lotion or perfume, there are very few compounds that people have found that actually repel them or prevent them from finding you. So, no, it’s like– some random perfume off the shelf is really not going to help. 

IRA FLATOW: With most scientists studying insects, they’re either trying to protect them or kill them, right? Why were you interested in studying mosquitoes? Were you with the protection or the killing side? 

CAROLYN LINDY MCBRIDE: I was actually neither. I was really interested in how insects evolve, and how they adapt to the things that they eat. And I used to study butterflies adapting to different host plants. And then I heard about this mosquito, and I just thought it was so fascinating how it’s evolved to specialize in, essentially, eating humans. And also it’s so easy to rear in the lab. They’re all of these genetic tools that are available in the species. So, actually, the idea of getting rid of them or preventing them from biting humans was an afterthought initially. Although, now, it’s an important part of what motivates us. 

IRA FLATOW: You’re also looking at odors for other animals. What do you hope to learn from that? 

CAROLYN LINDY MCBRIDE: Yeah, a lot of people have analyzed human odor, and you’ll see studies here and there on the odor of some various animals. But there haven’t been really comprehensive studies of how exactly human odor is unique. Just statistically speaking, what would be the most efficient or logical way to reliably find humans? And for that, we really need to describe what we call as host odor space– the odor of all the different potential hosts a mosquito might bite. 

IRA FLATOW: How many odors do you think there are in total? 1,000? 

CAROLYN LINDY MCBRIDE: Oh, jeez. Oh, yeah. 

IRA FLATOW: Yeah. 

CAROLYN LINDY MCBRIDE: My chemistry is actually quite poor, but yes– thousands, absolutely. Odorants. There’s hundreds of thousands of compounds, or millions. But not all of them can be smelled. Not all of them are volatile. 

IRA FLATOW: Have you ever looked into or thought about looking into changing our odor by maybe what we eat? 

CAROLYN LINDY MCBRIDE: I haven’t looked into that myself. You hear old wives’ tale about if you eat bananas, you’ll be less attracted to mosquitoes. If you eat a lot of garlic, maybe you’ll be less attractive. There haven’t been big well-designed studies to test those things. I’m personally dubious. 

IRA FLATOW: What would you like to study the most? If I had a blank check and– I sometimes ask people, what would you do with it? What would you like to do that you can’t do now? 

CAROLYN LINDY MCBRIDE: Well, I mean, I guess I’m in the lucky position of saying that we are doing the things we really want to do, and that is understanding exactly how the neural circuits in the brain of these mosquitoes have evolved, understanding where this preference for humans evolved. We have one project that’s collecting mosquitoes and testing their preference in various parts of Africa to understand exactly what were the initial drivers that caused the species to specialize in biting humans. And also– 

IRA FLATOW: Yeah. 

CAROLYN LINDY MCBRIDE: Yeah. 

IRA FLATOW: So, wow, well, why do they want to bite humans? I hope you find out and come back and tell us, OK? 

CAROLYN LINDY MCBRIDE: Yeah, we’ll try. 

IRA FLATOW: All right. Dr. Carolyn Lindy McBride, assistant professor at Princeton University.

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