Researchers Make A Stink To Fight Mosquitoes

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When they're hungry for blood, mosquitoes can sniff out a human meal. Some researchers are using that sense of smell against the pests. University of California Riverside entomologist Anandasankar Ray explains how odors might one day be used to repel and trap mosquitoes.

IRA FLATOW, host:

This is SCIENCE FRIDAY. I'm Ira Flatow.

Summer has many wonderful sounds, but this is not one of them.

(Soundbite of buzzing mosquitoes)

FLATOW: Ah, yes. I thought I got rid of that guy at the beginning of the show. Mosquitoes back with a friend.

(Soundbite of laughter)

FLATOW: Yeah. Stalking us at campfires and picnics and snacking on our blood and leaving behind that itchy welt. But while they are a nuisance here, for much of the rest of the world, mosquitoes are more than a backyard pest, they are a serious health threat. Around the world, hundreds of thousands of people die every year from mosquito-borne diseases, including malaria.

Our best defense against mosquitoes are repellants that have to be applied to the skin or you have your low-tech solutions like mosquito netting. But now, some researchers are trying a new approach, using the mosquito's own sense of smell, the one it uses to find us, against it.

Joining us now to talk about this new kind of chemical warfare is Dr. Anandasankar Ray, an assistant professor of entomology at the University of California, Riverside. He joins us from a studio on the campus.

Welcome, Dr. Ray, to SCIENCE FRIDAY.

Dr. ANANDASANKAR RAY (Assistant Professor of Entomology, University of California, Riverside): Thank you very much for having me online.

FLATOW: Tell us about - how - let's go to the ABCs of mosquitoes first. How do they find us?

Dr. RAY: So a female mosquito, when it's hungry and flying around can smell us from a far-off distance. And the most important odor that we emit that mosquitoes can sense from a distance is carbon dioxide. This is a compound, a gas, which is present in our exhaled air.

So we exhale copious amounts of carbon dioxide, which is carried by the wind many, many meters away. And this female hungry mosquito smells it and starts flying upwind, just as you would see a lion, you know, sensing a deer from a distance and moving upwind. In the same way, this female mosquito will track the carbon dioxide plume that is carried by the wind from our nose and mouth and come closer and closer to us.

And when it's really close, it can start sensing odors from our skin, from our sweat, and it can also sense the heat emitted by our body. And then, it becomes like a guided missile and hones in on us and then takes a blood meal.

FLATOW: So it's got multisensors then?

Dr. RAY: Oh, yes, absolutely. It's a very sophisticated sensing system, and this allows the mosquito - and this is true for other blood-feeding flying insects as well. It allows the mosquito to be able to hone in on us or its other hosts.

FLATOW: Mm-hmm. And you have decided that, well, one of the best ways to fight the mosquito is to turn it upon itself.

Dr. RAY: Yes, absolutely. So what we thought was if there was a simple way to confuse the mosquito by blocking its ability to smell us, to be able to smell us out from a distance, then perhaps, we could create some kind of an invisibility cloak or some kind of invisible net of odors that would mask human beings from the mosquito or rather the mosquito will be unable to smell us from a distance.

So we decided to go after carbon dioxide because years and years of study have shown that the most significant and the most important attractive odor from us is carbon dioxide. So we thought perhaps if we could find ways to fool the carbon dioxide sensor in the mosquito's nose, we may be able to confuse them and be able to completely disrupt their attraction behavior towards us. And that's exactly what we've done in this study that was recently published.

FLATOW: Mm-hmm. 1-800-989-8255 is our number if you'd like to talk about mosquitoes, everything you wanted to know about mosquitoes and mosquito repellents. What are the main repellants used today? I know people use DEET, right? That...

Dr. RAY: Yes. Absolutely. DEET has been the gold standard in repellency for many, many years now. And recently, there have been other compounds like piperidine, which is present in the market.

Now, one of the main problems with these types of repellants, in particular in being able to be widely used in the tropical countries and also actually here in the United States or in Western Europe, is that you have to apply copious amounts of it directly onto your skin. And believe it or not, to us, it might sound affordable, but it's out of reach for the common people in tropical areas because of the amount that you need to lather on.

The other thing is, you have to cover every part of your body. You have to cover your left arm, your right arm, your neck, your face, which makes it a little less convenient. And also, in a way, you know, people really don't like to - the lesser contact we have with chemicals, the better it is.

So our strategy is to try - and so, therefore, there are two major challenges in developing a repellent strategy that can be useful globally in the fight against malaria or the fight against dengue. And one, cost, so something needs to be affordable. And two, is not applying it directly onto the skin, so something that may be able to protect a large number of individuals in an area.

And so, these two factors have been a real motivation for us. Because what we have been trying to do is use odor molecules - now, these are scents that can be effective in very, very small quantities and to disrupt behavior. These are very simple chemicals. And potentially, when we get to something that - to an application, we would have safe chemicals that are environmentally safe, extremely affordable, useful in small quantities and most importantly, something which we would try to design that does not need to be applied directly to the skin. So something that can protect a large number of individuals.

FLATOW: So this would be an odor? What does it smell like? I'm sure you're experimenting with it.

Dr. RAY: So we have three classes of odors actually. So it's not just one strategy that we've designed. We've designed two alternative strategies using these three classes of odors. So the first class of odor, it's very simple. These are chemicals that mimic carbon dioxide. And as I just mentioned, carbon dioxide is such a good lure and attractant that we feel that these compounds that mimic - these odors that mimic carbon dioxide can be used as lures to create cheap traps. These cheap traps would potentially be useful in developing countries, as well as in our backyards and barbecues and wherever else to clear out mosquitoes from an environment.

Now, mosquito traps have been around for many years. But, again, the problem is, in order to generate carbon dioxide, which is the lure for most of these traps, you need to either burn a tank of propane to - or get dry ice, which is basically frozen carbon dioxide. These are expensive and so the traps round into hundreds of dollars.

If we can replace the lure with the mimic odor of carbon dioxide, we could probably have an evaporating lure, something which could simply evaporate and attract mosquitoes to it. So that's the first strategy of using a mimicking odor to use as a cheap and effective trapping solution. The second...

FLATOW: Do you have one of those that works?

Dr. RAY: We do have a number of odors now that can activate the carbon dioxide sensor in the mosquito's nose in the same fashion that carbon dioxide does. And we are testing them at the moment to see whether they are - which one of those are safest and can cause the maximum attraction. So hopefully, within a short of period of time, within a few years, we should be able to get to a safe and effective solution for this approach.

FLATOW: Mm-hmm.

Dr. RAY: Now, the second approach is to mask the mosquito's ability to smell us in a large area, like a backyard. Or if it were in Africa or Asia, it would be protecting an entire abode, like a hut. These would be chemicals that can block the carbon dioxide sensor so the mosquito nose would not be able to detect us from a distance. And these would be evaporated in an area, like a backyard, and create a plume or a protective cover around us.

Now, these odors are really interesting. They come in two flavors. One, inhibitors, so they outright block the carbon dioxide receptor. And the second, are blinders. These are - this was really unexpected, actually, which is very new in our study. These blinder class of chemicals, what they do is they activate the sensor in the mosquito's nose that detects carbon dioxide so strongly and for such a long period of time, it's almost as if the mosquito is blinded with activity and is unable to detect from which direction carbon dioxide is coming.

In another word, it would be sooner if we had white light all around us, really bright, white light. We wouldn't be able to find the direction of light at the end of a tunnel. So for a mosquito, the ability to smell carbon dioxide is just so fundamentally important that by blinding that sensation the mosquitoes are completely confused and are unable to find the source of carbon dioxide. So these compounds we envision will be evaporated or can be applied through a sprays, perhaps even through long releases - long-term release through paints.

Some of the chemicals that we report in the study, that came out in Nature from my lab at UC Riverside, do not have the perfect characteristics yet, but they provide a very important first step into...

FLATOW: And so, how soon is it going to be before we get these...

Dr. RAY: So that's...

FLATOW: ...on the market or...

Dr. RAY: So we have been...

FLATOW: I go to Zig's(ph) Sporting Goods, I'm not finding this on the shelf there, right?

Dr. RAY: No, not yet. Not yet. So this is very cutting edge. So what we've done is we've taken an approach that pharmaceutical companies have been doing for drug discovery for a while. We've taken a target, the carbon dioxide receptor, which is the most important sensor for mosquitoes. And now, we have shown, that as a proof of principle, that we can block its activity or blind its activity, and we can get complete behavior disruption inside a semi-field situation where they are attracted to a hut or not or, you know, wind tunnel, flight tunnel.

So this proof of principle allows us to do two things: One, show that it will work, this approach will work; and second, the structures of the chemicals that we have discovered enable us to apply very, very quick methods of chemical informatics, where we look at the chemical structure and design something better.

And we have already made a lot of progress down that road. We are continuing this progress in two different avenues, actually. We have the basic research going on in my lab, which is funded by the National Institutes of Health. All of this started with the Gates Foundation, actually, supporting us with some initial grant funding...

FLATOW: Dr. Ray, I'm going to have to interrupt because we've run out of time. But I'm glad - we're going to follow you on your research, and good luck to you.

Dr. RAY: Thank you very much. I would say in five years time, expect something on the shelves.

FLATOW: There you go. Dr. Anandasankar Ray is an assistant professor of entomology at the University of California at Riverside. I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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