Natural Mosquito Repellents
Chris - Tell us about your research. How did you actually get into this and what have you found?
John - Well there are a lot of stories about people being differently attractive to mosquitoes and other nuisance flies. The perceived wisdom is that the people who aren't attacked lack some attractant material. In fact what we've found is quite the opposite. People who aren't attacked produce some extra chemistry and put off the mosquitoes and other flies. The way we found this was to look first of all at the emanations that come out of people's skin. We chose a number of volunteers and looked at the way the mosquitoes would fly towards their arms in a device called a wind tunnel.
Chris - Presumably you had to pay people quite a lot to take part in this research!
John - No, they volunteered, and we didn't actually allow them to be attacked by the insects. We stopped them before they reached the arms so we could count them as they approached. What we did then was to take the volatile chemicals coming out of the skin of the volunteers. We did this by putting the volunteers into a kind of bag, which we kept a positive pressure into, so we could draw air out of it and take out the chemicals that the volunteers were putting into the air. By that means we could show that the same people that were not attacked by the mosquitoes also produced a range of chemicals that had the same effect.
Chris - How did you actually identify what the chemicals were?
John - Well that's even more clever. We use a lot of chemical analysis in this kind of work, but we can also use the insect itself. We can put small microelectrodes into the antennae of the mosquito and find out which chemicals in the complicated mixture given off by the skin were actually repelling the mosquitoes.
Chris - So essentially you attach a wire onto a nerve fibre coming from the antennae and record from that when you present different chemicals to the mosquito and see what the response is electrically.
John - Yes. We can either record from individual neurons or we can record from the whole antenna. In fact, we used a preparation that used the whole antenna in this work.
Chris - And if a mosquito is particularly put off by a smell, does you still get a positive response from the antennae or does it switch off?
John - The antennae is just a recording device. It's the central nervous system in the animal that decides whether an animal is going to approach the source of the material, or whether it goes away. What we've done is looked at the chemicals that are produced by those people who are attractive, and then we've tried to find the extra ones that the mosquitoes respond to from people who are not attractive. These have then been tried out in this wind tunnel device to see if they will put mosquitoes off from going to the attractive people's hands. That works.
Chris - So you can confer unattractiveness on individuals by decorating them with this smell.
John - Yes, and in the field. With help from our colleagues at the University of Aberdeen where they have the Scottish biting midge, we've shown that you can reduce almost to nothing the normal attacks that you get from the Scottish biting midge.
Chris - Why do some people have these chemicals, but not others? You would have thought that it would be a huge advantage if you think of places like Africa where they have malaria. If you have gene that means that you have components in your sweat that put mosquitoes off, it would be so advantageous that everyone should have it.
John - Yes that's right. The close relationship between some animals and these biting flies have meant that the mosquitoes are always keeping up with how we try to evolve away from them. But the kind of chemistry we're seeing sees to relate to stress. Of course, mosquitoes have to put in quite an investment to get a blood meal from somebody. They have to inject I chemicals that dilate the blood vessels and stop the blood coagulating. That's quite an expense for the mosquito, so they're very keen not to go to somebody who's stressed. We think that might be the reason for it, but we're not certain that this is true at the moment.
Kat - Are these pheromones going to be commercially available?
John - Well it's creating a lot of interest at the moment. We've been able to use one of the main commercial repellents, deet, in our comparative work in Scotland and we can certainly beat that in the short term. So there's a lot of interest. What we're doing at the moment is writing up the work and patenting it. That will of course facilitate commercial development.