What makes some people tastier to mosquitoes?

23 April 2018
Presented by Chris Smith.

Every year millions of people contract malaria, which is a blood parasite infection spread by mosquitoes. And part of the reason why the infection spreads so successfully, scientists now know, is because the parasite makes an infected human over-produce certain skin odours that are irresistible to a mosquito. Ailie Robinson, from the London School of Hygiene and Tropical Medicine, is the lead author on the new study that's uncovered how this happens, as Chris Smith found out…

Ailie - Well, there's been quite a few studies that show that people who are infected become more attractive, both in animal models of malaria and also in human malaria; a few studies have thought about the mechanism, but nobody's looked at skin odour as a mechanism for that.
 
Chris - Why should - from an evolutionary point of view - it be advantageous to, say, a parasite or a disease to make a victim more attractive to a biting insect?
 
Ailie - The malaria parasite passes from a mosquito to a person and back to the mosquito. Ultimately, it always just wants to get through to the next stage of its life cycle and while it's in the human the next stage involves passing through to the mosquito. So if you can attract more mosquito vectors to the human, then the chances are you'll increase the frequency of mosquito-human contacts, so more mosquitoes will bite that person. Then there is a possibility that those parasites will be passed back to the mosquito.
 
Chris - So how did you put this to the test to find out if it's a real phenomenon?
 
Ailie - Yeah. So the first thing we did is to confirm the phenomenon that infected people are more attractive. So we took children who had malaria and children who didn't have malaria and we sampled their skin odour using the sock method. So we had all of the children wear a sock for 20 hours; and then those who had malaria we treated with anti-malarials; then, 20 days later, we did the same process - so we gave them a sock to wear for 20 hours - and then we tested the two socks together...
 
Chris - So, the sock they wore when they were infected and then the sock they wore once you'd cured them?
 
Ailie - Exactly, or, for the negative individuals just the before and after sock: there was never a parasite.  And we tested those in what we call the duel choice chamber - so you have a sock in each chamber and you put mosquitoes in the middle and you see where the mosquitoes go; and we confirmed previous findings that the infected ones were more attractive, because in the children who had malaria it was the sock from the malaria timepoint that was more attractive; whereas in the children who had never had malaria then mosquitoes went evenly to either sock...
 
Chris - So there's some chemical losing out of the foot and into the sock - when an individual is carrying malaria, which is attractive to mosquitoes because that's why they make a bee-line, if that's the right phrase to use, for the sock that they wore when they were infected; that tells you there must be some smell, because there has to be an odour or some volatile chemical that was in the foot odour or their sweat?
 
Ailie - Exactly. And I think that was one thing that was a bit different about that study is that we've just looked at the skin odour; so we used a sock. Wherea previous studies which had done that work had looked at the whole body odour, which includes breath. So we'd isolated the phenomenon down to skin odour and that's what prompted the next series of experiments where, again, we sample skin odor in it but in a different way and then tested the chemicals in that odour.
 
Chris - And you get the same result that there is something about an infected individual that they are producing something on their skin which lures in mosquitoes?
 
Ailie - Yes. So we did a chemical analysis of all the chemicals inside the odour samples, and we found actually that it wasn't that there was novel chemicals in the infected odour profiles. It was the same chemicals that were present in all the different infected and uninfected but certain chemicals were produced in greater quantities by those who were infected firstly and secondly which was really exciting is that some of those chemicals were produced more highly by individuals who had more parasites. So that really tied in the phenomenon with the parasite itself.
 
Chris - Now is the parasite making those additional chemicals which then who's out in the sweat, or is the parasite making the person make more of those chemicals do you know?
 
Ailie - That's a very good question and no we don't know the answer to that. There is actually kind of evidence to suggest both.
 
Chris - And what are the implications of what you found?
 
Ailie - Well we found that the chemicals over-produced by people who have malaria are attractive to mosquitoes and one direct application would be to use those chemicals to improve synthetic lures. We already have lures which mimic the odour of a healthy person so there are attractive chemical lures and we use these to trap mosquitoes. And actually you can trap mosquitoes at a high enough level so that it reduces the population and actually reduces the prevalence of disease. So that's got a really important application. Another thing we could do is potentially use these chemicals as biomarkers for malaria. So we might be able to detect a person who has malaria due to overproduction of those compounds...

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