Why mosquitoes prefer some people to others

Scientists think they may have found out why mosquitoes bite some people - but leave others alone. Apparently, it’s all about the type of chemicals that are excreted by our bodies. The discovery was made by scientists at Johns Hopkins University, who built a large open-air arena in Zambia as part of their study. The architect of the project is Conor McMeniman.

Conor - We were really excited to develop a new system to decode what molecules emitted in our skin, odour, and breath are very attractive or perhaps not so attractive to the mosquito that transmits malaria throughout Africa.

Chris - I thought this had been looked at in some detail in decades past. Experiments where people were put in bubbles and all the smells that come off them were taken away and then presented to hungry mosquitoes to see what they did.

Conor - Yeah. There's been a wide range of studies conducted over the past hundred years or so. Most of these have been performed actually at small scales in the laboratory and in this study we built something much, much larger in a malaria endemic region of Africa in Zambia.

Chris - So this is real world data. It's the mosquitoes and the people that would normally interact.

Conor - Yeah. So to do this, we actually built an assay 2000 times larger than existing laboratory assays. This is about the size if we think about the area of two center courts in a tennis arena, basically a massive greenhouse facility. The arena itself is actually exposed to the elements, so wind currents can flow for this arena. The mosquitoes which we place inside are actually exposed to natural fluctuations in temperature and also humidity. And the foresight of this massive greenhouse, we have positioned 15 metres away, one person tents. And in these tents we can actually place individual humans. We actually connect these one person tents to the greenhouse by air conditioning ducting. Humans who occupy the tents fill the atmosphere of that tent with all of their scent, this rich bouquet of around 300 different chemicals that are released from the skin and the breath into the air. And then once it gets into the field cage, we have hot plates warm to human skin temperature. And on top of the hot plate is actually an infrared camera, which monitors the landing of mosquitoes on this hot plate. The scent from those tents is actually wafted over the surface of this hot plate, mimicking human skin. We simply then record throughout the nighttime period how many mosquitoes land on these hot plates. And each of these tents will have individual humans. We can compete up to six humans' scent profiles against each other all at once.

Chris - And are you taking snapshots of what those smells are coming from each of the individual humans? So you've got samples you can then analyse?

Conor - Yeah, that's exactly what we did. So concurrently we also actually sample the air from that ducting path to gain a snapshot of what chemicals are being emitted in the scent signature of that individual. And we can quantify the number of mosquitoes that really love that scent or dislike that scent by recording the number of landings. By analyzing that chemical sample, we can actually discern what the chemical constitution of the human scent signature is that actually is associated with being highly attractive or not so attractive at all.

Chris - Are there push pull factors here? Are some people just really attractive to mosquitoes, others less so? Or are there people at the other end of the spectrum that are actively repugnant, at least in small terms, to a mosquito?

Conor - Right. During the initial validation of this system, we were really excited to see that, night after night, we found individuals that were highly attractive to mosquitoes. In particular, we found one individual that was a mosquito magnet. On the other end of the spectrum, we found an individual that really had a radically different scent signature to the other humans. And this individual wasn't very frequently targeted by mosquitoes.

Chris - What about the fact that we know that certain human conditions may also predispose people to being more attractive to mosquitoes? Malaria itself might do that. Pregnancy also allegedly does this.

Conor - The comparative power of this system, the ability to test larger groups of humans all at the same time, will really allow us to start to test some of these anecdotes and also observations from scientific data from the field that has been previously observed. It's really worth noting that pregnant women are at a high risk of sickness from infection with malaria. So that would be a really important one to test. Also, whether your blood type correlates with how attractive you are to mosquitoes or other sort of anecdotes that get thrown around, for instance, my blood is sweeter than another person's and therefore I'm more attractive to mosquitoes. We can actually now test whether the blood of individuals that seem to be more highly attractive might have higher levels of sugars or other metabolites in their blood.

Chris - And presumably it's an opportunity to test novel ways to repel mosquitoes as well.

Conor - Yeah, absolutely. I think this system has the potential to reveal new components of human scent that might be produced by the human body naturally or alternatively acquired in our diet, for instance, that might make us less attractive to mosquitoes. By identifying what those molecules are, perhaps we could find ways to add them to existing repellent formulations that we currently apply to our skin and buy from the shops that could enhance the activity and efficacy of those particular products. So I think this system also provides a really great way to test repellants and benchmark their activity and efficacy in an exposure free and risk free way for humans. Everyone hates being bitten by mosquitoes.