Feel the heat: mosquitoes sense warmth

What's the most dangerous animal in the world? Most people immediately picture snakes, sharks or venemous spiders, but the real answer is something much smaller and easy to overlook: it's the mosquito. This insect group is singly responsible for millions of human deaths every year because, when they bite us, they spread infectious diseases like malaria, dengue and yellow fever. But how do they find us in the first place? Smell plays a big part, but so does temperature, as Leslie Vosshall explains to Chris Smith...

Leslie - The day job of the female mosquito is to go out and find as many humans as possible and bite them. And so, part of that is being attracted to the warm skin that humans have. So, we've discovered the function of a single gene is required for these animals to go after specifically human body heat and not overshoot and go toward things that are inappropriately hot.

Chris - How do they feel the heat? Is that through their feet, antennae, other body parts? Do you know?

Leslie - That's a really good question. So we don't know and that's something that definitely needs to be resolved in the future so we think at a minimum at the very tips of their antennae, they have cells that can sort of feel the heat as they're flying around. So, those cells are sensitive to heat.

Chris - And what are those cells doing to actually detect temperature?

Leslie - For an insect to figure out whether it's coming close to a warm human, it needs to be able to have a protein that will respond to heat. And so, the particular protein that we've been studying is called TRPA1. So, it's a protein that when you heat it up, it becomes activated so it changes its shape. It causes ions to flow into the cell and bottom line, it makes the neuron very excited and that excitement is then transferred to the brain.

Chris - How did you then work out what role that was playing in mosquitoes tracking down hot bodies?

Leslie - The first thing that we did was go in. My student Roman Corfas who is responsible for the entire study. He first really did a detailed analysis of what exactly are mosquitoes doing when there's something hot. So he figured out exactly how sensitive they are to heat. So just 2.5 degrees Celsius above room temperature, they already know there's something interesting and they fly toward it. And then when the surface gets hotter than human skin, a normal mosquito will avoid it. They don't like things that are too hot. He then used these amazing new techniques to delete a single gene. Now we have a mutant mosquito that's missing TRPA1 and we ask, "What does it do?" So, our TRPA1 mutants, they still really love humans and they still will fly toward warm surfaces. But when that surface gets superhot, they don't care. They like it. They like it more and they hang around until you make it crazy hot and then they leave. From that, we assume that TRPA1 now is a major player in telling the mosquito, "Okay, stop. It's too hot. Go away!"

Chris - At the same time though they still fancied the hot surface which could be a warm body, didn't they? So there must be something else going on as well that enables them to detect lower levels of heat. So you found a thing that repels them from very hot surfaces and helps them make the discrimination but it's not the whole story.

Leslie - That's exactly right. So there's two halves to the story. We've solved the second ones. So the first mysterious part, how do they recognise that there's something a little bit warmer than the ambient air and to detect that and like it, and fly toward it? We are continuing to look actively for what that magic molecule is. That's a very important piece of the puzzle for how mosquitoes become attracted to humans. So, we found the other part of the puzzle: How do they have like going to things that are warmer than human skin? Also extremely important because if they get distracted by things that are too hot, it would make them inefficient hunters of humans.

Chris - And you could potentially exploit both of those in order to work out ways to deter them from biting humans. 

Leslie - Absolutely, and so everybody in the whole world particularly my relatives at the holidays ask have we figured out a way to stop humans from being bitten. So it's a big imperative for every human on Earth. And so, knowing what the strategies are by which mosquitoes use temperature to find humans is really a useful piece of information. And so, a lot of the traps that are being designed to trap out mosquitoes are starting to play with these parameters to mimic humans so heat, carbon dioxide from the breath, stinky human odour. Our discovery indicates that you have to be very careful in tuning the temperature of these traps.