Itchy mosquito bites boost infection

The itchiness of a mosquito bite isn't just an aggravation: scientists have discovered that the inflammation that makes you want to scratch, dramatically boosts the infection rate for viruses like Zika, dengue and yellow fever, that the insects can carry. Clive McKimmie is at the University of Leeds, and explained his research to Chris Smith...

Clive - When mosquitoes bite you, what they're doing is they're trying to take a blood meal from your skin but, in the process, are spitting out saliva into your skin. This rather disgusting to think about, but this saliva actually contains a lot of disease causing viruses and some of these are quite well known such as the zika virus, dengue virus, and chikungunya and together that infects maybe several hundred million people each year. What we do know is that the mosquito bite somehow tends to be helping virus infection and giving it a boost.

Chris - So, over and above the physical fact you've got this flying hypodermic needle that comes along and injects you with virus particles, there is an effect in addition to just putting the virus into you whereby the mosquito increases your likelihood of catching whatever it happens to be carrying itself?

Clive - That's it. We actually know very little really about what happens during the early stages of infection, and what we've done in our recent paper is to show how -  what we call the mechanistic basis - by which the mosquito bite really seems to help the virus infection along.

Chris - And how does it, what does it do?

Clive - There's two things going in here. Firstly you have a bite and I think anyone who's been bitten by a mosquito knows what that's like. You get a horrible red swelling, there's some what we call inflammation. What your body is doing actually is perfectly normal. Whenever you get injured or cut, what happens is your immune cells, these are the cells that help to defend your body against infection, they rush to the site of the damage which is the mosquito bite in this case and they're trying to stop any infection that's there, but actually what happens is quite strange. These immune cells that we call leukocytes actually seem to get infected by the virus if it's present, and the virus takes over these cells and uses to replicate so there's more and more copies of the virus in your skin. So if you can stop those cells coming into the bite site, then you can also stop the virus from getting that extra boost.

Chris - Does this mean then we might have a new way of arresting the spread of some of these viral agents so that you might, for instance, have something that you could rub on which would cut down the risk of a virus infecting you via that route?

Clive - Well, we've certainly shown in the laboratory setting where everything's very well controlled that if you can stop that bite inflammation, then you can stop the virus from causing disease. Now the next step, obviously, is to work out how best we can do that before we could even begin to imagine doing any studies in humans. Because I think we have to be very careful about any form of immune suppression because that can actually be quite dangerous, even if it is a topical cream. We're very keen now that the next step is to say can we use this knowledge to stop the virus from spreading by targeting the bite inflammation, and that's particularly exciting because it's common to a lot of these infections. Whether it's zika or dengue, they're all transmitted by the same mosquito into the bite site.

Chris - That was going to be my next question which is, is this generalisable? Viruses spread by mosquitoes and there are many types of them, do they all exploit this mechanism?

Clive - Well we've looked at two very genetically distinct viruses. These are viruses that have nothing in common with each other, but what they do have in common is they're spread by the same mosquito. We showed that in this paper that this mechanism, this way by which the bite boosts the infection, does seem to be the same. And there's actually a paper out just this week from another group in America which is working on dengue, and they find something similar where mosquito saliva is boosting the dengue infection.

Chris - Is there any evidence that, in turn, the virus is manipulating the mosquito for instance, to make it's saliva more inflammatory so that the mosquito makes your bites itch more, so more immune cells are effectively coming and, therefore, you increase even further the likelihood you'll pick up that infection?

Clive - Well that sounds like an excellent question. I don't think there's any data out on that just yet. There is some data which shows that viruses do change what genes and proteins are made by the mosquito but we have yet to look whether that's actually directly related to how itchy or how inflamed a bit can get, but it's certainly a very good question.

Chris - Clive McKimmie, and that discovery was published in the journal Immunity.