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Mosquito fickleness explained at last

There are people—I seem to be one of them—who attract mosquitoes like magnets attract iron filings. And then there are people who claim never to be bothered by them at all.

It’s a phenomenon that has long been recognized but never understood—until now.

British scientists from Rothamsted Research in Harpenden and the University of Aberdeen have discovered that the difference between the mosquito-attractive and the mosquito-ignored lies in the smell of their sweat.

The research was led by James Logan. He built a Y-shaped chamber and had volunteers place their hands at the ends, then released mosquitoes (of the yellow fever-carrying species Aedes aegypti) into the chamber. They flew down the branch of the Y that led to the human they preferred.

He then analyzed the body odour of the people who failed to attract mosquitoes by sealing them up in a foil sack tied under the chin, then collecting and distilling their resulting sweat.

To find out which chemicals in the sweat repelled mosquitoes, the researchers strapped miniature electrodes to the mosquitoes’ antennae (which sounds like almost as much fun as being stuffed into a foil sack) and checked their reactions to various compounds.

What they found was that, while everyone produces mosquito-attractive smelly chemicals in their sweat (e.g., lactic acid), other chemicals found in sweat repel mosquitoes—and some people produce enough of these mosquito-repellant chemicals to mask the mosquito-attractive ones.

The research team hopes that these newly identified chemicals may point the way to better and safer insect repellants.

Currently, of course, the active ingredient in almost all insect repellants—the effective ones, anyway–is DEET: N, N-diethyl-m-toluamide.

DEET was first synthesized in the 1950s, and insects hate it. It repels a variety of mosquitoes, chiggers, ticks, fleas and biting flies (but not the stinging kind—no skin-applied repellant is effective against wasps, bees and hornets).

The concentration of DEET in insect repellents varies. The U.S. Army uses a mixture of 75-percent DEET in ethanol, ansd some commercial products that are even stronger. Deep Woods Off!, for instance, is 95 percent DEET, and Muskol is 100 percent DEET. (Pure DEET is unpleasantly oily, however, and lower concentrations work just about as well under most circumstances.)

Millions of people use DEET every year with no problems, but a few have unpleasant reactions to it. The Journal of Toxicology and Environmental Health estimates that as much of 56 percent of all DEET applied to the skin is rapidly absorbed and enters the bloodstream; 10 to 15 percent of each dose can be recovered from the urine. The rest remains in the body for up to two months. Because of this, even people who aren’t sensitive to the chemical should use it only lightly, and wash it off promptly once they’ve returned indoors.

Children seem to be particularly prone to adverse reactions to DEET, probably because of their small size, so only products containing lower concentrations of DEET should be used on children, and then only sparingly.

One way to minimize skin exposure to DEET is to spray clothing instead. Unfortunately, DEET can damage spandex, rayon and acetate, as well as the plastic in sunglasses and vinyl car seats.

These problems mean that a replacement for DEET is potentially of huge commercial value.

Which is why Logan isn’t divulging the names of the chemicals his team identified in sweat until they can be patented. However, for what it’s worth, Logan has indicated that in concentrated form the chemicals, normally undetectable by the human nose, have a fruity smell.

He also says, in a New Scientist online article about his research, that one of the key repellent chemicals identified is “a natural food additive, so has proven safety,” and notes, “because it can be made by plants, it may one day be possible to mass produce it cheaply.

That key chemical is currently being tested as a repellent in Africa in a study involving 16 volunteers. The researchers are also testing the repellent against other biting insects. They’ve already found that it also works against a kind of biting midge that makes people miserable along the west coast of Scotland. They’re hopeful it will also work against the species of mosquito that carry malaria.

But until it becomes a commercial product, those of us unlucky enough to lack our own built-in mosquito-repellant chemicals will have to make do with DEET.

The fault lies not in our stars, but in our sweat, that we are pincushions.

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1 comment

    • Nicole on July 4, 2006 at 8:06 pm
    • Reply

    Well, I must be pretty tasty, because they love to bite me! I look forward to a new product, but for now, No thanks to West Nile, and bring on the deet.
    Hop you had a happy Canada Day!

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