By Will Dunham
WASHINGTON, March 13 (Reuters) - The bug repellent DEET works by making mosquitoes and their brethren unable to smell the sweet aroma of human sweat that alerts them that a meal of blood is nearby, scientists said on Thursday.
This knowledge may help guide the creation of new repellents based on the same principle but without possible health worries, they said.
The chemical was first produced by government scientists in 1946 after the jungle warfare of World War II hammered home the need to prevent mosquito and other insect bites. It has been available to the public since 1957.
It is the world’s most widely-used topical agent against mosquitoes and other blood-eating insects. It repels mosquitoes which spread malaria, a leading killer in parts of the developing world, as well as ticks which spread Lyme disease. But it does not kill them.
Until now scientists had not grasped exactly how it works.
Mosquitoes are crazy for two human scents: sweaty body odor and the carbon dioxide in breath. They use different receptors in their olfactory system — the sensing system for smell — to detect these odors to locate their prey.
"Humans are very smelly, so there’s a large number of compounds that attract mosquitoes," Leslie Vosshall, head of Rockefeller University’s Laboratory of Neurogenetics and Behavior, said in a telephone interview.
In experiments on mosquitoes and fruit flies, her team determined that DEET serves sort of a chemical cloak, masking certain human odors.
"You become invisible to them," Vosshall said.
The scientists tracked the electrical activity of cells in the mosquito olfactory system while exposing them to DEET. They found that it blocked bugs from smelling body odor but not the carbon dioxide in breath.
Without the scent of body odor, the bugs cannot figure out that a succulent person is nearby, the scientists said.
"Lots of things out in nature will release carbon dioxide — swamps, for example — and that’s not a good thing to go after if you’re a mosquito," said Vosshall, whose findings were published in the journal Science.
The U.S. Environmental Protection Agency said about a third of the U.S. population is expected to use DEET annually. The agency said about 140 products containing DEET are registered with the EPA by about 39 different companies.
A safety review concluded in 1998 that insect repellents containing DEET are not a health threat, but DEET should not be used on children under two months old, the EPA said.
"There are problems with DEET," Vosshall said. "It’s oily, it melts plastic, you can’t use it on babies, you have to reapply it on every piece of exposed skin frequently. It does actually penetrate your skin and enter your blood stream."
She said there have been rare cases in which people had seizures after putting on too large a dose.
(Editing by Alan Elsner and Maggie Fox)