DEET, the active ingredient in many bug repellents, doesn’t only cripple mosquitoes — it also meddles with mammals. A new study examining DEET’s effects on insects, mice and human proteins reports that the chemical interferes with a prominent central nervous system enzyme. This effect is magnified when exposure to DEET is combined with exposure to certain pesticides, researchers report online August 4 in BMC Biology.

The results are consistent with previous studies, says Bahie Abou-Donia of the Duke University Medical Center in Durham, N.C, who was not involved in the new work.

“DEET is a good chemical for protection against insects,” he says. “But prolonged exposure results in neurological damage, and this is enhanced by other chemicals and medications.”

Led by Vincent Corbel of the Institut de Recherche pour le Développement in Montpellier and Bruno Lapied of the University of Angers in France, the researchers examined DEET’s effects on mosquitoes, cockroach nerves, mouse muscles, and enzymes purified from fruit flies and humans. Applications of DEET slowed or halted the actions of the enzyme acetylcholinesterase. This enzyme hangs out between nerve and muscle cells, breaking down a messenger molecule after it has passed information from one cell to another. If this messenger isn’t properly recycled, it can build up and lead to paralysis.

Certain pesticides are designed to shut down this enzyme in insects, which may explain DEET’s enhanced toxicity when used by someone already exposed to these chemicals.

Abou-Donia says that these effects should be clearly labeled on products containing DEET, or N,N-diethyl-3-methylbenzamide. He notes that in Canada, insect repellents can contain no more than 30 percent DEET. The United States — where 100 percent DEET repellents are available — should consider such restrictions, he says.

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