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Lice Resistant to Common Treatments

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WebMD Health News

April 6, 2000 (Washington) -- Anguished parents who pour head lice treatments on their kids only to find live critters a few minutes or hours later already have experienced what scientists in Massachusetts have proven: Many lice have mutated and are now resistant to permethrin, the main chemical ingredient in the most popular lice products.

And in the same way that super-bacteria are invincible to common antibiotics, the lowly louse may ultimately outwit all efforts to kill it, unless smarter -- and perhaps simpler -- strategies are found.

"We are just beginning to get a picture of how widespread [permethrin] resistance is," J. Marshall Clark, PhD, tells WebMD. "Clearly the resistance is there and it is established and it is a real problem." Clark was the lead investigator of the study, published in a recent pesticide journal. He is the director of the Massachusetts Pesticide Analysis Laboratory and an entomology professor at the University of Massachusetts, both in Amherst.

Clark and his team collected live head and body lice from individuals in Israel and Panama, and from school children in England, Massachusetts, and Florida. Lice were housed in incubators or kept free on volunteers, and were fed four to five times daily on humans.

Nymphs and mature lice were exposed to concentrations of permethrin equivalent to those found in over-the-counter lice products, such as Nix. They measured the time it took for the lice to die, and used DNA testing to examine the lice genes. They found two gene mutations in the lice that were not killed by permethrin.

"Resistance levels of the Florida head lice and the Massachusetts head lice were 132 and 223 times higher than that of the Israel population and 41 and 68 times higher than that of the Panama head lice population," the researchers reported.

In response to a request for an interview, Warner-Lambert, the manufacturer of Nix, issued a written statement to WebMD. The statement commends Clark and the other authors for developing a technique "for examining resistance in head louse populations." It also calls the findings of resistance "interesting," but maintains the lice samples were not representative of the entire nation.

"I do think there is a problem with permethrin-resistant lice, but no one knows how large the problem is," Sharon Raimer, MD, tells WebMD. " Until we know, permethrin is still a reasonable treatment with which to start because it is safe and easy to use." Raimer is a professor in the department of dermatology at the University of Texas Medical Branch in Galveston.

However, other experts say that research convinced them that most permethrin products are probably ineffective. Jerry F. Butler, PhD, an entomologist at the University of Florida, who reviewed the study for WebMD, says the research shows "massive failure" of permethrin and confirms what Florida officials have long suspected.

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