Head lice populations in at least 25 states have developed resistance to over-the-counter treatments still widely recommended by physicians and schools, according to research presented Aug. 19 in Boston at the 250th National Meeting & Exposition of the American Chemical Society.
“We are the first group to collect lice samples from a large number of populations across the U.S.,” Southern Illinois University-Edwardsville researcher Kyong Yoon, PhD, said in a news release. “What we found was that 104 out of the 109 lice populations we tested had high levels of gene mutations, which have been linked to resistance to pyrethroids.”
Pyrethroids are a family of insecticides used widely indoors and outdoors to control mosquitoes and other insects. It includes permethrin, the active ingredient in some of the most common lice treatments sold at drug stores.
Yoon said the momentum toward widespread pyrethroid-resistant lice has been building for years. The first report on this development came from Israel in the late 1990s. Yoon became one of the first to report the phenomenon in the U.S. in 2000 when he was a graduate student at the University of Massachusetts in Amherst. “I was working on insecticide metabolism in a potato beetle when my mentor, John Clark, suggested I look into the resurgence of head lice,” he said in the release. “I asked him in what country, and was surprised when he said the U.S.”
Yoon contacted schools near the university to collect samples. He suspected the lice had developed resistance to the most common insecticides people were using to combat them, so he tested the pests for a trio of genetic mutations known collectively as kdr, which stands for “knock-down resistance.” Kdr mutations had initially been found in house flies in the late 1970s after farmers and others had shifted to pyrethroids from DDT and other harsh insecticides, according to the release.
Yoon found that many of the lice had kdr mutations, which affect an insect’s nervous system and desensitize them to pyrethroids. Since then, he has expanded his survey.
The most recent study gathered lice from 30 states with the help of a broad network of public health workers. Population samples with all three genetic mutations associated with kdr came from 25 states, including California, Texas, Florida and Maine. Having all the mutations means these populations are the most resistant to pyrethroids. Samples from four states — New York, New Jersey, New Mexico and Oregon — had one, two or three mutations. The only state with a population of lice still largely susceptible to the insecticide was Michigan. Why lice haven’t developed resistance there is still under investigation, Yoon said in the release.
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