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© Ajay Verma / Reuters
Lice populations in at least 25 US states have developed resistance to traditional treatments commonly used by doctors, parents and schools, scientists warn. They say the threat of the itch-inducing pests is looming.

"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," Dr. Kyong Yoon of Southern Illinois University said, adding that his team is the first to collect lice samples from a large number of populations across the US.

Pyrethroids are a family of synthetic chemicals with insect repellent properties. Permethrin, which also belongs to pyrethroids, is an active ingredient in some of the most common lice treatments, such as lotions and sprays, sold at drug stores. The bugs are spread by head-to-head contact and are notoriously common among primary school kids.

According to Yoon, the momentum toward pyrethroid-resistant lice has been mounting for years, with the first report coming from Israel in the late 1990s. In 2000, when Yoon was a graduate student at the University of Massachusetts, he became one of the first to report the phenomenon in America.

"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 a press release.

"I asked him in what country and was surprised when he said the US."

Yoon contacted schools near the university to collect samples. He imagined that the lice had probably already developed resistance to the most common insecticides aimed at combating nasty bugs. So he decided to test the pests for a trio of genetic mutations known as kdr (knock-down resistance). Yoon found that many of the lice did as a matter of fact have those kdr mutations, which affect a bug's nervous system, desensitizing them to pyrethroids.

In his most recent study, Yoon gathered lice from as many as 30 states in America. Population samples with all three genetic mutations associated with kdr came from 25 states, including California, Texas, Florida and Maine. Samples from four states, such as 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. Yoon says he is still investigating, why lice haven't developed resistance there.

The scientist reassures that lice can still be treated using different chemicals, some of which are actually available only by prescription. He cautioned that if you use a chemical over and over, "these little creatures will eventually develop resistance," however.

The only good news is that head lice don't carry disease. "They're more a nuisance than anything else," Yoon wrapped up.