Resistant lice that no longer responds to common over-the-counter treatments are thriving in many U.S. states. With six million to 12 million children infested with head lice each year, per the Centers of Disease Control and Prevention, the scenario does not look so good.

Researchers examined lice populations in various states and found that the insects have developed a resistance to OTC treatments, such as those that use permethrin, the American Chemical Society said in a press release Tuesday.

Permethrin is an insecticide from the pyrethoid family commonly used to kill disease-carrying insects like bed bugs. This treatment became available on the market in the early 1990s. Observations of head lice developing genetic mutations that resist the insecticide have been reported as early as 2000.

"It's a very classic resistance story," lead study author Kyong Yoon, assistant professor in the biological sciences and environmental sciences program at Southern Illinois University, told Health Day.

The research team collected lice samples from large populations across the U.S. and found many of them had "knock-down resistant mutations" - three genetic mutations that make them highly resistant to pyrethoids. These three genetic mutations were observed in lice collected from 25 states, including North and South Carolina, California, Georgia, Arizona and Texas.

"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," Yoon said.

In New York, New Jersey, Oregon and New Mexico, the lice have only developed partial resistance. Lice from Michigan, on the other hand, have developed no resistance at all, something that researchers are still looking into.

So what can be done to get rid of lice?

"If you use a chemical over and over, these little creatures will eventually develop resistance," Yoon said. "So we have to think before we use a treatment."

He recommended using different chemicals, such as treatments that can be obtained by prescription.

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

The research was presented Tuesday at a meeting of the American Chemical Society in Boston.