It's the note every parent dreads: Head lice are making the rounds at your kid's school.

The tiny bugs are a huge problem. Head lice infect somewhere between 6 million and 12 million kids from the ages of 3 and 11 each year, according to the Centers for Disease Control and Prevention.

male human head louse magnified Just in time for back-to-school season, a new study finds that lice in at least 25 states have become resistant to over-the-counter treatments recommended by doctors.

Researchers from Southern Illinois University gathered lice from 30 states with the help of public health workers. The research was presented at a meeting of the American Chemical Society.

"We are the first group to collect lice samples from a large number of populations across the U.S.," said researcher Kyong Yoon, Ph.D., in a press 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 type of insecticide commonly used to control insects. It includes permethrin, the active ingredient in many common lice treatments sold in stores.

Yoon and his research team discovered that lice had developed three gene mutations, known as knock-down resistance (kdr) against the pyrethroids — making the drugs ineffective.

The first reports of pyrethroid resistance in lice came from Israeli researchers in the 1990s. Yoon was one of the first researchers to report on the resistant lice in the U.S. in 2000 in Massachusetts. Since then, he has expanded his survey.

map of states with drug-resistant lice populationsLice populations in the states in pink have developed a high level of resistance to some of the most common treatments. (Photo: Kyong Yoon, Ph.D.)
In this most recent study, researchers found lice with all three genetic mutations in 25 states, including California, Texas, Florida and Maine. Having all the mutations meant those lice were the most resistant to pyrethroids. Lice in four states — New York, New Jersey, New Mexico and Oregon — had one to three mutations.

Michigan was the only state of the 30 involved in the study that had a lice population that was still largely susceptible to the insecticide. Researchers are studying why lice there haven't developed resistance, Yoon says.

So what should a parent do?

Head lice can still be controlled with different chemicals, says Yoon, some of which are available only by prescription.

The American Academy of Pediatrics says:

In areas with known resistance to an over-the-counter pediculicide, or when parents' efforts on their own do not work, parents should involve their pediatrician for treatment with a prescription medication such as spinosad or topical ivermectin.

The AAP says parents should take other measures such as removing nits (lice eggs) by hand, using a nit-removal comb. Other recommendations include washing pillow cases and treating natural bristle brushes, but not using home pesticides.

Yoon points out that there's a lesson to be learned in the tale of the newly hardy head louse.

"If you use a chemical over and over, these little creatures will eventually develop resistance," he says. "So we have to think before we use a treatment. The good news is head lice don't carry disease. They're more a nuisance than anything else."

Head louse photo: Gilles San Martin/flickr

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.