Here’s some lousy news: Lice in half of America—at least 25 states—are now resistant to over-the-counter treatments. That’s according to new research presented at the American Chemical Society’s national meeting.

Study author Kyong S. Yoon, PhD, assistant professor in the Biological Sciences and Environmental Sciences Program at Southern Illinois University, has been researching lice since 2000. (“My PhD entirely focused on head lice,” he says with a laugh.) Using the services of professional nitpickers across the country, Yoon decided to take an American lice census by collecting pest populations from every state.

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His research is still ongoing, but what he’s found so far in 109 samples from 30 states is startling: the vast majority of lice now carry genes that are super-resistant to the over-the-counter treatment used against them.

Lice is commonly treated by a group of insecticides called pyrethroids, used for mosquito control. One of those, permethrin, is the active ingredient in some anti-lice treatments—but lice populations can develop a trio of mutations that make it resistant to pyrethroids.

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In 25 of the states, lice samples had all three of these genetic mutations, making them the most resistant to treatment. Lice populations from four other states had one, two or three mutations, and in just one state—Michigan—were the pests not resistant at all to the insecticide.

“It’s a really, really serious problem right now in the U.S.,” Yoon says. Though head lice aren’t known to transmit any diseases, they can be an itchy nuisance—and now, they’re harder to kill. Yoon suggests prescription-based products, like ivermectin or spinosad, if pyrethroid-based treatments don’t work.

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Write to Mandy Oaklander at mandy.oaklander@time.com.