FRIDAY, March 14, 2014 (HealthDay News) -- Most head lice found in North America now carry a gene mutation that makes them resistant to standard over-the-counter treatments, a new study cautions.

Head lice infestation is a major public-health issue, the researchers said, with roughly 10 percent of all American school-aged children missing school due to the intense itching and secondary infections that signal exposure.

The problem: Years of relentless exposure to a single treatment option has given rise to a surviving head lice population that is armed with what geneticists call "knockdown resistance," in the form of the TI genetic mutation.

This gives most of today's head lice an ability to withstand exposure to the main -- and previously effective -- ingredients found in most nonprescription head lice drugs: "pyrethroid" compounds such as permethrin.

"This isn't really controversial," said study co-author John Clark, a professor of environmental toxicology and chemistry at the University of Massachusetts, Amherst. "This is a problem we've been showing in development over a period of about 20 years. But our new work now shows that head lice are now almost 100 percent [knockdown resistant]. That means there's an awful lot of resistant insects out there in the U.S. and elsewhere."

Clark and his colleagues discussed their findings in the March issue of the Journal of Medical Entomology.

In the early 1990s, over-the-counter permethrin-based formulations entered widespread use, Clark said. Designed to kill lice by essentially short-circuiting their nervous system, such drugs became the standard of care.

But the seeds of permethrin's future failure were planted decades earlier, in the immediate post-WWII period, he said, when much of the population was first exposed to the pesticide DDT in an effort to reign in body lice infestations.

Head lice got caught up in the process, giving rise to a surviving population that silently, but increasingly, began to carry the protective TI mutation decades before the introduction of permethrin products, according to the study.

With that in mind, investigators set out to assess the current resistance status of North American head lice by conducting a genetic analysis of lice samples collected from 32 mostly urban locations across the United States and Canada.