Two head lice seen under an electron microscope. (Centers for Disease Control and Prevention)

Head lice aren't particularly dangerous, but they are a nuisance -- one that has become such a common part of life with young children that multiple over-the-counter remedies are available to parents.

That very fact has made the increase in prevalence of lice cases since the 1990s -- despite the corresponding increased access to effective treatments -- so puzzling.

A new study that will be presented at the National Meeting & Exposition of the American Chemical Society on Tuesday suggests one reason for the increase: After decades of treatment with anti-pest remedies sold in drugstores across America, head lice are evolving to resist our efforts to snuff them out.

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The study found that of the head lice samples collected across 30 states, all but five showed signs of a very high level of resistance to pyrethroids -- the chemicals contained in some of the most common over-the-counter treatments.

"We are the first group to collect lice samples from a large number of populations across the U.S.," said Southern Illinois University at Edwardsville researcher Kyong Yoon. "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."


Lice populations in the states in pink have developed a high level of resistance to some of the most common treatments. (Kyong Yoon, Ph.D.)

The trio of mutations -- called kdr, for "knock down resistance" -- affects the insect's nervous system and makes them less sensitive to the insecticide chemicals that are found in lice treatments and also in mosquito repellant or fly spray, for example.

In four states, head lice were found to have one, two or three of the mutations. Only one state -- Michigan -- had head lice samples that didn't show any signs of widespread resistance to treatment; Yoon added that the reason for that is unclear.

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Head lice affect somewhere between 6 million and 12 million children between the ages of 3 and 11 every year, according to the U.S. Centers for Disease Control and Prevention. Over-the-counter medications that contain pyrethroids include Pronto, R&C, Rid, and Triple X.

In recent decades, pyrethroids have been increasingly used as a pesticide as part of a broader effort to shift away from harsher chemicals like DDT. Pyrethroids resistance has been found, for example, in house flies.

Anecdotal evidence and previous studies have also suggested that head lice were also increasingly becoming resistant. A 1999 study, for example, found evidence of resistance to pyrethroids in the United States in a small sample of kids who had contracted and had been treated for lice multiple times in the past.

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The solution, Yoon says, may be to treat lice with other chemicals that are more likely to be available with a prescription.

"If you use a chemical over and over, these little creatures will eventually develop resistance," Yoon added. "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."

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