With the start of the new school year already upon us, parents and healthcare professionals are becoming worried to learn that head lice have developed resistance to some of the most popular treatments.

Research suggests that lice in at least 25 states have developed a “high level” of resistance to pyrethroids, a family of insecticides that includes permethrin, the active ingredient used to kill lice in common over-the-counter treatments.

“We are the first group to collect lice samples from a large number of populations across the United States,” Kyong Yoon, a professor at Southern Illinois University, Edwardsville, said in a statement from the American Chemical Society about the study. “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.”

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The news is alarming to parents in the United States, where an estimated 6 to 12 million cases of lice occur among children aged 3 to 11 every year.

Yoon says, however, that lice have been developing pyrethroid resistance for a number of years now. The first report of this kind was published by Israeli researchers in the late 1990s, and Yoon became one of the first to report on the phenomenon in the United States in 2000 when he was a graduate student at the University of Massachusetts, Amherst.

Yoon tested lice for 3 genetic mutations, collectively known as kdr, which means “knock-down resistance.” The kdr mutation was first found in house flies in the 1970s after farmers began using pyrethroids. Yoon found that many of the lice also carried the mutations that desensitized them to the insecticide.

In the most recent study, Yoon gathered the itch-inducing pests from 30 states and discovered that samples from 25 of them had all 3 genetic mutations associated with kdr. Samples from 4 states varied between carrying any number of the mutations, whereas Michigan was the only state with a population of lice still largely susceptible to pyrethroids.

“In the United Kingdom and Europe, they don’t even use pyrethroids anymore. Virtually everyone but the United States and Canada have given up using these over-the-counter products,” said John Clark, a professor of environmental toxicology and chemistry who worked on the research.

Treatment options using different chemicals are still effective in controlling head lice, although some are only available by prescription and can be more expensive. The good news, says Yoon, is that lice don’t carry disease, making lice more of an annoyance than anything else.

The American Academy of Pediatrics suggests that parents who suspect head lice should contact their pediatricians. In areas where there is no known resistance to pyrethroids, using the over-the-counter treatments is acceptable. In areas where there is known resistance to the insecticides, alternative prescription products may be used.


  1. Aleccia J. The start of school, and a growing resistance to pesticides, mean an uptick in lice. Providence Journal website. http://www.providencejournal.com/article/20150824/ENTERTAINMENTLIFE/150829807. Published August 24, 2015. Accessed September 1, 2015.
  2. Grisham L. Super lice: what you need to know. USA Today website. http://www.usatoday.com/story/news/nation-now/2015/08/24/super-lice-treatment-pyrethroids/32255719/. Published August 24, 2015. Accessed September 1, 2015.
  3. Lice in at least 25 states show resistance to common treatments. American Chemical Society website. http://www.acs.org/content/acs/en/pressroom/newsreleases/2015/august/lice.html. Published August 18, 2015. Accessed September 1, 2015.
  4. Super head lice warning as scientists discover almost all species are resistant to treatments. The Telegraph website. http://www.telegraph.co.uk/news/health/news/11810582/Super-head-lice-warning-as-scientists-discover-almost-all-species-are-resistant-to-treatments.html. Published August 18, 2015.