Just in time for the new school year, head lice have become literal superbugs.
Lice have developed a high level of resistance to the most common over-the-counter treatments in 25 states, according to research presented at a meeting of the American Chemical Society.
“If you overuse a product, over time, the selection pressure will cause insects to develop resistance to it,” Kyong Yoon, an assistant professor of biological and environmental science at Southern Illinois University-Edwardsville and lead author of this ongoing research, told a group of journalists.
Head lice are most commonly treated with pyrethroids, a widely used class of indoor-outdoor insecticides, often used for mosquito control. Yoon and his team found that lice developed a gene mutation, known as knock-down resistance (kdr) against the pyrethroids — rendering them ineffective. In 104 out of the 109 lice populations Yoon tested, the insects were resistant to pyrethroids.
“Lice don’t have wings, and they don’t jump, so they move where people move,” Yoon said.
That’s why the new school year is such a vulnerable time for head lice transmission. Kids bring lice back from vacations and into the classroom, where they pass the infestation on to their classmates.
“It’s a really, really serious problem right now in the U.S.,” Yoon told Time. “Though head lice aren’t known to transmit any diseases, they can be an itchy nuisance — and now, they’re harder to kill.”
Pyrethroid-resistant lice were first discovered in Israel in the 1990s and Yoon was one of the first researchers to report on pyrethroid-resistant lice in the United States, in Massachusetts in 2000.
“At this particular time, we have five states that are not completely saturated with the kdr mutations,” said co-author, J. Marshall Clark, a professor of veterinary and animal sciences at the University of Massachusetts.
Among those five states, there’s only one state — Michigan — where the population of lice are still largely susceptible to pyrethroids.
There are other chemicals, such asivermectin or spinosad, which are available by prescription, that can kill head lice, but they’re not as safe or as gentle as the pyrethroids that doctors and schools currently recommend.
“For the past 20 to 25 years, we only used the products containing over-the-counter pyrethroids,” Clark said. “One, because they were very available. Two, because they are incredibly safe.”
To prevent head lice from mutating again the future, said Clark, there need to be a number of anti-lice products on the market at the same time, so that lice can’t select against one particular mutation, the way they did with pyrethroids.
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