Scientists have discovered that lice populations in at least 25 states have mutated to develop a resistance to over-the-counter treatments still widely recommended by doctors and schools.
Researchers found that 104 out of the 109 lice populations tested in 30 states had high levels of gene mutations, which have been linked to resistance to pyrethroids. Pyrethroids are a family of insecticides used widely indoors and outdoors to control mosquitoes and other insects. It includes permethrin, the active ingredient in some of the most common lice treatments sold at drug stores.
Kyong Yoon, Ph.D., a researcher with Southern Illinois University, Edwardsville, explained that the momentum toward widespread pyrethroid-resistant lice has been building for years. The first report on this development came from Israel in the late 1990’s. Yoon became one of the first to report the phenomenon in the U.S. in 2000 when he was a graduate student at the University of Massachusetts, Amherst.
“I was working on insecticide metabolism in a potato beetle when my mentor, John Clark, suggested I look into the resurgence of head lice,” he says. “I asked him in what country and was surprised when he said the U.S.”
Intrigued, Yoon followed up on the lead and contacted schools near the university to collect samples. He suspected that the lice had developed resistance to the most common insecticides people were using to combat the bugs. So he tested the pests for a trio of genetic mutations known collectively as kdr, which stands for “knock-down resistance.” kdr mutations had initially been found in house flies in the late ’70s after farmers and others had shifted to pyrethroids from DDT and other harsh insecticides.
Yoon found that many of the lice did indeed have kdr mutations, which affect an insect’s nervous system and desensitize them to pyrethroids. Since then, he has expanded his survey.
|Lice populations in the states colored pink have mutated to develop a high level of resistance to some of the most common treatments. Credit: Kyong Yoon, Ph.D. |
In the most recent study, Yoon gathered lice from 30 states with the help of a broad network of public health workers. Population samples with all three genetic mutations associated with kdr came from 25 states, including California, Texas, Florida and Maine. Having all the mutations means these populations are the most resistant to pyrethroids. Samples from four states — New York, New Jersey, New Mexico and Oregon — had one, two or three mutations. The only state with a population of lice still largely susceptible to the insecticide was Michigan. Why lice haven’t developed resistance there is still under investigation, Yoon says.
The solution? Yoon says that lice can still be controlled by using different chemicals, some of which are available only by prescription.
But the situation also offers a cautionary tale. “If you use a chemical over and over, these little creatures will eventually develop resistance,” Yoon says. “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.”