Division of Agricultural Sciences and Natural Resources
Permethrin resistance behind so-called ‘super’ lice
“There is nothing really ‘super’ about them, of course; they are simply lice that are more resistant than previous generations to permethrin, the active ingredient in many head lice treatments,” said Justin Tally, Oklahoma State University Cooperative Extension entomologist.
The decades-long popularity of permethrin in combating head lice is essentially the problem.
“A synthetic chemical, permethrin has been incredibly effective throughout most of the past quarter century, plus it boasted low toxicity to humans and pets,” Talley said. “Over time, however, only lice that survived permethrin treatments went on to reproduce, passing on their innate resistance to the ingredient.”
Permethrin- and malathion-resistant lice have been found in the United States since the mid-1990s. However, it is important to remember that misuse of products when treating a head lice infestation can be as significant an issue as overuse of products.
“One of the major issues is over-the-counter treatment medications that are not applied correctly,” Talley said. “A conditioning shampoo will act as a barrier that keeps the head lice medication from adhering to the hair, so do not use a conditioner or a shampoo with conditioner when treating for lice.”
Another issue is people failing to apply the second treatment to kill the eggs of the lice approximately nine days after the initial treatment. Then there is the “just wanting to get it done” scenario, often viewed with fidgety children undergoing the laborious treatment process.
“As an example, parents may think their child has been re-infested with lice after a successful treatment but it may be the result of washing the child’s hair too soon after the treatment, which reduces the residual killing activity of the medicine,” Talley said.
Detecting insecticide resistance is not easy. The persistence of living head lice after using a lice medication may be caused by a lack of adherence on the part of the patient or parents to the treatment protocol, incorrect treatment, lack of residual properties of the product, re-infestation from other people after successful treatment or authentic resistance of lice to the medication.
“Yes, there is increased resistance of lice to medications showing up but we don’t know regional resistance levels in head lice, thus my initial questions typically revolve around determining how the person applied the product to see if they applied it correctly,” Talley said.
In addition, it is important to remember lice are common and do not necessarily reflect a household or individual’s hygiene. Also, while lice infestations can be traumatic, especially to young children, they are not actually dangerous.
The most common incidents of infestation in the United States occur with school-age kids, so Talley recommends parents help their offspring practice “lice avoidance techniques.”
“No sharing of hats, brushes, combs, headbands, headphones or other items that come into close contact with their hair,” he said. “If a child does contract lice, parents should start treatment with permethrin products – as there is still nothing better – and then follow up with manual louse and louse-egg removal.”
Regardless, parents will have to manually comb and remove lice eggs from their child’s head because no product currently on the market kills lice eggs.
“The major difference from previous decades is that with so-called super lice, parents may find themselves removing more live lice when they are removing the eggs,” Talley said. “As always, follow all label instructions with permethrin products to ensure safe and effective treatment.”
Manual removal is tedious but effective. Be sure to regularly inspect the child’s head and follow up as needed every two to three days, using a nit comb to ensure the best possible chance of removal.
“Again, head lice is not an indicator of cleanliness or home health care,” Talley said. “Anyone can get head lice. Always contact your physician to get proper treatment recommendations. Do not simply rely on the Internet for recommendations.”
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