Head Lice Are Resistant to Over-the-Counter Treatments

By Chris Williams on September 16, 2015.

Kids are back in school and you can breathe a sigh of relief…except now you have to worry about head lice. School kids and head lice go hand-in-hand. An infestation can mean an embarrassing, grueling, messy episode of treatments that often must be repeated. And guess what, the treatments are becoming less effective.


A little over a year ago, we reported in our blog that head lice were showing resistance to permethrin, the over-the-counter insecticide used to kill the lice (see In the News – It’s Getting Harder to Kill Head Lice). Permethrin belongs to a class of insecticides called pyrethroids that desperate parents and schools began using in the 1990’s as a replacement for a prescription head louse insecticide.


Historical Use of DDT is Partly to Blame

“Resistance” means that the lice have genetically mutated and are able to survive the insecticide. But it didn’t happen overnight. The use of DDT against bed bugs in the early 1900s played a role in today’s head lice resistance to permethrin. The ancestors of today’s head lice were incidentally exposed to DDT when homes were treated for bed bugs. Since both insecticides have the same mode of action, lice surviving the DDT also carried a mutation that gave them and their offspring cross-resistance to permethrin.


New research has confirmed what we already knew. Head lice are highly resistant to the only over-the-counter insecticide available to kill them. Kyong Yoon of Southern Illinois University employed a nationwide team of lice pickers who collected lice from every state. Of 109 louse populations tested so far, 104 had mutations that gave them resistance to permethrin. Twenty-five states, including Massachusetts, have documented resistant louse populations. New Hampshire and the remaining states were still being tested, but the researchers expect to find almost 100% resistance.


Then, How Do I Control Head Lice?

So what can you do if your child has a head louse infestation? Follow the advice of the school. If you use an over-the-counter product, be diligent, make sure you also use a special “louse comb,” and be prepared to repeat treatment. There are now new prescription treatments for head lice that you can get from your physician that do not contain permethrin and do not have the resistance problem. Do not use any product that is not specifically labeled for head lice.


One thing you shouldn’t do is spray insecticide on bedding, or clothing, or in your home in an attempt to kill head lice (see Head Lice – Sprays Are Not the Answer). Head lice remain on a person’s head and control must be directed towards the infested head, not the premises. Head lice are a personal medical issue, not a pest control issue, and any reputable pest control company will advise you that treating the child’s environment accomplishes nothing. Sorry, but we can offer advice and sympathy only.



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