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Lice Resistant to Common Treatments

Development of New Safe, Effective Treatments Needed

By Theresa Defino
WebMD Medical News

April 6, 2000 (Washington) -- Anguished parents who pour head lice treatments on their kids only to find live critters a few minutes or hours later already have experienced what scientists in Massachusetts have proven: Many lice have mutated and are now resistant to permethrin, the main chemical ingredient in the most popular lice products.

And in the same way that super-bacteria are invincible to common antibiotics, the lowly louse may ultimately outwit all efforts to kill it, unless smarter -- and perhaps simpler -- strategies are found.

"We are just beginning to get a picture of how widespread [permethrin] resistance is," J. Marshall Clark, PhD, tells WebMD. "Clearly the resistance is there and it is established and it is a real problem."

Clark was the lead investigator of the study, published in a recent pesticide journal. He is the director of the Massachusetts Pesticide Analysis Laboratory and an entomology professor at the University of Massachusetts, both in Amherst. Clark and his team collected live head and body lice from individuals in Israel and Panama, and from school children in England, Massachusetts, and Florida. Lice were housed in incubators or kept free on volunteers, and were fed four to five times daily on humans.

Nymphs and mature lice were exposed to concentrations of permethrin equivalent to those found in over-the-counter lice products, such as Nix. They measured the time it took for the lice to die, and used DNA testing to examine the lice genes. They found two gene mutations in the lice that were not killed by permethrin.

"Resistance levels of the Florida head lice and the Massachusetts head lice were 132 and 223 times higher than that of the Israel population and 41 and 68 times higher than that of the Panama head lice population," the researchers reported.

In response to a request for an interview, Warner-Lambert, the manufacturer of Nix, issued a written statement to WebMD. The statement commends Clark and the other authors for developing a technique "for examining resistance in head louse populations." It also calls the findings of resistance "interesting," but maintains the lice samples were not representative of the entire nation.

"I do think there is a problem with permethrin-resistant lice, but no one knows how large the problem is," Sharon Raimer, MD, tells WebMD. " Until we know, permethrin is still a reasonable treatment with which to start because it is safe and easy to use." Raimer is a professor in the department of dermatology at the University of Texas Medical Branch in Galveston.

However, other experts say that research convinced them that most permethrin products are probably ineffective. Jerry F. Butler, PhD, an entomologist at the University of Florida, who reviewed the study for WebMD, says the research shows "massive failure" of permethrin and confirms what Florida officials have long suspected.

"I think this is groundbreaking work," adds W. Steven Pray, PhD, RPh, who also reviewed the study for WebMD. "I have always seen resistance as a problem." Pray is professor of nonprescription products and devices at the School of Pharmacy at Southwestern Oklahoma State University in Weatherford.

Drugstores are often the first place parents or other adults run to after getting a note of a lice spread at school or finding a louse on a child. Therefore, pharmacists have a big role to play in educating parents, schools, and other health care providers about the proper way to fight lice, Pray adds. The message that he tries to get across to consumers and pharmacy students is that the weapons in the battle against lice are fingernails and a comb -- no chemicals, oils, salves, shampoos, or shower caps.

"We don't recommend Nix and Rid anymore, because of the resistance issue. I get people who look at me over the counter and tell me, 'I have used these six, seven, 10 times. I promise you I am using them right. And still I see lice.' They are using them everyday!" says Pray.

"And I tell them right off the bat, 'Let's get away from pesticides completely, and just go into combing,'" he says. "It is more effective to comb. All you need is a good comb and it doesn't involve any danger. Otherwise you are putting pesticides on the child's head, repeatedly. Even once is too much for me. And all we are doing is setting up more resistance."

Physicians may be asked to treat patients who have had no success with over-the-counter products, but few prescription products are available today, and all are controversial. Many physicians, when faced with this problem, simply give their patients literature from the lice treatment manufacturers. If they do write a prescription, it may be for Lindane, which used to be available without a prescription under the name Kwell. But Lindane has been linked to serious nerve damage and deaths and is not recommended by most experts.

The development of new, safe, and effective lice treatments has been hampered by a lack of funding for research facilities where lice can be raised and thoroughly studied, says Clark.

"We have never really given enough thought to a resistance management strategy," says Clark. "What we do is market one thing at a time and use it until it becomes useless. What I find very unacceptable is that everyone knows [resistance] is out there and nobody is doing anything about it."

"We still have the old tried-and-true method, which was tried on me in 1940, and that is to cut all the hair off," says Butler. "And hand-combing. That has been used forever. Even the Pharaohs were buried with combs, and they found nits on the combs."

The study was funded by the National Pediculosis Association.

Vital Information:

  • Researchers have shown that lice in this country have become resistant to permethrin, the active ingredient found in the most popular lice treatment products.
  • One pharmacist recommends that the best way to treat lice is with a comb and fingernails.
  • Prescription products are available, but their use is controversial because of negative side effects.

© 2000 Healtheon/WebMD. All rights reserved.



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