Press Releases

1999 Releases

Pesticide-Resistant Head Lice Found in US

For immediate release: September 14, 1999 

Boston, MA--Researchers from the Harvard School of Public Health have verified what many parents have suspected: There are head lice in this country that are not susceptible to one of the most frequently used pediculicides (anti-louse insecticides). The report, "Differential Permethrin Susceptibility of Head Lice Sampled in the United States and Borneo," is printed in the September issue of the Archives of Pediatric and Adolescent Medicine.

The researchers collected head lice from infested patients in two areas of the US: Massachusetts and Idaho. In a laboratory, the lice were exposed to progressively higher doses of permethrin--the active ingredient in one of the most popular pediculicides.

Most of the lice collected from the two sites in the US were not killed by the permethrin.

In comparison, and to validate the assay, the researchers collected lice from Borneo, where permethrin is rarely used as a pediculicide. The lice from Borneo were quickly killed by permethrin.

Lead author Richard Pollack, instructor in the Department of Immunology and Infectious Diseases, said, "When exposing lice from Borneo to permethrin, we saw a classic dose-response curve, meaning that the greater the dosage of permethrin, the more effective it was at killing the lice."

"With the American lice, however," he said, "the dose-response curve was relatively flat. If a little permethrin wasn't effective, neither was a larger dose."

These findings have implications for the treatment of lice in the US. Traditionally, if over-the-counter medications failed to kill lice, then patients would seek prescription-only alternatives. Some prescribed medications simply contain a higher concentration of permethrin.

Pollack said: "If you have head lice and an over-the-counter medication containing permethrin doesn't solve your problem, then neither will a prescription for a higher dose of permethrin. If you have permethrin-resistant lice, then it's best to try something else."

Alternatives to permethrin include pediculicides containing lindane and malathion, both available by prescription.

"Perhaps the most critical action to take to end a head louse infestation, regardless of the use of any pediculicide," said Andrew Spielman, professor, Department of Immunology and Infectious Diseases, "is to practice good, anti-louse grooming." Special combs are available for combing living lice and their eggs from the scalp and hair. However, because the effectiveness of these combs depends upon their construction of tightly spaced teeth, they are difficult or impossible to use with some hair types or styles. "These combs are of limited use for people with curly, coarse hair, or for those with braids or other styles that can't be combed through," said Spielman.

Extent of Resistance Unknown

The US lice tested were collected from the heads of children by nurses assigned to schools in Massachusetts and in Idaho. The parents or guardians of these children were asked to answer a questionnaire regarding family knowledge of this and any prior louse infestations. Nearly every child had been previously treated with a pediculicide.

"Our sample of US lice was biased towards those taken from children who have already been exposed to pediculicides," said Pollack. "Therefore, what we have shown is that if an over-the-counter preparation doesn't end an infestation, then additional treatments with the same product or higher concentrations of the same active ingredient won't help either."

"But what our study hasn't addressed," he continued, "is the prevalence of permethrin-resistant lice in the United States. Research to assess the extent of this problem is currently under way."

Permethrin is a synthesized pyrethroid chemical formulation. Other over-the-counter anti-louse therapies contain related chemicals such as synergized pyrethrum extracts. These act in similar ways to permethrin, but are chemically different. This research does not address the efficacy of synergized pyrethrum extracts.

For newly diagnosed head-lice infestations, these researchers recommend that grooming and over-the-counter pediculicides remain the first choices for eradication.

Pollack and Spielman maintain a head lice information resource on the World Wide Web at This site contains information about the life stages of head lice and what is known about treatment methods.

For further information, please contact:

Robin Herman
Harvard School of Public Health
677 Huntington Avenue
Boston, MA 02115
Phone: 617-432-4752