HSPH Researchers Find Pesticide-Resistant Head Lice in US

Infestations of head lice create hardships for many parents of school-age children. Many school districts have "no nits" policies that prohibit children from attending school if they have evidence of a louse infestation--usually found either in the form of louse eggs, called nits, or living lice in the hair.

For the past several years, anecdotal evidence from parents has proposed that some head lice in the US were not affected by some of the primary over-the-counter anti-louse pesticides, called pediculicides. Now, researchers from HSPH's Department of Immunology and Infectious Diseases have provided quantitative evidence of pesticide-resistant lice. Their report is printed in the September issue of the Archives of Pediatric and Adolescent Medicine.

Researchers from the Department of Immunology and Infectious Diseases, Richard Pollack (left) and Andrew Spielman, discussing a lousy subjectthe resistance of US head lice to a frequently used anti-louse insecticide. Pollack is an instructor in the department; Spielman is a professor of tropical public health.

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 permethrinthe 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.

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, Pollack recommends that over-the-counter pediculicides and good, anti-louse grooming remain the first choices for eradication. Anti-louse grooming is performed with a special comb designed with tightly spaced teeth that remove lice and nits from the hair follicles.