Permethrin No Longer as Effective Against Head Lice

Laurie Barclay, MD

December 09, 2002

Dec. 9, 2002 — The most commonly used pediculicide for lice is permethrin 1% cream (PLT), now available over the counter. At its introduction, it was 97% effective, but a study in the November issue of the Journal of Pediatrics showed that it is currently significantly less effective, even when combined with nit removal.

"Multiple studies were published at the time of the initial regulatory approval of 1% PLT as a prescription pediculicide documenting success rates of at least 97% when patients were evaluated at both 7 and 14 days after a single 10-minute treatment," write Terri L. Meinking, BA, from the University of Miami School of Medicine in Florida, and colleagues. "Increasingly, clinical resistance to treatment is being reported in the U.S. and multiple other countries, as well as reduced efficacy of permethrin and derivatives in vitro."

In this randomized, observer-blinded study, 95 infested adults and children were treated with 1% PLT on day 1 and, if still infested, on day 8. One-third of households were randomized to combing, and two-thirds did not attempt to remove nits by combing.

In the no-combing group, the lice-free rate determined by visual inspection and shampooing/straining was 83.1% on day 2 (95% confidence interval [CI], 71.0 - 91.6), 45.8% on day 8 before the second treatment (95% CI, 32.7 - 59.2), 77.6% on day 9 (95% CI, 64.7 - 87.5), and 78.3% on day 15 (95% CI, 65.8 - 87.9). Efficacy was no better on any given day in the combing group.

Most subjects required a second treatment on day 8. Of the 56 participants who were retreated, 15 still had lice on day 15, yielding a failure rate of 27%. One subject had mild scalp folliculitis and rash thought to be possibly related to PLT, and two others had self-limited vomiting and upper respiratory infection.

"In this population, 1% PLT was significantly less than 95% effective and suggests resistance to 1% PLT," the authors write. "The failure of nit removal combing by nonprofessional caregivers to improve efficacy demonstrates the unreliability of combing as adjunctive treatment in this setting."

Merck Research Laboratories supported this study and employs some of its authors.

J Pediatr. 2002;141:665-670

Reviewed by Gary D. Vogin, MD

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