Permethrin-resistant human head lice, Pediculus capitis, and their treatment

Objective: To compare the pediculicidal activity of Ovide lotion and its active ingredient, 0.5% malathion, with Nix and its active ingredient, 1% permethrin, in permethrin-resistant head lice. Design: In vitro pediculicidal product and active ingredient comparison. The presence of knockdown resistance-type mutations (T929I and L932F) was validated by DNA sequencing. Setting: University of Massachusetts-Amherst; University of Miami.School of Medicine, Miami, Fla; Plantation and Homestead, Fla; and Mathis, Tex. Other Participants: Lice were collected in 3 geographical regions within the United States and in Yamburna, Ecuador, from healthy but infested individuals. Intervention: Within 3 to 6 hours of collection, lice were given a blood meal, exposed to products or active ingredients, and observed at regular intervals. Main Outcome Measures: Percent mortality of lice at regular intervals after exposure to products or active ingredients and presence of T929I and L932F mutations. Results: South Florida lice exhibited a significantly slower mortality response to permethrin compared with susceptible Ecuadorian lice. Ovide and malathion killed permethrin-resistant lice faster than Nix or permethrin. The presence of T9291 and L932F in permethrin-resistant south Florida lice was confirmed by DNA sequencing. The population of Texas lice from Mathis was slightly resistant to permethrin and included 13% with resistant genotypes. Conclusions: The presence of the T9291 and L932F mutations was confirmed by DNA sequencing in lice-collected from children in south Florida that were resistant to the pediculicidal effects of permethrin and the leading permethrin-based head lice product, Nix. Malathion resistance was not observed in this study. The data also show that Ovide killed these same permethrin-resistant head lice approximately 10 times faster than permethrin or Nix.