Differential permethrin susceptibility of head lice sampled in the United States and Borneo

Background: Pediculiasis is treated aggressively in the United States, main ly with permethrin- and pyrethrin-containing pediculicides. Increasingly fr equent anecdotal reports of treatment failure suggest the emergence of inse cticidal resistance by these lice. Objective: To confirm or refute the susceptibility of head life sampled in the United States to permethrin. Design: Survey. Head lice were removed from children residing where pedicul icides are readily available and where such products are essentially unknow n. Their survival was compared following exposure to residues of graded dos es of permethrin in an in vitro bioassay. Setting: School children from Massachusetts, Idaho, and Sabah (Malaysian Bo rneo). Subjects: In the United States, 75 children aged 5 to 8 years. In Sabah, 59 boys aged 6 to 13 years. Virtually all sampled US children had previously been treated with pediculicides containing pyrethrins or permethrin; none o f the Sabahan children were so exposed. Main Outcome Measure: Survival of head lice exposed to permethrin. Results: Permethrin did not affect head lice sampled from chronically infes ted US children who had previously been treated for pediculiasis. The slope of the dose-response regression line for these lice did not differ signifi cantly from zero (P = .66). This pediculicide immobilized lice sampled in S abah. Mortality correlated closely with permethrin concentration (P = .008) . Conclusions: Head lice in the United States are less susceptible to permeth rin than are those in Sabah. The pyrethroid susceptibility of the general p opulation of head lice in the United States, however, remains poorly define d. Accordingly, these relatively safe over-the-counter preparations may rem ain the pediculicides of choice for newly recognized louse infestations.