Comparative in vitro pediculicidal efficacy of treatments in a resistant head lice population in the United States.

OBJECTIVE To compare the pediculicidal activity of 5 head lice products available in the United States on head lice from south Florida. DESIGN In vitro pediculicidal product comparison. SETTING Lice Source Services, Inc, located in Plantation, Fla, a clinic for the treatment and grooming of individuals with pediculosis capitis. PARTICIPANTS Head lice were collected from healthy clients with Pediculus capitis that came to Lice Source Services, Inc, to seek their services. INTERVENTIONS Within 2 to 6 hours of capture, lice were placed in continuous direct contact with the pediculicide products and observed at regular intervals. Results were compared with findings of a recent study of a treatment-sensitive population of head lice conducted in Panama. MAIN OUTCOME MEASURE Percentage of lice dead at regular observation intervals between 5 minutes and 3 hours of continuous exposure to the pediculicides. RESULTS Two prescription products, Ovide lotion (0.5% malathion) and 1% lindane shampoo, were ranked in the same order as in 2 previous studies (first and last, respectively). The order of effectiveness from most to least effective was as follows: Ovide lotion, A-200 shampoo (a natural pyrethrin product synergized with piperonyl butoxide), undiluted Nix (1% permethrin), diluted Nix, RID (a natural pyrethrin product synergized with piperonyl butoxide), and 1% lindane shampoo. CONCLUSIONS There were statistically significant differences in the efficacy of all the products when compared with the results found in the recent study in Panama, except for Ovide lotion. Of those tested, Ovide was the only pediculicide in the United States that had not become less effective. The difference in efficacy of 1% lindane, Nix, and pyrethrins between the Panama and Florida studies supports the argument that some head lice in the United States have become resistant to these treatments.