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Dead Head: Ivermectin vs. Malathion for Hard-to-Treat Lice

Oral ivermectin is an effective backup when topical treatment fails.

Head lice infest the scalps of millions each year, especially children. Researchers conducted a multicenter, randomized, double-blind, controlled comparison of oral ivermectin (400 µg/kg) and malathion lotion 0.5% in index patients and family members with previous inadequate response. Ivermectin is an oral inhibitor of parasite neurotransmitters, and malathion is a topical organophosphate neuroinhibitor. Randomization was by household to prevent contamination between treatment groups. Staff administered malathion or placebo lotion and ivermectin or placebo pills to all subjects on days 1 and 8. Lotion was applied to dry hair and scalp until thoroughly moistened and then air dried. The study included 812 patients (mean age, 10 years; female sex, 87%) from 376 households.

On day 15, visual inspection and fine combing demonstrated that 93% of ivermectin recipients and 85% of malathion recipients were lice free (P<0.001; mean difference, 10.2 percentage points; 95% confidence interval, 4.6–15.7). By household, 92% of 185 ivermectin households and 79% of 191 malathion households were lice free (mean difference, 13.4 percentage points; CI, 6.4–20.4). On day 15, 8 ivermectin nonresponders and 39 malathion nonresponders crossed over to the other treatment. Except for one patient treated with malathion, all patients were lice free at day 29. Adverse events occurred in fewer than 2% of patients in each group; a seizure in a 7-year-old ivermectin recipient and a severe headache in a 6-year-old malathion recipient are reported but not specifically as treatment-related events.

Comment: This well-designed, well-executed study demonstrates the efficacy of both medications. The decision to use a pill rather than a lotion may depend more on situation than on expectation of cure. These findings showed both the noninferiority and the superiority to malathion of oral ivermectin at a dose of 400 µg/kg. It appears to be fairly safe to use in children as young as 2 years and weighing at least 15 kg. Neither treatment had much efficacy against nits, which continue to hatch for 11 days. A second treatment is indicated, which the authors prescribed after 7 days. Ivermectin resistance may eventually develop; the authors therefore advocate its restriction to treatment of patients for whom topical insecticide has failed.

— Mark V. Dahl, MD

Published in Journal Watch Dermatology March 10, 2010

Citation(s):

Chosidow O et al. Oral ivermectin versus malathion lotion for difficult-to-treat head lice. N Engl J Med 2010 Mar 11; 362:896.

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