Topical Ivermectin Kills Most Lice With a Single Blow

Jenni Laidman

November 01, 2012

Topical ivermectin proved effective against head lice after a single treatment, according to results from a pair of placebo-controlled trials published in the November 1 issue of the New England Journal of Medicine. The new data suggest the drug could help knock down the pest, which is often resistant to first-line treatments.

In February 2012, the US Food and Drug Administration approved the lotion, sold under the trade name Sklice lotion (Sanofi Pasteur) for the treatment of lice based on these data.

David M. Pariser, MD, professor in the Department of Dermatology, Eastern Virginia Medical School, Norfolk, and colleagues conducted 2 multisite, randomized, double-blind studies that compared a single topical application of 0.5% ivermectin lotion with a vehicle control among 765 patients aged 6 months or older. The ivermectin lotion or a sham treatment was applied once to dry hair, left for 10 minutes, and then rinsed out.

Every member of a household who had 1 louse or more received the same treatment. In these studies, the researchers analyzed the percentage of index patients (the youngest person in the household with 3 or more live lice) who were louse-free the day after treatment and remained so on days 8 and 15. There were 132 index patients, 66 in each study group.

Ivermectin-treated patients fared considerably better than those using the vehicle control, with 94.9% louse free the day after treatment compared with 31.3% of the control patients. On day 8, 85.2% of the index patients were louse-free compared with 20.8% of the control group, and on day 15, 73.8% of the ivermectin patients were louse-free compared with 17.6% of the control group (P < .001 for each comparison).

Although the authors note that at least 2 prior reports have shown efficacy for topical ivermectin, neither report was adequate to determine the degree of efficacy or the possibility of reinfestation. More commonly, ivermectin has been tested as an oral medication and has been found effective against head lice.

Richard Pollack, PhD, a public health entomologist and president of IdentifyUs LLC, says ivermectin "has been used many, many years off label." This study shows "[i]t's another tool in the arsenal of products from which a prescriber can select to help eliminate lice."

More lice are showing resistance to permethrin and pyrethrins, which have long been the first choice against head lice. The US Food and Drug Administration recently approved 2 new head lice treatments: benzyl alcohol and spinosad. Other treatments, such as lindane and malathion, raise some concerns about safety, flammability, and unpleasant odor, the authors state.

Even these products may not be enough, the authors warn. "However, the short generation time of head lice and the exposure of all life-cycle stages to any applied treatment are predisposing factors to the emergence of resistance; therefore, new therapies are needed," the authors write.

The authors add that ivermectin performed as well as the recently approved benzyl alcohol lotion and spinosad suspension against head lice. They also speculate that the gradual increase in the number of live lice found as the time from treatment lengthened may be the result of several factors, including improper product application, inadequate exposure of louse eggs to ivermectin, and reinfestation of patients who are still exposed to head lice from other sources.

Topaz Pharmaceuticals provided study medications and one author received consulting fees from Topaz and Sanofi Pasteur. One author is a former Topaz employee and has received consulting fees and stock or stock options from Topaz and consulting fees and payment for manuscript preparation from Sanofi Pasteur. Dr. Pollack has received consulting fees from several pharmaceutical companies in the past, including from Topaz in 2008.

N Engl J Med. 2012;367:1687-1693. Abstract