What to Do About Mutant "Super Lice"?

Researchers have confirmed what pediatricians have suspected for years: As many as 99.6% of head lice in the United States and Canada are now genetically resistant to over-the-counter pyrethroid pediculicides.

A team of researchers examined lice from 32 North American sites from 2007 to 2009 and found that almost all of the parasitic insects have a genetic mutation making them immune from the effects of the topical pyrethrin- and permethrin-based medications.¹

Analysis of the lice's DNA showed that 88.4% of insects in the United States and Canada carried the immunity-conferring TI genetic mutation. The researchers found that overall, in the 10 years between 1999 and 2009, 84.4% of U.S. lice carried the mutation, but this figure doesn't tell the whole story: In the 2 years from 2007 to 2009, the prevalence of the TI mutation had increased to 99.6% in U.S. lice; in 2008, 97.1% of Canadian lice had the mutation.

The findings of their research were published in the March issue of the Journal of Medical Entomology.

While not associated with significant morbidity, lice infestation has major public health consequences. The Centers for Disease Control and Prevention estimates that 6 million to 12 million infestations occur annually in the United States among children 3 to 11 years of age.² Pediculosis and its characteristic intense itching and possible associated secondary infections are among the top reasons for U.S. children missing school.

Head lice (Pediculus humanus capitis) is a model of "survival of the fittest": While years ago nearly all lice were susceptible to the single widely used pediculicide, the individual parasites best equipped to survive exposure have passed their immunity-conferring TI mutation down to subsequent generations of lice over the years, leading to near-universal "knockdown resistance" today. In fact, the authors note that the progression to today's "super lice" began shortly after World War II, well before the introduction of permethrin, when DDT came into use as a treatment for infestations of body lice (Pediculus humanus humanus). DDT exposure also began to select for the TI mutation-containing population of the very closely related head louse.

The authors conclude, "Alternative approaches to treatment of head lice infestations are critically needed." While lice have become knockdown resistant to pyrethroid compounds, a number of new and effective prescription pediculicides have become available in recent years, including benzyl alcohol formulations (e.g., Ulesfi), ivermectin (e.g., Sklice), and spinosad (e.g., Natroba). None of these medications are available over-the-counter, however.

—Michael Gerchufsky

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References:

1. Yoon KS, Previte DJ, Hodgdon HE, et al. Knockdown resistance allele frequencies in North American head louse (Anoplura: Pediculidae) populations. J Med Entomol. 2014;51(2):450-457.

2. US Centers for Disease Control and Prevention. Parasites – Lice. http://www.cdc.gov/parasites/lice/. Updated September 24, 2013. Accessed March 18, 2014.

CDC PHOTO