Researchers pinpoint cause of resistance to lice treatments

The use of pyrethrins and pyrethoid-based products to treat head lice may have helped to create selection pressure for resistant strains of the pest in North America, according to new research.

A research team led by Kyong-Sup Yoon, a postdoctoral fellow at the University of Massachusetts Amherst, did genotyping on DNA extracted from samples of human head lice collected from 32 locations in the United States and Canada. They found that the frequency of a knockdown-type resistance allele T917I (TI) was high in North American lice (88.4 percent). When they focused on U.S. lice, TI frequency overall was 84.4 percent from 1999 to 2009 and increased to 99.6 percent from 2007 to 2009. In Canadian lice, TI frequency was 97.1 percent in 2008.

The researchers conclude that the high frequency of TI mutation in human head lice in North America may be the result of selection pressure caused by the widespread longtime use of pyrethrins- or pyrethroid-based pediculicides.

“The frequencies of TI in North American head louse populations were found to be uniformly high, which … is likely a main cause of increased pediculosis and failure of pyrethrins- or permethrin-based products in Canada and the United States,” the authors wrote. “Alternative approaches to treatment of head lice infestations are critically needed.”

The study was published in the Journal of Medical Entomology.