Overexposure to insecticides has bred resistance in the parasites, making it harder than ever to treat infestation
By Karen Weintraub | Scientific American June 2017 Issue
Karen Sokoloff finds a certain satisfaction in picking lice off a person’s scalp, smoothing olive oil into the hair strands and carefully pulling a metal comb through them to catch the stragglers. It’s a good thing she enjoys it: Sokoloff co-founded LiceDoctors, one of a handful of national chains of lice pickers, and business is booming, in part because conventional treatments have become largely ineffective.
For decades people have turned to special over-the-counter shampoos containing plant-derived insecticides known as pyrethrins or their synthetic counterparts, called pyrethroids, to treat cases of head lice. When they first came to market, these products worked well. But sustained use of these same few chemicals has allowed the blood-sucking parasites to evolve widespread resistance to them. Indeed, a recent study of lice in the U.S. carried out by pesticide toxicologist John Marshall Clark of the University of Massachusetts Amherst and his colleagues found that two thirds to three quarters of them are immune to the effects of these insecticides. They have become “super lice.”