Head lice resistance is pervasive.
Over the past 40 years, several mechanisms for resistance have developed in human head lice rendering anti-parasitic agents ineffective—including Malathion, Lindane and now the most common OTC formulations.1
Neurotoxic agents (pyrethrins, pyrethroids, and DDT) that bind to voltage-sensitive sodium channels (VSSC), are rendered ineffective due to target site insensitivity known as knockdown resistance or kdr – one of the most common mechanisms of resistance in parasites.1
Increasing frequency of kdr-type mutations in head lice parallels the decreasing effectiveness of OTC formulations containing permethrin and pyrethroids.1,2
Since the late 1990’s, eight clinical studies demonstrated a declining rate of effectiveness of the permethrin-containing Nix® formulation, which no longer provides a level of effective control.1
View an in vitro assessment of knockdown resistance in action1
The three pyrethrin- and pyrethroid-resistant gene mutations are observed in an average of 98% of lice gathered from 48 states1 like these, shown 15 minutes after treatment with Nix® (permethrin 1%) as compared to an untreated group.*
Without administration of active agent.
Following administration of permethrin 1%.
*Source: Videos created in collaboration with J. Marshal Clark, et al, and the Department of Veterinary Animal Sciences, Molecular Pharmacology, Microbiology, and Biochemistry and Molecular Biology at the University of Massachusetts, Amherst
Nix® is a registered trademark of Prestige Brands, Inc.
- Gellatly KJ, Krim S,Palenchar DJ, et al. Expansion of the knockdown resistance frequency map for human head lice (phthiraptera: pediculidae) in the United States using quantitative sequencing. Journal of Medical Entomology, 2016:1-7.
- Yoon KS, Previte DJ, Hodgdon HE, et al. Knockdown resistence allele frequencies in North American head louse populations. J Med Entomology. 2014;51(2):450-457.