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Head Lice Now Resistant to Common Treatments, Study Says

Head Lice Now Resistant to Common Treatments, Study Says

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The human head louse is now mostly resistant to common insecticide treatments, say researchers in a recent article in the Journal of Medical Entomology.1 Pediculus humanus capitis is well known to cause itching and annoyance, particularly for school children around the world, leading it to be one of the most prevalent human parasitic infestations. Here in the US, widely available over-the-counter topical treatments like Nix or Rid have been relied upon for several decades to rid human head hair of the small dark colored insect. The active ingredient of most of those OTC insecticides are pyrethrins or synthetic pyrethroids, the latter including permethrin and phenothrin.

For this study, volunteer school nurses and professional lice combers collected lice from 138 sites in 48 states between 2013 and 2015. Researchers then extracted and analyzed the DNA of the lice to determine the frequency of knockdown resistance (kdr)-type mutations. Such a mutation is associated with nerve insensitivity to insecticides like pyrethroids, pyrethrins, and DDT, chemicals that target the nervous system. Kdr-type mutations have already been identified in the common house fly and among the potato tuber moth, an agricultural pest.

While the identification of kdr-type mutations might not necessarily predict the failure of one of these insecticides to kill lice, its identification seems correlated with reports of product failures, the authors note. And product failure has been on the rise. Between 1984 and 1995, studies consistently found permethrin to have a 96 to 100 percent effectiveness in killing lice, but by 2001, a study reported the effectiveness at just 80 percent. The researchers describe recent studies that have shown effectiveness rates between 28 and 55 percent.2-6

This report’s findings were even graver. The overall mean percent resistance allele frequency — meaning, the percentage of lice with genes that were found with at least one kdr-type mutation — was 98.6 percent. Only one of the 138 sites collected lice with no mutations. Forty-two of the 48 states sampled had a mean resistance of 100 percent. Urban, suburban, and rural lice showed similar levels of resistance.

The study demonstrates that permethrinbased insecticides like Nix are no longer effective as a lice treatment. The study does not analyze or recommend alternative treatments nor does it explore the insecticides’ impact on human health.

The Centers for Disease Control and Prevention continues to recommend FDA-approved OTC treatments containing pyrethrins or permethrins, or to ask a doctor for a prescription medication. While some small peer-reviewed studies exist, more research needs to be conducted into alternative products.



  1. 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. J Med Entomol. 2016. Available online at: Accessed 31 October 2016.

  2. Burkhart CG, Burkhart CN. Clinical evidence of lice resistance to over-the-counter products. J Cutan Med Surg. 2000;4:199-201. 

  3. Hipolito R, Mallorca F, Zuniga-Macaraig Z, polinario P, Wheeler-Sherman J. Head lice infestation: single drug versus combination therapy with one percent permethrin and trimethoprim/sulfamethoxazole. Pediatrics. 2001;107:E30. 

  4. Meiking T, Vicaria M, Eyerdam D, Villar M, Reyna S, Suarez G. Efficacy of a reduced application time of Ovide lotion (0.5% malathion) compared to Nix creme rinse (1% permethrin) for the treatment of head lice. Pediatr Dermatol. 2004;21:670-674. 

  5. Stough D, Shellabarger S, Quiring J, Gabrielsen A. Efficacy and safety of spinosad and permethrin creme rinses for Pediculosis capitis (head lice). Pediatrics. 2009;124:e389-e395. 

  6. Kim C. Biostatistics team leader memorandum. US Department of Health and Human Services. Center for Drug Evaluation and Research. 2010. Available online at: Accessed 31 October 2016. 




[The material presented in this portion of Streamline is supported by a grant from theEnvironmental Protection Agency, Office of Pesticide Programs, Grant # x8-83487601.]


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