Almost fixation in knockdown resistance alleles to pyrethroids in head louse (Pediculus humanus capitis) populations from Argentina

The human head louse, Pediculus humanus capitis De Geer (Phthiraptera: Pediculidae), is an obligate ectoparasite that causes Pediculosis capitis and has been in contact with humans since the beginning of humankind. Head louse infestations are widespread throughout the world, and have been increasing since the beginning of the 1990s partially due to ineffectiveness pediculicides. In Argentina, the overuse of products containing permethrin (a common used synthetic pyrethroid) in the last three decades has led to the development of resistant louse populations. Pyrethroid insecticides act on the nervous system affecting voltage-sensitive sodium channels (VSSC). Three point mutations at the corresponding amino acid sequence positions M815I, T917I and L920F in the voltage-gated sodium channel gene are responsible for contributing to knockdown resistance (kdr). The management of pyrethroid resistance requires either early detection or the characterization of the mechanisms involved in head louse populations that are essential to perform optimal control strategies. In the present study, we estimated the geographical distribution of kdr alleles in 154 head lice from six geographic regions throughout Argentina. Pyrethroid resistance kdr alleles were found in high frequencies ranging from 67 to 100%, reaching almost fixation. Of these, 131(85.1%) were homozygous resistant, 13 (8.4%) were homozygous susceptible, and 10 (6.5%) were heterozygous. Exact tests for Hardy-Weinberg equilibrium for each location showed that genotype frequencies differed significantly from expectation in 4 of the 6 sites studied (67%). These results show that pyrethroid resistance is well established and has evolved rapidly among head louse populations from Argentina.