Measuring local genetic variation in permethrin resistant head lice, pediculus humanus capitis (Phthiraptera: Pediculidae), from Buenos Aires, Argentina

Authors: TOLOZA, ARIEL - National Scientific And Technical Research Council (CONICET); Ascunce, Marina; REED, DAVID - University Of Florida

Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2024
Publication Date: 4/18/2024
Citation: Toloza, A.T., Ascunce, M.S., Reed, D. 2024. Measuring local genetic variation in permethrin resistant head lice, pediculus humanus capitis (Phthiraptera: Pediculidae), from Buenos Aires, Argentina. Journal of Medical Entomology. 18(11). https://doi.org/10.1093/jme/tjae048.
DOI: https://doi.org/10.1093/jme/tjae048

Interpretive Summary: Human lice are blood-sucking, wingless, ectoparasites of humans that affect mostly school-aged children with hundreds of millions of infestations reported every year. Methods of control include the use of pediculicides based on the chemical permethrin. When this chemical is used repeatedly over time to control insects, genetic mutations arise among the insect populations that allow the insects to become resistant to permethrin. This is what has been happening with human lice, and now in many regions of the world, louse populations are resistant to permethrin. In this study, we focused on understanding the relationship between the toxicological profile through topical applications of permethrin to each louse, then we used the same louse to do DNA extraction and obtained the description of the alleles responsible of the permethrin resistance, as well as its genetic diversity at nuclear polymorphic loci. We included lice found on heads of school children from four schools in the city of Buenos Aires. The frequency of the resistant mutation was 87.31% and school louse populations showed different resistance level to permethrin. Nuclear genetic diversity indicated that louse populations underwent recent reductions in population sizes, which may be the result of permethrin use in the past. However, there was also ongoing gene flow among those schools, suggesting that as a whole all the schools could represent a meta-population, where the different schools behave as a subpopulation. These results have implications for the spread of resistant alleles and should be considered to improve lice control and resistance monitoring programs.

Technical Abstract: The cosmopolitan human head louse, Pediculus humanus capitis (De Geer), is an ectoparasite affecting mostly school-aged children with infestations reported every year partially due to the failure of control methods as a consequence of louse resistance to pyrethroids. The main resistant mechanism of pyrethroids consists of the target site insensitivity (kdr) that is caused by single nucleotide point mutations (SNPs) located in the voltage-sensitive sodium channel (VSSC). In this study, we analyzed individual head louse toxicologically via the description of its susceptibility profile to permethrin and genetically through the genotypification of its kdr-alleles as well as nuclear microsatellite loci. The resistant ratios varied from 33.3 to 71.4% with a frequency of the T917I kdr mutation of 87.31% and with 83.6% of the head lice being homozygous resistant to pyrethroids. Microsatellite data indicated that all the populations had genotype proportions that deviated from Hardy-Weinberg expectations with FIS > 0 reflecting a deficit of heterozygotes. Bottleneck analysis suggested that all louse school populations underwent recent reduction/s in population sizes, while three out of the four schools had gene flow values around 1, indicating ongoing gene flow among those schools. Our study suggests that louse school populations may form a metapopulation, where each school represents a small population that undergoes extinction and re-colonization processes under strong permethrin selection. This is the first multilevel analysis integrating toxicological, kdr-genotyping and microsatellites data in human louse populations. Its results underpin the importance of this multilevel approach to improve lice control programs and resistance monitoring.