Molecular detection of the kdr T917I mutation in head lice (psocodea: pediculidae) from Malaysia and its increasing frequency over a decade.

Pediculosis capitis remains a common ectoparasitic infestation in Malaysia, particularly among children in overcrowded and resource-limited settings. Permethrin-based pediculicides continue to be widely used; however, global reports of knockdown resistance (kdr) mutations in the voltage-sensitive sodium channel gene raise concerns regarding reduced treatment efficacy. Despite extensive epidemiological data on infestation by head lice (Pediculus humanus capitis De Geer) in Malaysia, the molecular kdr status of local populations has not previously been examined. This study provides a temporal molecular evaluation of the kdr T917I mutation by comparing head lice collected from underprivileged children in the Klang Valley in 2015 and 2025. Genotyping was performed using Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-RFLP) targeting the T917I locus, with bidirectional sequencing for confirmation. In 2015, head lice were predominantly homozygous susceptible (SS, 96.8%), with a low proportion of heterozygous (RS, 3.2%) and no homozygous resistant (RR) genotypes detected. In contrast, the 2025 collection showed a marked shift in genotypic distribution, with SS decreasing to 60.6% and RS increasing to 39.4%, while RR genotypes remained absent. Correspondingly, the resistance-associated allele frequency increased from 1.6% in 2015 to 19.7% in 2025. Several establishments sampled in 2025 exhibited heterozygote excess and deviations from Hardy-Weinberg equilibrium; however, given differences in sampling structure between collection periods, these patterns should be interpreted cautiously and may reflect heterozygous treatment exposure or population substructure rather than directional selection alone. This study provides the first molecular detection and temporal comparison of the T917I mutation in Malaysian head lice populations and establishes baseline data for future resistance monitoring.