Treatment failure of over-the-counter pediculicide shampoos in Thai schoolgirls: Evidence of insecticide resistance

The increasing resistance to chemical pediculicides has raised concerns about the efficacy of head lice treatments, including in Thailand. This study evaluated the efficacy of three commercially available pediculicide shampoos (0.75% permethrin, 0.6% carbaryl, and 0.12% Stemona root crude extract) among infested children in Chonburi Province, Thailand. A pre-test/post-test experimental design was conducted with 135 infested female children assigned to three treatment groups (n = 45 each), with no significant differences in baseline demographic characteristics (level of head lice infestations, hairstyle, and hair length). Each group received two applications (Day 0 and Day 7) of the allocated pediculicide shampoo, following the manufacturer's instructions. The final proportion of cured individuals was assessed on Day 14 after treatment using a fine-tooth comb. The observed percentages of cured individuals were 42.22% for carbaryl shampoo, 24.44% for Stemona shampoo, and 6.67% for permethrin shampoo. No side effects were reported, although none of the shampoos achieved complete eradication, with particularly poor outcomes in children with heavy infestations. These findings provide clinical evidence of reduced efficacy of over-the-counter pediculicides in Thailand, consistent with emerging resistance trends. More effective alternatives with different mechanisms of action, including ivermectin, abametapir, and dimeticone-based products, should be considered, although their current availability in Thailand remains limited. In the interim, development of locally available herbal formulations, together with school-based screening and simultaneous treatment, represents more feasible strategies. These results highlight the need for updated treatment guidelines, restriction of ineffective products, and regular resistance surveillance to ensure effective control.