Permethrin failed two-thirds of children with head lice

Design

single-blind randomised controlled trial with allocation by school across three primary schools in Thailand

Population

85 girls aged 6-13 with active pediculosis capitis; 66 in the per-protocol analysis

Primary outcome

cure at day 9, defined as absence of live lice and viable nits

Effect

oral ivermectin 95.5% (21/22), permethrin 1% 37.0% (10/27), single-dose 4% dimethicone 29.4% (5/17), p<0.001

Permethrin 1% remains the default for head lice in most of Asia, and resistance has been reported for years without much changing in practice. This single-blind randomised trial in three primary schools in Chachoengsao province, Thailand, enrolled 85 girls aged 6 to 13 with active infestation, allocating schools to oral ivermectin 200 micrograms per kilogram on days 0 and 7, permethrin 1% shampoo on days 0 and 7, or a single application of 4% dimethicone gel.

Of 66 analysed per protocol, cure at day 9 — no live lice and no viable nits — was achieved in 95.5% on ivermectin (21 of 22), 37.0% on permethrin (10 of 27) and 29.4% on dimethicone (5 of 17), p<0.001. Dimethicone cleared 88.2% of parasites on day 0 but 58.8% were reinfested by day 9, which says more about a single-dose protocol and an unchanged environment than about the agent. Adverse events were mild in all groups.

Randomisation was by school rather than by child, so the comparison carries whatever differed between the three schools, and 66 children in three arms is small. But a 37% cure rate for the standard of care is not a subtle finding, and it matches what clinicians in similar settings describe. Where permethrin is failing repeatedly, oral ivermectin two doses a week apart is a reasonable next step in children over 15 kg, alongside treating the household and wet combing.

• Where permethrin has failed once, do not simply repeat it — resistance is the likelier explanation
• Oral ivermectin 200 micrograms/kg on days 0 and 7 was far more effective in this population
• Ivermectin is not recommended under 15 kg; check weight before prescribing
• A single dose of dimethicone is insufficient — reinfestation reached 58.8% by day 9
• Randomisation was by school, not child, and the trial is small; treat the size of the difference, not the exact rate, as the finding

The statistics, in plain English

The analysis is per protocol on 66 of 85 enrolled, which favours the treatments people completed and excludes those who dropped out — an intention-to-treat analysis would likely narrow the gap. Allocating whole schools rather than individual children means the arms may differ systematically in crowding, income and reinfestation pressure, and with only three clusters that cannot be adjusted for. Cure rates of 95.5% from 22 children and 29.4% from 17 carry wide uncertainty, but a difference of nearly 60 percentage points is far larger than that uncertainty.

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