Follow up for 6 days is inadequate because louse eggs normally take 7 days to hatch. In cases where a second application of insecticide is required it should be given 7 days after the first treatment. Most investigators agree that assessment for absence of infestation at 14 days after treatment is appropriate to determine the primary endpoint of a study.

The three trials comparing chemical treatments included in the most recent systematic review were conducted in developing countries, where insecticide treatments were not regularly available.2 This may have resulted in greater efficacy, because the insects had not been subjected to any kind of selection pressure.

No RCT has yet considered that the formulation of a pediculicide might affect its activity. Studies in vitro suggest that other components of products (for example terpenoids and solvents) may contribute significantly to pediculicide activity, in some cases more than the insecticide itself.12

Resistance to one or more insecticides has now been identified in several countries.13-19 There are no data available to indicate the prevalence of resistance, and most studies have collected insects from only a few problem cases in making their evaluation of resistance. These cases are unlikely to be representative of the population at large. However, one RCT (193 people) compared malathion (0-5% lotion with terpenoids) with phenothrin (0-3% lotion) in a community where lice were identified in vitro as being tolerant of phenothrin.13 One day after treatment more people treated with phenothrin had lice (8/95 (8%) versus 59/98 (60%); RR 0-14, CI 0-07-0-3; NNT 2, CI 4-14) and this difference had increased by day 7 (6/95 (6%) versus 40/98

(41%); RR 0-15, CI 0-07-0-3; NNT 3, CI 3-15). However, some children not free from lice on day 1 had become louse free by day 7 in both groups, suggesting some parental intervention had influenced the results. Nevertheless, this study indicates that resistance to pyrethroid insecticide may have influenced around 60% of the treatments.

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