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Permethrin malathion resistance in head lice confirmed in Paris

Head lice collected from schoolchildren in Paris showed resistance to the topical insecticides permethrin and malathion that was supported by molecular testing in a recent study.

Researchers conducted the prospective, observational study of children from 74 Parisian elementary schools from January 2008 to June 2009. From 14,436 visually screened children, researchers collected 7,400 live head lice from 574 participants (3.98% prevalence; 95% CI, 3.45-4.50). The lice were submitted to ex vivo bioassays or underwent individual DNA extraction where fragments of knockdown resistance (kdr)-like gene were amplified and compared with wild-type sequences.

Using ex vivo assaying on 3,797 lice, none survived after 1 hour of exposure to 1% malathion lotion. When 1,531 lice were exposed to 0.3% permethrin lotion and 1% piperonyl butoxide for 1 hour, 1,312 died (85.7% effectiveness; 95% CI, 83.9-87.5). At shorter exposures, fewer head lice died, particularly when subjected to permethrin/piperonyl butoxide compared with malathion (15 minutes, 74.9% effectiveness vs. 87.3%).

Among the 670 head lice with workable DNA sequences, 661 lice (98.7% prevalence; 95% CI, 97.7-99.3) had homozygous kdr mutations.

“The high occurrence of kdr mutant allele suggests an already strongly established insecticide resistance in Paris,” the researchers said, “but this may not correlate with treatment failure in prospective studies. The very high proportion of kdr mutant allele … would advocate for the abandonment of pyrethroids as head lice treatment, at least in the studied geographic area.”