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Study entitled Evidence for double resistance to
permethrin and malathion in head lice published in British Journal of
Dermatology 1999 Sep;141(3):508-511.
Downs AM, Stafford KA, Harvey I, Coles GC
Departments of Dermatology
A rising prevalence of head lice among school children
and rising sales of insecticides with anecdotal evidence of their treatment
failure, led us to examine whether head lice in Bristol and Bath were resistant
to the insecticides available for treating head lice. Ten schools in Bristol and
Bath were visited to collect field samples of head lice. A comparison was made
of the survival rates of fully sensitive laboratory reared body lice and field
samples of head lice on insecticide exposure. To confirm the in vitro relevance
of these tests we performed supervised treatments of affected subjects with
malathion or permethrin. There were significant differences (P < 10-6 Fishers
exact test) between head and body lice survival for malathion and permethrin
exposure, but not for carbaryl. There was an 87% failure rate for permethrin and
a 64% failure rate for malathion with the topical treatment of a selected number
of infested school children. We conclude that there is a high resistance to
permethrin and malathion, but head lice remain fully sensitive to carbaryl. This
is the first report of doubly resistant head lice. As permethrin, phenothrin (a
very similar synthetic pyrethroid) or malathion are the active ingredients in
all the over-the-counter head lice treatments in the U.K., then it is likely
that head lice prevalence will continue to increase. The resistance against
permethrin employed by the head louse is probably the kdr (knockdown resistance)
mechanism, and an enzyme-mediated malathion-specific esterase is the likely
mechanism against malathion.
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