Head lice resistant to both malathion and permethrin have been reported in a study by Dr Anthony Downs (department of dermatology, Bristol Royal infirmary) and colleagues. In addition, they found a high level of resistance to both insecticides, although head lice remained fully sensitive to carbaryl (British Journal of Dermatology 1999;141:508). This is the first report of doubly resistant head lice, they say.
The authors compared the insecticide survival rates of samples of head lice collected from 3,300 children at schools in Bath and Bristol with laboratory reared, fully insecticide-sensitive body lice. They report that there were significant differences between the survival times of laboratory and collected lice for malathion and permethrin, but not for carbaryl. They also tested two over-the-counter (OTC) products for head lice (1 per cent aqueous malathion and 1 per cent aqueous permethrin) in children aged seven to nine years. In this treatment group, there was an 87 per cent failure rate for permethrin and a 64 per cent failure rate for malathion.
Excessive application of head lice treatments increases the development of insecticide resistance, the authors say. There are no safeguards to prevent the "indiscriminate use" of malathion and permethrin due to their availability as OTC products. The product with no resistance problems, carbaryl, is prescription only. Its use should remain restricted because of its mutagenic properties, they add. The authors suggest that a specific malathion resistance mechanism could be in operation because it binds to acetylcholinesterase at the same site as carbaryl, yet malathion-resistant insects remained sensitive to carbaryl. They conclude that new products, for example the novel insecticides fipronil and imidacloprid,.need to be rapidly evaluated for introduction into the market.