Physically acting products for head lice – the end of the beginning

Treatment of head louse infestation has evolved from widespread use of neurotoxic insecticides that have been extensively affected by resistance since the mid-1990s into the use of so-called physically acting treatments. It is widely believed that physically acting products are effectively “resistance proofed” because they do not act to inhibit any particular physiological mechanism and most have some kind of occlusive effect on the target organism. Over the past 20 years various new active materials have been utilized ranging from natural oils, synthetic oils, through to surfactants both as excipients and active substances. Relatively few of these products have been adequately tested clinically and, of those that have, there is now some indication that they are less effective than when first introduced. The question therefore arises whether lice can become resistant to these physically acting products. Only adequate testing both in the laboratory and in clinical trials can determine their real effectiveness and claiming efficacy based on the presence of a named chemical rather than demonstrated activity may result in acquired resistance to these types of product also.