Clinical update on resistance and treatment of Pediculosis capitis.

The anatomy and physiology of head lice make them extremely adaptable to their human hosts but also difficult to eradicate. Their coloring and small size make them difficult to see, and the larvae, or nymphs, have multiple exoskeletons for protection. When exposed to pediculicides, a nymph can shed its outer exoskeleton and receive only a sublethal dose. Through natural selection, head lice have developed resistance to commonly used pediculicides, making it increasingly difficult to eliminate infestations. Other contributors to resistance include changes in formulations of pediculicides and improper use. Over the last 20 years, the efficacy of pyrethrins has declined because of necessary safety changes in formulations and unintentional effects of new packaging on chemical components of these products. Pediculicides designed to be applied to wet hair may become too diluted to produce the desired effect. Patients also may use too little product to conserve costs. The combination of decreased product efficacy and exposure of head lice to diluted or insufficient amounts of pediculicides has reduced their effectiveness substantially. Studies comparing the current efficacy of several over-the-counter and prescription pediculicides with effectiveness in the 1980s has shown that OVIDE (malathion) Lotion, 0.5% is the only product that has retained its efficacy over time. The efficacy of over-the-counter pyrethrin products had declined significantly, and lindane, another prescription product, was least effective of all products tested both in the 1980s and in more recent studies. In addition to retaining its efficacy, malathion 0.5% also is effective when applied for only 20 minutes. The shorter application time is safer for children and also decreases the likelihood that residual product remains in the hair, further contributing to resistance.