Treating Pediculosis—Those Nit‐Picking Details

In this issue of Pediatric Dermatology, Meinking et al present data regarding the efficacy of a gel formulation of malathion and remind us of the importance of emerging resistance to pediculicides. Permethrin resistance was reported in Israel in 1994, where evidence of delayed in vitro killing was correlated with reports of clinical failure (1). Assays for judging resistance have assessed either the concentration required to kill lice or the time required to kill lice at a fixed concentration. Both methods are valid and neither is perfect. Assays for genetic mutations offer a third method for assessing the emergence of resistant lice. In particular, gene mutations such as T929I and L932F have been associated with knockdown resistance to pyrethroids (2). Knockdown resistance relates to voltage-gated sodium channels and translates to nerve insensitivity to an agent. It manifests as a lack of immobilization of the lice, but also results in delayed killing of lice. Cross-resistance among pyrethroids is common (3). Therefore, new pyrethroid preparations are likely to add little to our existing armamentarium. In contrast, malathion resistance appears to be relatively uncommon in the United States at present, and new forms of malathion that are easier to use are a welcome addition. Agents such as carbaryl and pirimiphos often remain active in the face of pyrethroid resistance, but such agents are not readily available in the United States (4,5). In Britain, d-phenothrin resistancewas reported in 1994 (6). Malathion retained activity at that time, but double resistance to permethrin and malathion is becoming increasingly common in Britain (7). Multidrug-resistant lice are likely to become an increasing problem in the United States as well. Considering our limited choices for the treatment of lice, new alternatives are badly needed. Hipolito et al reported efficacyof trimethoprim/sulfamethoxazole when added to a permethrin regimen (8). Unfortunately, a more recent study found no benefit to adding sulfa to a lindane regimen (9). Veterinary agents, such as fipronil (commonly used as a topical flea-preventative) and imidacloprid, show some promise (10) but human toxicology data are sparse and I do not advocate use of these agents on children. Piperonal has been used as a pediculicide and also exhibits a repellent action against lice (11). Data regarding piperonal are limited, and no commercial form of this agent is available in the United States. Essential oils have been shown to have some efficacy against lice (12), but these agents are potential contact allergens. Although chemical agents remain the current gold standard for lousemanagement, physical agentswith less potential toxicity have considerable appeal. Goates et al (13) evaluated a variety of ‘‘hot air’’ devices and found that all six methods tested resulted in high egg mortality, but the methods showed more variable success in killing hatched lice. Any method that kills ova is helpful, as ova are relatively resistant to our existing chemical agents. The downside of hot-air treatment is that a 30-minute application may be necessary, and compliance may be poor. Cetaphil lotion and a blow dryer were reported to be effective in one study, but the relative contribution of each remains to be studied (14).