Portal de Periódicos da CAPES

Head Lice

2002; American Academy of Pediatrics; Volume: 23; Issue: 1 Linguagem: Inglês

10.1542/pir.23-1-34

1529-7233

Diane E. Bloomfield, Holger Adam,

Dermatological diseases and infestations

Pediculosis is a skin infestation by blood-sucking lice, the most common of which is the head louse (Pediculus humanus var capitis). In the United States, an estimated 6 to 12 million people, predominately school-age children, become infested with head lice each year, with transmission occurring almost always by direct head-to-head contact. Infestation is not associated with either poor hygiene or socioeconomic grouping. Head lice do not carry disease, but infestation can cause significant pruritus, excoriation, scalp impetigo, and cervical and occipital lymphadenopathy. In addition, the condition can be a significant cause of school absences because children commonly are forbidden to attend school until free from all evidence of infestation.Successful treatment depends on an appreciation of the life cycle of the louse, which exists in three forms: egg, nymph, and adult. The adult is 3 mm long and has three pairs of legs that end in powerful claws. Adult lice must feed every 4 to 6 hours and can survive only for 24 hours away from their host. During its 1- to 2-month lifetime, the adult lays 300 ovoid, translucent eggs, which are cemented with chitin to the hair shafts at their proximal ends. The lice develop for 7 to 10 days within the operculated eggs, emerging as nymphs and leaving behind empty white shells, or nits, still attached to the hair shafts. The nymphs mature while feeding on the host scalp and become reproductive adults within 1 to 2 weeks.Through the stages of its life cycle, the louse benefits from a number of adaptive factors. The egg’s proximity to the scalp and its lack of color make early detection difficult, the embryo’s incomplete nervous system lessens its sensitivity to neurotoxic pediculicides, and the mature louse’s ability to close down its respiratory system for as long as 4 hours when exposed to water protects it during routine shampooing.The diagnosis of pediculosis, requiring evidence both of adult lice and their eggs, is made best by carefully inspecting the hair of a patient in a well-lit area and using a fine-tooth comb to examine the entire head. The presence of nits is not sufficient proof of an active infestation; egg remnants can persist even after treatment.Many pediculicides are available to eradicate head lice, with the effectiveness depending on both the individual agent and its formulation. Treatment should aim for 100% ovidical and insecticidal activity. In general, lotions and cream rinses, which deliver a high concentration of toxin in one application, are preferable to lower-concentration shampoos. All products should be applied to dry hair because adult lice become dormant when exposed to water, making them less susceptible to insecticide activity.The pyrethrins and permethrin are the agents used most commonly in the United States. Natural extracts from chrysanthemums, the pyrethrins are combined with a synergistic insecticide, piperonyl butoxide, either as a liquid or a shampoo. With limited ovicidal activity, pyrethrin-based products cannot be guaranteed to kill unhatched eggs and must be reapplied 1 week after the initial treatment. Permethrin, available as a 1% cream rinse, is a synthetic pyrethrin that has greater potency and less systemic absorption than its parent compound. It is both insecticidal and ovicidal at a rate greater than 90% and has the additional advantage of remaining active for 2 weeks after application. Nonetheless, a second treatment often is advised to kill any residual lice. Unfortunately, resistance to permethrin appears to be spreading.Lindane 1% shampoo has become a second-line treatment because of concerns about its neurotoxicity. In fact, toxicity rarely has been associated with proper use; it is more likely by far to result from overt ingestion or from prolonged exposure. All the same, it should be used with particular caution in children younger than 2 years of age and not at all in preterm infants or children who have seizure disorders. Because its ovicidal activity is limited, lindane should be reapplied 7 to 10 days after the first dose.Malathion, an organophosphate that irreversibly inhibits cholinesterase, is used commonly in the United Kingdom to treat head lice. It recently became available in the United States as a 0.5% lotion. In a double-blind trial, reported in The Medical Letter in 1999, a single 12-hour application of malathion was fully insecticidal, with 95% of patients remaining lice-free after 7 days. Another new treatment is oral ivermectin, an antihelminthic agent that is structurally similar to the macrolide antibiotics and can be effective as a single dose, but should be repeated after 10 days.In recent years, reports of treatment failure have become increasingly common worldwide. Although reinfestation and improper application of a pediculicide frequently underlie what appears to be a failure of treatment, growing resistance to pyrethrins and permethrin has become a reality. Strategies for treating resistant lice include switching to a different product or retrying the initial agent at a higher strength or for a longer application time.Following treatment, hair should be shampooed and combed while wet with a fine-tooth comb to remove lice and nits. A comb called the LiceMeister, with 1.5-inch long, tightly spaced teeth, is available from the nonprofit Pediculosis Association. Remaining nits can be soaked in a 1:1 mixture of white vinegar and water, followed by recombing. All children and adults in the household should be examined, although there is no advantage to treating close contacts unless infestation is detected. Although spread of lice almost always is by direct contact, many families feel more comfortable if clothing, bed linens, hair implements, and other possible fomites are disinfested; washing in hot water (>128°F [53.4°C]) or dry cleaning is effective.Comment: Probably the greatest morbidity stemming from louse infestation is school absence. Requiring children to be “nit-free” before returning to school is more of a harm than a help. Nits, the egg casings in which lice develop, are generally empty and do not contribute to reinfestation. Nits that actually harbor incubating lice are attached to hairs at the scalp line and are very difficult to remove by combing. A second application of pediculicide, 7 to 10 days after the first, will kill any newly emerged lice before they mature and lay eggs of their own. So much for nit-picking! (By the way, an “operculated egg” has a lid or cover capping it—I had to look it up.)