Most persons with head lice have fewer than 20 mature lice present at any given time, although the female louse can produce more than 100 eggs in her brief one-month life span. The eggs are glued in nit sacks to hair shafts. These nits typically are found near the scalp and hatch in about nine days. Away from a host, adult lice can survive up to three days. Fomite transmission of lice is thought to be rare. Lice usually are spread through direct head-to-head contact, a route that is efficient in children. Growing resistance to many of the pediculicides commonly used to treat head lice may contribute to the recently noted increase in cases of infestation. Jones and English summarize a Cochrane meta-analysis of head lice treatment and present a general review of the topic. The authors note that, because of strict inclusion criteria, the Cochrane meta-analysis included only four of the 71 randomized controlled studies available on head lice treatment.
The first pediculicide covered in the review is lindane, which was once widely used for treatment of lice infestation. Concerns regarding neurotoxicity (e.g., seizures) and increasing levels of resistance now have sharply limited its use.
Pyrethrins are available over-the-counter for the treatment of head lice. Two applications administered one week apart are required for treatment, because the compound does not kill the nit phase of the louse. Pyrethrins have no major safety concerns other than mild skin irritation, which occurs with other pediculicide agents as well. Some studies have shown low eradication rates, even when the lice do not demonstrate pyrethrin resistance.
Permethrin, a synthetic reformulation of pyrethrin that has enhanced stability, is presented by the authors as the treatment of choice for head lice. The Cochrane meta-analysis did not find any acceptable studies that showed permethrin to have superior efficacy. Two treatments one week apart are still recommended for permethrin. If treatment failure occurs, the authors suggest extending the time of topical application to overnight treatment under a shower cap or switching to malathion.
Malathion is rapidly lethal to lice, but because of safety concerns about flammability, the authors recommend it for second-line use only in cases refractory to permethrin treatment. Use of hair dryers or curling or straightening irons should be avoided during treatment. Malathion is not approved by the U.S. Food and Drug Administration (FDA) for use in children younger than two years.
Oral ivermectin is used for treatment of intestinal parasites, but recently has been proposed as an alternative agent for head lice infestation. An oral dosage of 200 mcg per kg, repeated in 10 days has been shown to be effective. To date, no serious side effects have been reported in persons using oral ivermectin for lice infestation, although it has not been approved by the FDA for this indication.
Regardless of the treatment mode, use of nit combs enhances the eradication rate. Application of vinegar or 8 percent formic acid to the hair for 10 minutes, followed by rinsing and drying, softens the nit glue and may aid in removal.