26 Resistance to 1% permethrin has been reported, 6, 27, – 30 but its prevalence is unknown. 6, 25 An alternate treatment schedule on days 0, 7, and 13 to 15 has been proposed on the basis of the longest possible life cycle of lice for this and other nonovicidal agents (eg, pyrethrins plus piperonyl butoxide). 6 Although many repeat the application sometime between day 7 to 10 after treatment if live lice are seen, new evidence based on the life cycle of lice suggests that retreatment at day 9 is optimal. 24 However, conditioners and silicone-based additives present in almost all currently available shampoos impair permethrin adherence to the hair shaft and reduce its residual effect. Permethrin leaves a residue on the hair that is designed to kill nymphs emerging from the 20% to 30% of eggs not killed with the first application. It is left on for 10 minutes and then rinsed off. The product is applied to damp hair that is first shampooed with a nonconditioning shampoo and then towel dried. Permethrin is less allergenic than pyrethrins and does not cause allergic reactions in individuals with plant allergies. Reported adverse effects include pruritus, erythema, and edema. 23 Permethrin is a synthetic pyrethroid with extremely low mammalian toxicity. One percent permethrin lotion is one of the drugs available to treat head lice. 18 Introduced in 1986 as a prescription-only treatment, 1% permethrin lotion was approved for OTC use in 1990 and is marketed as a “creme rinse” (Nix Insight Pharmaceuticals, Trevose, PA). Permethrin has been the most studied pediculicide in the United States and is the least toxic to humans. 5 In general, eggs found more than 1 cm from the scalp are unlikely to be viable, although some researchers in warmer climates have found viable eggs farther from the scalp. Many presumed “lice” and “nits” submitted by physicians, nurses, teachers, and parents to a laboratory for identification were found to be artifacts, such as dandruff, hairspray droplets, scabs, dirt, or other insects (eg, aphids blown by the wind and caught in the hair). It is also important not to confuse live eggs with dead or empty egg cases (nits). It is important not to confuse eggs or nits, which are firmly affixed to the hair shaft, with dandruff, hair casts, or other hair debris, which are not. 17 Tiny eggs may be easier to spot at the nape of the neck or behind the ears, within 1 cm of the scalp. 16 Some experts have suggested using a lubricant (water, oil, or conditioner) to “slow down” the movement of lice and eliminate the possibility of static electricity. Studies have revealed that diagnosis of infestation by using a louse comb is quicker and more efficient. This can be difficult sometimes because lice avoid light and can crawl quickly. Identification of eggs (nits), nymphs, or adult lice with the naked eye establishes the diagnosis. If not treated, the cycle repeats itself approximately every 3 weeks. The female louse can mate and begin to lay viable eggs approximately 1.5 days after becoming an adult. Once it hatches, a nymph leaves the shell casing and passes through a total of 3 nymph stages (instars) during the next 9 to 12 days before reaching the adult stage. The eggs are incubated by body heat and typically hatch in 8 to 9 days, but hatching can vary from 7 to 12 days depending on whether the ambient climate is hot or cold. (Note that some experts refer to “eggs” as containing the developing nymph and use “nits” to refer to empty egg casings others use the term “nits” to refer to both eggs and the empty casings). Empty egg casings (nits) are easier to see because they appear white against darker hair. Viable eggs camouflaged with pigment to match the hair color of the infested person often are seen more easily at the posterior hairline. These tiny eggs are firmly attached to the base of the hair shaft within approximately 4 mm of the scalp with a gluelike substance produced by the louse. The female lives up to 3 to 4 weeks and, once mature, can lay up to 10 eggs per day. The adult head louse is 2 to 3 mm long (the size of a sesame seed), has 6 legs, and is usually tan to grayish-white in color.
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