24 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. 23 A repeat application is recommended between day 9 and 10 after initial treatment if live lice are seen in the interim, with evidence based on the life cycle of head lice suggesting that retreatment at day 9 may be ideal. Permethrin leaves a residue on the hair that is designed to kill nymphs emerging from the 20% to 30% of nits not killed with the first application. Hair should not be shampooed for 24 to 48 hours after application. After washing the hair with a nonconditioning shampoo and towel drying, the product is applied to damp hair (saturating the scalp and working outward to the ends of the hair), left on for 10 minutes, and then rinsed off. Conditioners and silicone-based additives present in many shampoos should be avoided on the day of application, because these impair permethrin adherence to the hair shaft and reduce its residual effect. It is approved for use for individuals 2 months and older and is regarded as the drug of choice for treatment of head lice during pregnancy. 22 Permethrin lotion (1%) is marketed as a “creme rinse” (Nix and generics). Permethrin, a pyrethroid, is the most widely used and studied pediculicide in the United States. In general, nits found more than 1 cm from the scalp are unlikely to be viable. 5 It is also important not to confuse live nits with dead nits, because the latter do not represent a treatment failure. Many presumed lice and nits submitted by physicians, nurses, teachers, and caregivers 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 important not to confuse nits, which are firmly affixed to the hair shaft, with dandruff, hair casts (pseudonits), or other hair debris (including piedra, which is a fungal infection of the hair shaft), which are not. 17 Tiny nits may be easier to spot at the nape of the neck or behind the ears, within 1 cm of the scalp. Some experts have suggested using a lubricant (water, oil, or conditioner) to “slow down” the movement of lice. The ends of the comb teeth may be sharp, so care should be taken to not scrape or abrade the scalp. 16 The ends of the comb teeth should contact the scalp initially before being pulled toward the end of the hair. 15 Studies have revealed that diagnosis of infestation by using a louse comb is quicker and more efficient than a standard comb, because the very fine teeth of the louse comb are able to separate the hair strands to pull off the lice and nits. Viewing lice on the head can be difficult sometimes because lice avoid light and can crawl quickly, and a typical affected scalp will have fewer than 10 live lice. Identification of nits, nymphs, or adult lice with the naked eye, which can be assisted by handheld magnification, 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 ∼ 1.5 days after becoming an adult. Once it hatches, a nymph leaves the nit and passes through a total of 3 nymph stages (instars) during the next 9 to 12 days before reaching the adult stage. These 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. Empty 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 ∼ 2 mm of the scalp with a glue-like substance produced by the louse. The female louse lives up to 3 to 4 weeks and, once mature, can lay up to 10 eggs per day. 5 Although some experts use “nits” to refer to only empty casings, this report will use nits to encompass both eggs and empty shell casings, with some exceptions to use “eggs” when describing the reproductive cycle of the louse. 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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