Head Lice: Diagnosis and Therapy

Gabriel J. Martinez-Diaz, MD; Anthony J. Mancini, MD


Dermatology Nursing 

In This Article

Etiology and Pathophysiology

The head louse is 1–2 mm long, wingless, and white-to-gray in color, living on average 30 days. It has a long, dorso-ventrally flattened, segmented abdomen with 3 pairs of clawed legs. The louse inserts its mouthparts into the skin to feed on the blood of the human host every 4–6 hours. In doing so, it injects saliva which results in an inflammatory reaction with resultant pruritus. Head lice move rapidly, traveling up to 23 cm/min, by grasping hairs and generally remaining close to the scalp (Ko & Elston, 2004). The female head louse lays eggs at night and can produce as many as 10 eggs per day. There is a predilection for the posterior hairline and post-auricular areas, and the eggs are deposited at the base of the hair shafts, within 1–2 mm of the scalp. The nit is attached with a highly insoluble cement. The end of the nit is topped with a tough but porous cap known as the operculum. This structure allows for gas exchange while the nymph develops in the casing (Rubeiz & Kibbi, 2009). The nit requires optimum conditions of 30° C and 70% humidity to hatch within an average time frame of 8–10 days; the incubation period may be longer at lower temperatures, and hatching usually does not occur if the temperature is lower than 22° C. Nits may survive for as long as 1 month away from the human host. After incubation, the nymph is hatched and undergoes three stages (first, second, and third "instar forms") of development before reaching its adult form 8–10 days later (Frankowski & Weiner, 2002; Janniger & Kuflik, 1993; Rubeiz & Kibbi, 2009). In general, nits located close to the scalp are viable and unhatched, although in warmer climates, viable ones may be found several inches away from the scalp (American Academy of Pediatrics [AAP], 2009).

Direct head-to-head contact is a common mode of transmission of head lice, but indirect transmission via fomites (such as brushes, combs, hair accessories, bedding, helmets, and headgear) is also well established (Burkhart & Burkhart, 2007). This fomite transmission contributes to the challenging cycle of head lice infestation. While hair length or frequency of grooming do not generally influence susceptibility to head lice infestation, cultures where hair grooming is performed frequently (such as in the United States) tend to have no more than a dozen live lice, in comparison to cultures with lessfrequent grooming practices, where a hundred or more live lice may be noted with an infestation (Frankowski & Weiner, 2002).


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