Etiology and Epidemiology
SUBSPECIESPediculus humanus capitis. Sesame seed size, 1 to 2 mm. Feed every 4 to 6 hours. Move by grasping hairs close to scalp; can crawl up to 23 cm/day. Lice lay nits within 1 to 2 mm of scalp. Nits are ova within chitinous case. Young lice hatch within 1 week, passing through nymphal stages and maturing to adults over a period of 1 week. One female can lay 50 to 150 ova during a 16-day lifetime. Survive only for a few hours off scalp. Transmission: Head-to-head contact; shared hats, caps, brushes, combs; theater seats; pillows. Head louse is not a vector of infectious disease.
DEMOGRAPHY Estimated that 6 to 12 million persons in the United States are infested annually. Claws have adapted to grip cylindrical hair; hair pomade may inhibit infestation. In Africa, pediculosis capitis is relatively uncommon.
SYMPTOMS Pruritus of the back and sides of scalp. Scratching and secondary infection associated with occipital and/or cervical lymphadenopathy. Some individuals exhibit obsessive-compulsive disorder or delusions of parasitosis after eradications of lice and nits.
INFESTATIONHead lice are identified by eye or by microscopy (hand lens or dermatoscope) but are difficult to find. Most patients have a population of < 10 head lice. Nits are the oval grayish-white egg capsules (1 mm long) firmly cemented to the hairs (Fig. 28-9); vary in number from only a few to thousands. Nits are deposited by head lice on the hair shaft as it emerges from the follicle. With current infestation, nits are near the scalp; with infestation of long standing, nits may be 10 to 15 cm from the scalp. In that scalp hair grows 0.5 mm daily, the presence of nits 15 cm from the scalp indicates that the infestation is approximately 9 months old. New viable eggs have a creamy-yellow color; empty eggshells are white. Sites of predilection: Head lice nearly always confined to the scalp, especially occipital and postauricular regions. Rarely, head lice infest the beard or other hairy sites. Although more common with crab lice, head lice can also infest the eyelashes (pediculosis palpebrarum).
SKIN LESIONSBite reactions: Papular urticaria on the neck. Reactions related to immune sensitivity/tolerance. Secondary lesions: Eczema, excoriation, or lichen simplex chronicus on the occipital scalp and neck secondary to chronic scratching/rubbing. Secondary infection with S. aureus of eczema or excoriations; may extend onto the neck, forehead, face, or ears. Posterior occipital lymphadenopathy.
MICROSCOPYNits 0.5-mm oval, whitish eggs (Fig. 28-9B). Nonviable nits show an absence of an embryo or operculum. Louse. Insect with six legs, 1 to 2 mm in length, wingless, translucent grayish-white body that is red when engorged with blood.
TOPICALLY APPLIED INSECTICIDESPermethrin, malathion, pyrethrin, ivermectin, and spinosad.
SYSTEMIC Oral ivermectin, levamisole, and albendazole.