Nymphs and adults of all three kinds of human lice feed at least once a day, ingesting human blood exclusively. Head lice (Pediculus capitis) infest mainly the hair of the scalp, body lice (Pediculus humanus) the clothing, and crab or pubic lice (Pthirus pubis) mainly the hair of the pubis.

The saliva of lice produces a pruritic morbilliform or urticarial rash in some sensitized persons. Female head and pubic lice cement their eggs (nits) firmly to hair, whereas female body lice cement their eggs to clothing, particularly to threads along clothing seams. After ~10 days of development within the egg, a nymph hatches. Empty eggs may remain affixed for months thereafter.

In North America, head lice infest ~1% of elementary school-age children. Head lice are transmitted mainly by direct head-to-head contact rather than by fomites such as shared headgear, bed linens, hairbrushes, and other grooming implements. Chronic infestations by head lice tend to be asymptomatic. Pruritus, due mainly to hypersensitivity to the louse’s saliva, generally is transient and mild. Head lice removed from a person succumb to desiccation and starvation within ~1 day. Head lice are not known to serve as a natural vector for any pathogens.

Body lice remain on clothing except when feeding and generally succumb in ≤2 days if separated from their host. In most Western countries, body lice are generally found on a small proportion of indigent persons but may become increasingly prevalent after upheaval associated with natural or human-caused disasters, when homeless victims are in close contact with infested individuals with whom they share accommodations. Body lice are acquired by direct contact or by sharing of infested clothing and bedding. These lice are vectors for the agents of louse-borne (epidemic) typhus), louse-borne relapsing fever, and trench fever. Pruritic lesions from their bites are particularly common around the neckline. Chronic infestations result in a postinflammatory hyperpigmentation and thickening of skin known as vagabond’s disease.

The crab or pubic louse is transmitted mainly by sexual contact. These lice occur predominantly on pubic hair and less frequently on axillary or facial hair, including the eyelashes. Children and adults may acquire pubic lice by sexual or close nonsexual contact. Intensely pruritic, bluish macules ~3 mm in diameter (maculae ceruleae) develop at the site of bites. Blepharitis commonly accompanies infestations of the eyelashes.

Pediculiasis is often suspected upon the detection of nits firmly cemented to hairs or in clothing. Many bona fide nits, however, are dead or hatched relics of prior infestation, and pseudo-nits are frequently misconstrued to be signs of a louse infestation. Confirmation of a louse infestation, therefore, best relies on the discovery of a live louse.