pediculosis

**Etiology / Organisms (obligate human ectoparasites, wingless insects)** - *Pediculus humanus capitis* (head louse): scalp and hair. Spread by direct head-to-head contact and fomites (combs, hats). Most common in school-aged children. Not a vector for disease. Less common in people of African descent because of hair shaft shape. - *Pediculus humanus corporis* (body louse): lives in clothing seams and visits the skin only to feed. Associated with poor hygiene, overcrowding, homelessness, war, and refugee camps. - *Pthirus pubis* (pubic or "crab" louse): pubic hair, but also axillae, eyelashes, and eyebrows. Transmitted sexually, so screen for other STIs. Infestation of the eyelashes in a child raises concern for sexual abuse.

**Life cycle** - Eggs (nits) are cemented to the hair shaft near the scalp. They hatch in about 7–10 days into nymphs, which mature in about 7–10 days. - Adults live about 30 days on the host but die within 1–2 days off the host. - Head lice do not jump or fly. They crawl.

**Pathophysiology** - Lice feed on blood. The itch comes from a hypersensitivity reaction to saliva and feces. - Chronic body louse infestation can cause "vagabond's disease": hyperpigmented, thickened, excoriated skin. - Pubic lice can leave bluish-gray macules (maculae ceruleae) at bite sites.

**Clinical features** - Pruritus, often worst at night, with excoriations and possible secondary bacterial infection (impetigo). - Head lice: itching of the scalp, posterior neck, and behind the ears. Nits are visible on the hair shaft. - Body lice: itching on the trunk, axillae, and waist, with excoriations. Lice are found in clothing seams. - Pubic lice: pubic itching. Lice or nits are seen on the hair.

**Diagnosis** - Clinical. Finding a live louse confirms active infestation, and a fine-toothed comb makes this easier. - Nits alone, especially those more than 1 cm from the scalp, may represent old or non-viable infestation. - Microscopy of the louse or nit can confirm it.

**Vector-borne diseases (body louse)** - *Rickettsia prowazekii*: epidemic typhus. It presents with fever, headache, and a centrifugal rash that begins on the trunk and spreads to the extremities. Recrudescence years later is Brill-Zinsser disease. Treat with doxycycline. - *Bartonella quintana*: trench fever. It causes a relapsing fever every ~5 days and bacillary angiomatosis in immunocompromised patients. - *Borrelia recurrentis*: louse-borne relapsing fever. It relapses because of antigenic variation of surface proteins (variable major proteins).

**Treatment** - Head and pubic lice: topical permethrin 1% (first-line, repeat in 7–10 days) or pyrethrins with piperonyl butoxide. Alternatives include malathion, benzyl alcohol, spinosad, ivermectin lotion, and oral ivermectin. - Permethrin acts on neuronal sodium channels and causes paralysis. - Ivermectin acts on glutamate-gated chloride channels. - Malathion is an organophosphate that inhibits acetylcholinesterase. - Body lice: improve hygiene and launder clothing and bedding in hot water (at least 130°F/54°C) and dry on high heat. Pharmacologic treatment is usually unnecessary. - Treat close contacts and sexual partners. For pubic lice, evaluate for other STIs. - Eyelash involvement: use petrolatum ointment, not permethrin.

**High-yield pearls** - Body lice are the only lice that transmit disease. - Pediculosis = lice. Scabies (*Sarcoptes scabiei*) is a mite that burrows, with burrows in the finger webs and wrists. - Treatment-resistant head lice (resistance to pyrethroids) are increasingly common.