Pediculosis capitis is an infestation of the scalp by the head louse, which feeds on the human scalp and neck and deposits its eggs on the hair.
SYNONYMS Head lice, louse, nit.
PREVALENCE 12 million school children annually in the United States. Ten percent of children worldwide.
RACE Caucasian, Asians > blacks.
SEASON Year-round, but greatest in summer.
ETIOLOGY Pediculus humanus capitis (2-mm, six-legged, wingless insect; Fig. 25-1).
Six-legged wingless insect responsible for head lice.
Head lice are transmitted from person to person via shared hats, caps, brushes, combs, or head-to-head contact. The Pediculus humanus capitis female lays approximately 10 ova per day gluing its eggs to the hair within 1 to 2 mm of the scalp. The ova hatch in 10 days, the louse emerges as a nymph, reaches its adult form 10 days later, and has a life span of 30 days. Lice have anterior mouthparts that attach and feed on blood five times a day. Lice cannot survive for more than 3 days off the human head. Majority of patients have a population of fewer than 10 head lice.
Humans contract lice by sharing brushes, hats, close head-to-head contact, etc. The scalp louse deposits nits on the hair next to the scalp, and scalp hair grows 0.5 mm daily (thus, the presence of nits 15 cm from the scalp indicates an infestation that is approximately 9 months old). New viable eggs have a creamy-yellow color; empty eggshells are white. The infestation first manifests as severe pruritus of the back and sides of scalp. Crusts and secondarily impetiginized lesions are common and may extend onto the neck, forehead, face, and ears. In extreme cases, the scalp can become a mass of matted hair, lice, nits, and purulent discharge called plica polonica.
TYPE Lice, nits (1-mm eggs; Fig. 25-2), macules, papules, excoriations.
Numerous grayish-white lice egg capsules stuck firmly on the hair in a child with head lice.
SITES OF PREDILECTION Scalp: occipital, postauricular regions.
ASSOCIATED FINDINGS Cervical or posterior auricular lymphadenopathy may be present.
The differential diagnosis of pediculosis capitis includes hair casts, dried hairspray or gel, dandruff (epidermal scales), impetigo, seborrheic dermatitis, and tinea capitis.