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.
Etiology Pediculus humanus capitis
(2-mm, six-legged, wingless insect; Fig. 25-1).
Pediculus humanus 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 their 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, the louse cannot survive
for more than 3 days off the human head. Majority of patients have
a population of <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 that the infestation is approximately
9-month-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 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.
Head lice Numerous grayish-white
lice egg capsules stuck firmly on the hair in a child with head
Sites of Predilection Scalp: occipital,
The differential diagnosis of pediculosis capitis includes hair
casts, dried hairspray or gel, dandruff (epidermal scales), impetigo,
seborrheic dermatitis, and tinea capitis.
Dermatopathology Bite site will
show intradermal hemorrhage and a deep, wedge-shaped infiltrate
with eosinophils and lymphocytes.