GPAtlas🧭 When to suspect
Head lice (pediculosis capitis) are an infestation of the scalp by Pediculus humanus capitis, a wingless grey-brown insect about 2–3 mm long that lives only on the human head, feeds on blood, and dies within a day or two away from the scalp. It is one of the commonest conditions of childhood, peaking in children aged roughly 4–11 years and slightly more frequent in girls. Lice cannot fly, jump, or burrow; they spread almost entirely by head-to-head contact, which is why cases cluster within families and classrooms.
Suspect head lice in a child with an itchy scalp, classically behind the ears and at the nape of the neck. The itch is an allergic response to louse saliva and may take several weeks to develop, so asymptomatic infestation is common and itch is neither sensitive nor specific.
The single most important principle is diagnostic: treat only when a living, moving louse is found. Eggs alone – whether unhatched, or the empty white cases (nits) left after hatching – are not proof of active infestation, and itch by itself is not a reason to treat.
| What you find | What it means |
|---|---|
| Live, moving louse | Active infestation → treat |
| Unhatched eggs close to the scalp | Possible active infestation → confirm by finding a live louse before treating |
| Nits (empty white cases), often > 1 cm from the scalp | Old or resolved infestation → do not treat on this alone |
Source: DermNet · British Association of Dermatologists
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