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Pubic lice (Phthirus pubis)

Phthirus pubis ("crab louse") — ectoparasite

Overview

Infestation of coarse body hair (pubic, but also axillary, chest, and eyelashes) by the crab louse, spread by close/sexual contact. Causes itch; managed with a topical insecticide and treated as an STI (full screen + partner treatment).

Recognise

Itch in the pubic/genital area, often worse at night
Visible lice or nits (eggs) attached to coarse hairs; blue-grey macules (maculae ceruleae) at bite sites
Can involve eyelashes — especially relevant in children

Red flags

Eyelash involvement in a child → consider sexual abuse / safeguarding

Differentials & how to tell them apart

Scabiesburrows in finger-webs/wrists, generalised nocturnal itch (Sarcoptes) — different site and treatment

Eczema / folliculitisno lice or nits visible

Head lice (Pediculus capitis)scalp hair, a different louse and site

Investigations

Clinical — identify lice/nits on coarse hair (magnification helps). Screen for other STIs, as co-infection is common.

Management

Permethrin 5% (or malathion 0.5%) to all body hair, repeated after 7 days

1Topical insecticide (permethrin 5% or malathion 0.5%) to ALL body hair — not just pubic — and REPEAT after 7 days to kill newly-hatched lice.Gate: Eyelash involvement → white soft paraffin to the lashes (avoid insecticide near the eyes); eyelashes in a child → safeguarding referral
2Hot-wash clothing/bedding; treat sexual partners; screen for other STIs.

Permethrin 5% cream — first-line topical insecticide; apply to ALL body hair, wash off, repeat after 7 days

Malathion 0.5% aqueous — alternative insecticide

Key points

Treated as an STI: partner treatment + STI screen. The 7-day repeat is essential because insecticides do not reliably kill the eggs.

Monitor & prognosis

Re-examine if itch persists; confirm partner treatment and decontamination.

Cured with correct application, the repeat dose and partner treatment.

Source: NICE CKS Pubic lice; BASHH