Pubic lice are also known as pediculosis pubis or phthiriasis pubis. It is most commonly a sexually transmitted infestation of the pubic hair by Phthirus pubis also known as the 'crab louse'. The incidence of the condition has been steadily decreasing over the past decade, which is believed to be in part due to changes in the population's grooming habits with regard to pubic hair.

Epidemiology

• The majority of pubic lice infestations are due to sexual contact and so can affect sexually active patients of any age, however are more common in men and in people aged 15-40 years of age.
• The range of estimated incidence has previously been as high as 4.0%, with average worldwide incidence of 2.0% however one study from the UK in 2014 showed an incidence of only 0.07%, which is likely linked to increased pubic hair removal rates.

Risk factors

• The primary risk factor for pubic lice is sexual contact, however Phthirus pubis does occasionally infest other sites of hair and so sexual contact is not always the cause. Incubation takes a minimum of 5 days, but a longer asymptomatic period is possible so time between sexual contact and symptoms may be many weeks.

Patients typically present with:

• Itching/pruritus- typically worse at night: this may be localised to the pubic region or may affect any regions of body hair and rarely eyelashes and the head.
• Sometimes patient's may notice a 'black/rust-coloured powder' appearance in their underwear.

Signs:

• It may be possible to visualise individual lice, however microscopy may be required to see them.
• Excoriation marks due to repeated scratching due to pruritus.
• Blue macules (maculae ceruleae) may be visible: these are due to anticoagulant saliva injected by the lice during their feeding.

Investigations:

• The diagnosis is clinical.
• Microscopic/dermascopic evaluation of hair can help to evidence the presence of lice or eggs if they are not visible to the naked eye.
• Patients presenting with pubic lice should be offered a full sexual health screen.

Management

• Clothing and bed linen should be decontaminated, either by washing at >50ºC or by placing in a sealed plastic bag for >7 days.