Pediculosis pubis (also known as "crabs" and "pubic lice") is an infestation by the pubic louse, Pthirus pubis, a wingless insect which feeds on blood and lays its eggs (nits) on mainly pubic hair. Less commonly, hair near the anus, armpit, beard, eyebrows, moustache, and eyelashes may be involved. It is usually acquired during sex, but can be spread via bedding, clothing and towels, and is more common in crowded conditions where there is close contact between people.

The main symptom is an intense itch in the groin, particularly at night. There may be some grey-blue discolouration at the feeding site, and eggs and lice may be visible. Scratch marks, crusting and scarring may be seen, and there may be signs of secondary bacterial infection.

Diagnosis is by visualising the nits or live lice, either directly or with a magnifying glass. Investigations for other sexually transmitted infections (STIs) are usually performed.

First line treatment usually contains permethrin and is available over the counter. Two rounds of treatment at least a week apart are usually required to kill newly hatched nymphs. Washing bedding and clothing in hot water kills the lice, and transmission can be prevented by avoiding sexual contact until no signs of infestation exist. Eggs may be removed by combing pubic hair with a comb dipped in vinegar. Sexual partners should be evaluated and treated.

Infestation with pubic lice is found in all parts of the world and occurs in all ethnic groups and all levels of society. Worldwide, the condition affects about 2% of the population.

The onset of symptoms is typically three weeks after the first infestation of lice and is mainly an intense itch in the pubic area and groin, particularly at night, resulting from an allergic reaction to the saliva of feeding lice. In some infestations, a characteristic grey-blue or slate coloration macule appears (maculae caeruleae) at the feeding site, which may last for days. Nits or live lice may be seen crawling on the skin. Louse droppings may be noticed as a black powder in the underwear.

Scratch marks, crusting, scarring, rust-colored faecal material, blood stained underwear and secondary bacterial infection may sometimes be seen. Large lymph nodes in the groin and armpits may be felt. Some people with pubic lice infestation may not have any symptoms.

Complications are usually as a result of persistent scratching and include thickening of the skin, darkened skin, and secondary bacterial infection including impetigo, conjunctivitis and blepharitis.