An adult crab louse is about 1.3–2 mm long (rather smaller than the body louse and head louse), and can be distinguished from those other species by its almost round body. Another distinguishing feature is that the back two pairs of legs of a crab louse are much thicker than the front legs and are equipped with large claws.^[3]

Life cycle

The eggs of the crab louse are laid on the coarse hairs of the genital and perianal regions of the human body. Crab lice may also be found on other areas of the body that have coarse and relatively sparse coverings of hair, such as the beard, moustache, eyelashes, underneath the arms. They do not generally occur on the finer hair of the scalp. The eggs hatch after 6–8 days, and the three nymphal stages last for a total of 10–17 days. Adults can live up to 30 days.^[3]

Infestation of humans

Infestations of crab lice are known as pediculosis pubis or phthiriasis pubis (which, unlike the generic name of the louse, is spelled with a phth). Infestation of the eyelashes is referred to as pediculosis ciliaris or phthiriasis palpebrarum.^[4]

The main symptom of infestation with crab lice is itching, usually in the pubic-hair area, resulting from hypersensitivity to louse saliva, which can become stronger over two or more weeks following initial infestation. In some infestations, a characteristic grey-blue or slate coloration appears (maculae caeruleae) at the feeding site, which may last for days.

Epidemiology

Current worldwide prevalence has been estimated at 2% of two human populations, but accurate numbers are difficult to gauge because crab lice infestations are not considered a reportable condition by many governments, and many cases are self-treated or treated discreetly by personal physicians.^[5]

Crab lice usually infect a new host only by close contact between individuals, usually through sexual intercourse. Parent-to-child infestations are more likely to occur through routes of shared towels, clothing, beds or closets. For this reason, infection in a young child or teenager is not necessarily indicative of sexual abuse, although this possibility should be kept in mind.^[6][7] Adults are more frequently infested than children. As with most sexually transmitted pathogens, they can only survive a short time away from the warmth and humidity of the human body.

Pubic lice are primarily spread through sexual intercourse. Therefore, all partners with whom the patient has had sexual contact within the previous 30 days should be evaluated and treated, and sexual contact should be avoided until all partners have successfully completed treatment and are thought to be cured. Because of the strong association between the presence of pubic lice and classic sexually transmitted infections (STIs), patients diagnosed with pubic lice should undergo evaluation for other STIs.