Crab louse

The crab louse or pubic louse (Pthirus pubis) is an insect that is an obligate ectoparasite of humans, feeding exclusively on blood. The crab louse usually is found in the person's pubic hair. Although the louse cannot jump, it can also live in other areas of the body that are covered with coarse hair, such as the perianal area, the general body hair, and the eyelashes (in children).

Humans are the only known hosts of the crab louse, although a closely related species, Pthirus gorillae, infects gorillas. The human parasite is thought to have diverged from Pthirus gorillae approximately 3.3 million years ago. It is more distantly related to the genus Pediculus, which contains the human head and body lice and lice that affect chimpanzees and bonobos.

An adult crab louse is about 1.3–2 mm long (slightly 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 second and third pairs of legs of a crab louse are much thicker than the front legs and have large claws.

The eggs of the crab louse are laid usually on the coarse hairs of the genital and perianal regions of the human body. The female lays about three eggs a day. The eggs take 6–8 days to hatch, and there are three nymphal stages which together take 10–17 days before the adult develops, making a total life cycle from egg to adult of 16–25 days. Adults live for up to 30 days. Crab lice feed exclusively on blood, and take a blood meal 4–5 times daily. Outside the host they can survive for 24–48 hours. Crab lice are transmitted from person to person most commonly via sexual contact, although fomites (bedding, clothing) may play a minor role in their transmission.

Infestation of the eyelashes is referred to as pediculosis ciliaris or phthiriasis palpebrarum.

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 several days.^{[citation needed]}

The prevalence varies between 0.3% to 4.6% with an estimated average of 2% with an increase during war, disasters and in overcrowding. Crab louse infestations are not considered a reportable condition by many health authorities, and many cases are self-treated or treated discreetly by physicians.

It has been suggested that an increasing percentage of humans removing their pubic hair, especially in women, has led to reduced crab louse populations in some parts of the world.