Pediculus pubis Linnaeus, 1758
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 eyelashes. Humans are the only known hosts of the crab louse, although a closely related species, Pthirus gorillae, infects gorilla populations. The human parasite 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 a louse that affects chimpanzees and bonobos.
Infestation with crab lice is also called phthiriasis or phthiriasis pubis. The disease is spelled with phth, but the scientific name of the louse, Pthirus pubis, is spelled with pth, despite this being based on a misspelling that was nevertheless fixed as the official spelling in 1958.
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 rearmost two 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. 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 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.
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.
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.
Current worldwide prevalence has been estimated at 2% of the human population, 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 primary physicians.
Crab lice usually infect a new host only by close contact between individuals, usually through sexual intercourse. Adults are more frequently infested than children. Non-sexual transmissions may occur among family and roommates through the use of shared towels, clothing, beds or closets. They can only survive a short time away from the warmth and humidity of the human body.
It has been suggested that an increasing percentage of humans removing their pubic hair has led to reduced crab louse populations in some parts of the world. Because the World Health Organization and other authorities do not record statistics for pubic louse infestation, there is virtually no hard data to support this supposition.
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