Pediculus pubis Linnaeus, 1758
The crab louse (Pthirus pubis, frequently misspelled as Phthirus pubis), also known as the pubic louse, is an insect that is an obligate ectoparasite of humans. It is typically found in pubic hair, but may also live on other areas with coarse hair, including the eyelashes. They feed exclusively on blood. Humans are the only known hosts of this parasite, although a closely related species, Pthirus gorillae, infects gorilla populations. The species passed to humans 3.3 million years ago.
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.
Life cycle 
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 days.
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 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. As with most sexually transmitted pathogens, they can only survive a short time away from the warmth and humidity of the human body.
It has recently 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 don't record statistics for pubic louse infestation, there is virtually no hard data to support this supposition.
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