Pthirus pubis
pubic louse
(Also: crab louse)

Pthirus pubis is a host specific louse found throughout the world on humans. (Burkhart and Burkhart, MAR 1999; Scott, Radcliff, and Ahmed-Jushuf, AUG 1999)

Pthirus pubis lays its eggs on coarse body hair, especially in hair associated with the pubic regions, but also in the anal region, armpits, thighs, abdomen and will even infect eyelashes and beards. These regions have flattened hairs, which the claws of P. pubis are highly modified to grasp. These lice are mostly found in unsanitary or overcrowded conditions of human hosts. (Burkhart and Burkhart, MAR 1999; Kremer and Ball, NOV 1997; Mori, Hayakawa, and Koide, 1978; Roberts and Janovy, 2000; Scott, Radcliff, and Ahmed-Jushuf, AUG 1999)

Smaller than Pediculus humanus (head and body lice), gray "crab" lice are 1.25 to 2 mm long. Pthirus pubis has an oval body shape and is wider than it is long and these lice are given the nickname "crabs" from their shape and chelate tarsi. These lice also have small heads relative to their body size, simple eyes, and short antennae. The lice have six legs, each of which terminate with a tarsal claw. The claws on the 2nd and 3rd pairs of legs are huge compared to those on the pair of legs closest to the head, which are smaller and thinner. Pthirus pubis also has another modification of the claw region, which is actually an extension of the tibia, called the thumb of the tibia which allows it to grasp the flattened hairs of the pubic region of humans. Another distinguishing feature is the four pairs of tubercles, which stick out on each side of the animal's abdomen. Lice breathe through spiracles at the ends of these para-tergal sclerites leading to the tracheal system. (Opaneye and Jayaweera, FEB 1993; Roberts and Janovy, 2000)

All lice exhibit hemimetabolous development, consisting of three life stages: egg, nymph, and adult. The eggs of P. pubis hatch in six to eight days and the young lice then pass through three nymphal stages, lasting a total of 23 days, before becoming adults. (Roberts and Janovy, 2000)

These lice normally live for a little less than a month, dying soon after reproducing. However, when separated from a host, they live less than 24 hours.

These lice are much less mobile than Pediculus humanus and can remain attached to human skin for days at a time with their claws and mouthparts engaged in the hosts' skin. When feeding, P. pubis injects saliva into the host, causing itching and pruritus. The distinctive crab-like claws of P. pubis allow it to grasp the flattened hair of the host, and they have been known to travel up to 10 cm in a day. Only found on humans, crab lice are normally transmitted directly from person to person, especially during sexual contact. Some studies have shown infants contracting these lice at birth on their scalp from an infected mother. (Kremer and Ball, NOV 1997; Opaneye and Jayaweera, FEB 1993; Scott, Radcliff, and Ahmed-Jushuf, AUG 1999; Silburt and Parsons, SEP 1990)

Presumably, lice can tell when they have hit a blood vessel when beginning to feed by sensing chemicals released at the site of the wound. Also, lice have short antennae with chemoreceptors and tactile hairs, and some, such as P. pubis, have simple eyes. No information is available on how these lice communicate with one another. (Roberts and Janovy, 2000)

Pubic lice feed on human blood and will die within 24 hours without having a human blood meal. Species in the order Anoplura, to which P. pubis belongs, have a highly modified feeding mechanism. They lack mandibles like all other types of lice and they have a fascicle made of four stylets which they use to feed on the blood of their hosts. The haustellum derived from the labrum supports the fascicle. Two of the stylets derived from the maxillae lock together to form a food channel. A single hollow stylet derived from the hypopharynx connects with the salivary duct and conveys salivary materials into the wound. And a large flattened stylet derived form the labium cuts into tissue with a serrated tip and serves as a guide for the other stylets. The cibarium and pharynx in the head serve as a two-chambered pump, sucking material in through the mouth and passing it on to the esophagus. (Burkhart and Burkhart, MAR 1999; Kremer and Ball, NOV 1997; Roberts and Janovy, 2000)

Pthirus pubis is an obligate ectoparasite of humans.

Infestation of P. pubis is a sexually transmitted condition causing serious itching to its victims. Irritation is increased in the area with host scratching and the area can often become scabby with oozing lesions. Asymptomatic individuals may discover nits or lice on pubic hair, or black specks on underpants. Characteristic small blue-gray macules known as maculae caeruleae may appear at bitten sites. Complications including excoriations, secondary bacterial infections, and eczematization may ensue.

Pthirus pubis thrives in unsanitary, overcrowded living conditions and historically has been common in military, refugee, and concentration camps, prisons, and overcrowded city dwellings. Overcrowding favors P. pubis because it can migrate between hosts easily and is not always contracted through sexual contact.

Unlike body lice (Pediculus humanus), pubic lice are not known to spread typhus. (Burkhart and Burkhart, MAR 1999; Opaneye and Jayaweera, FEB 1993; Scott, Radcliff, and Ahmed-Jushuf, AUG 1999; Skinner, Viswalingam, and Goh, DEC 1994)

There is often a strong correlation between the infection of Pthirus pubis in the eyelashes and individuals having other sexually transmitted diseases. Therefore, Pthirus pubis can sometimes be used as an indicator of more serious problems. (Opaneye and Jayaweera, FEB 1993; Skinner, Viswalingam, and Goh, DEC 1994)

Diagnosis of an infestation is normally by a report by the victim or clinical observation of the symptoms. Doctors may perform a microscopic examination of the lice and nits for confirmation of the diagnosis.

Treatments for infestation of P. pubis include taking a shower and then applying a 1% gamma benzene hexachloride ointment or lotion. This is normally left on for 12 hours. Eyelash infestations are treated with white petrolatum ointment for 10 days. Clothing and bedding can be laundered in boiling water or dry cleaned and items inconvenient to clean should be stored away from other bedding and clothing for a month. Without blood, the lice will soon die. (Burkhart and Burkhart, MAR 1999; Mori, Hayakawa, and Koide, 1978; Skinner, Viswalingam, and Goh, DEC 1994)