Sydney University Department of Medical Entomology Westmead Hospital
Pubic Lice

Pubic lice

modified from: Service, M.W. (1980). A guide to Medical Entomology. Macmillan. London.

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Natural History

Pubic lice, or Pthirus pubis, are commonly referred to as crab lice or simply ‘crabs’. This name has come from the crab-like appearance and slower movement compared to other lice, such as the head louse, Pediculus capitis and the body louse, Pediculus humanus that can infest humans. Pubic lice are found worldwide, they are haematophagous (feeds on blood), and strictly host specific to humans. Typically, they infest the hair of the pubic and perianal regions but are occasionally found in other areas where the hair is sparse and coarse. This includes the hair of the beard, moustache, eyelashes, armpits, and sometimes the chest and abdomen. The hair on the scalp is usually unsuitable, because of its fine texture closeness of the shafts, but pubic lice are occasionally found at margins of the head on the hairline. If children are infested, the lice will generally be found only in the eyebrows or eyelashes.

The pubic louse is grey in colour, and smaller (1.25 -2.00mm) than the head and body louse. Their body shape is oval and broader than long, with four distinct lobe-like protuberances on each side of their abdomen. The lice have a small head with short antennae and simple eyes. Each of the six legs of the louse terminates in a claw, butPubic lice claws on the second and third pair of legs are huge compared to the first pair which are slender. Within each claw there is an associated thumb-like projection which enables the louse to grasp body hair, securing them whilst they feed on blood using their mouthparts which are especially adapted for piercing and sucking. The lice blood feed intermittently over several hours. Adults and the three smaller nymphal stages usually remain and feed in a settled position. Pubic (and other lice) lice cannot burrow into the skin and do not live under the skin.

The life span of adult lice is less than a month. A mature female louse will lay a total of 30 eggs (nits), laying up to 3 eggs a day. The eggs are smaller than the eggs of the other human lice, and are a darkish-brown with an opalescent sheen. Each egg is cemented to the shaft of coarse hair, and at skin temperature will hatch within 6-8 days. If the lice are forced off the host they will die within 24-48 hours. Pubic lice cannot infest the rooms or carpets of an infected person.

Pubic lice are usually transmitted by sexual contact, and although this is the most common method, it is incorrect to assume this is the only means of transmission. Shared bath-towels and clothing, discarded clothing hanging in overcrowded locker rooms, children sleeping with an infected parent, or bedding that has recently been vacated by an infected individual can lead to infestations. Pubic louse infestations cannot be transmitted from animals. No known disease causing organism has been transmitted by pubic lice in Australia.

Clinical Presentation

At each puncture site a red papule develops and the immediate area swells. Intense itching is common due to the host’s reaction to the foreign proteins in the saliva of the louse. If the infestation is left untreated, the infected individual can become sensitised. Continual scratching may lead to secondary infections, and in some cases swollen lymph glands, due to bacterial infection. If the infestation involves the eyelashes, and left untreated, the eyelids can become swollen and inflamed. With the majority of infestations, after the pubic lice have fed, a characteristic grey-blue or slate colouration appears at the feeding site, which may last for days. The coloured area can be 0.2-3.0cm in diameter, and may have an irregular outline deep in the surrounding tissues, although this does not always occur for each infestation of pubic lice, it is more characteristic of pubic than for body lice. This discolouration is thought to be a result of altered human blood pigments or a reaction to substances excreted from the louse’s saliva.

Laboratory Diagnosis

Identification of louse specimens is by light microscopy.

Treatment & Control

When pubic lice have been identified as the source of the problem, it is important that all sexual contacts of that person be made aware of the situation and treated if necessary. If one person within a family has pubic lice, all family members should be examined and treated, especially if the infected person shares a bed with other family members. Treatments for pubic lice are similar to those for head lice, and can be purchased from any pharmacy without a prescription. For eyelash and eyebrow infestations, an application of petroleum jelly twice a day for 7-10 days can be effective in controlling the lice. Careful mechanical removal of eggs will be required, as the lice apply cement like substances when gluing the eggs to the hair and they are difficult to remove. The infected person’s underwear and bed linen should be washed in hot water, followed by hot tumble drying to ensure all lice have been killed. Spraying rooms or beds with insecticides is unnecessary, as the lice can only survive for a limited time without a host.

Confirmation and Enquiries

Identification of pubic and other human lice, and all other medically important insects, is performed through the Medical Entomology Department at ICPMR, Westmead Hospital.

See 'Contacts' for further information.

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