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Crab Lice

Crab lice are one of three types of lice associated with people. The others are head lice and body lice. Head lice and body lice are very similar in appearance, but have different behavior. Head lice are usually found on the head of humans and remain on the host for their entire life. Body lice are only found on humans when they are feeding, hiding in the seams of clothing at other times.

Crab louse. Size compares to the year on a pennyAppearance

The crab or pubic louse gets its name because its body and enlarged pincher-like second and third pair of legs give it the appearance of an ocean-dwelling blue crab. However, crab lice are grayish in color and very small, with males and females only measuring about 1/16 inch.

Habits

Crab lice are blood-feeding external parasites, usually found on the hair in the pubic region of humans. Sometimes they migrate to other coarse body hairs such as on the chest, armpits, or even eyebrows. Crab lice are usually transferred from one individual to another through intimate contact. On rare occasions crab lice can be transferred if they are dislodged from the host and are in a place where they have almost immediate contact with a new host. Since they do not live more than 24-hours when away from the high temperature and humidity found on human skin, crab lice are not often transferred in this way.

Crab louse egg on hair shaft.Life cycle

Crab lice have a life cycle where the immature looks very similar to the adult. Each female lays 15 to 50 white eggs in small batches of two or three in a 24-hr period. The eggs are attached at the base of coarse hair. The immatures hatch in six to eight days and usually begin feeding within the first few hours. Once the immatures begin to feed they usually remain stationary, grasping the hair shaft with their crab-like legs. The entire life cycle of the crab louse from egg to adult is four to six weeks, with adults living an additional two to four weeks.

Diagnosis

Often the first indication of a crab louse infestation is irritation and itching at the site of feeding.  Sometimes small blue dots appear along with tenderness and swelling.  On closer inspection, eggs or nits as they are called, may be visible on the hair shaft where they have been cemented in place by the female. Small particles of blood and debris may also be seen as lice defecate while feeding.

Control

Control of crab lice is a medical issue and is resolved by the patient self-medicating with prescription or over-the-counter lotions or shampoos containing insecticides. There is no known resistance to insecticides among crab lice. Due to the life cycle of the louse it is often necessary to repeat the treatment after a week to ten days to kill any newly hatched nymphs. Consult your physician or pharmacist for a prescription or product recommendation. Since crab lice are transferred by close body contact, the infested person should notify anyone with whom they have been intimate. Any crab louse infestation on a child should be investigated due to the nature of infestation.


Prepared by Patricia Zungoli, Extension Entomologist/Professor, and Eric P. Benson, Extension Entomologist/Associate Professor, Department of Entomology, Soils, and Plant Sciences, Clemson University.
EIIS/MV-9 (New 12/1999).


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