Diagnosis of crabs (pubic lice)

Crabs or pubic lice infestation is transmitted through sexual or other close bodily contact. Although these are transmitted via sexual contact, they are not associated with carrying other sexually transmitted infections (STIs) or HIV AIDS from person to person.

These lice are transmitted from an infected person despite the use of condoms or other barrier contraceptives. Diagnosis of pubic lice mainly depends on physical examination and detection of the lice. There are no specific laboratory tests to detect these lice.

Where should a person go if they suspect they have crabs?

When a person suspects he or she has pubic lice infestation they should visit their general physicians, contraception clinic or sexual health clinic (also called a genitourinary medicine (GUM) clinic).

Sexual health and GUM clinics are often located in hospitals or health centres.

Diagnosis of pubic lice or crabs infestation includes outlining relevant history of exposure, examining affected areas and so forth. 1-6

Relevant history of exposure

A person may be asked about their relevant history of exposure (sexual or close bodily contact) with an infected person. These may be a non-sexual exposure like kissing or hugging a person with pubic lice in his beard or moustache.

Examination of the affected areas

The most commonly affected areas include:

  • the pubic region

  • armpits

  • coarse hair over the chest

  • abdomen

  • legs

  • back

  • beard or moustache

There are characteristic symptoms of itching, scratching and inflammation of the affected areas.

On examination of the areas, blue spots of lice bits, tiny bleeding spots of fresh lice bits may be detected easily.

Visualization of crabs

A magnifying glass is often used to look for tan, dusky grey or yellowish grey adult lice. These are usually 1 to 2m in length and often remain still to blend with the skin when there is light.

The adult pubic lice appear as tiny crabs. They have six legs, but their two front legs are very large and look like the pincher claws of a crab. This is how they got the nickname “crabs”.

Pubic lice may be also difficult to find because there may be only a few. Pubic lice often attach themselves to more than one hair and generally do not crawl as quickly as head and body lice.

For better visualization special fine toothed combs called louse combs may be used. After combing through the teeth are examined for living lice. The presence of a single live louse is adequate for the diagnosis of active infestation.

The magnifying glass is also used to look for greyish white or brown lice eggs or empty white eggshells (nits).

Detection of empty egg shells is not diagnostic of an active infestation. This is because the eggs have hatched and the shells may remain attached for long periods of time even after the infection has been successfully treated.

Tests for other sexually transmitted infections (STIs)

Those with pubic lice infestation need to be tested for other sexually transmitted infections (STIs) as well. This is done as a precaution.

Sexual partners within the last three months of patients are also tested and examined for crabs and other STIs. This is often needed to prevent recurrence of the infestation.

Further Reading

Last Updated: May 26, 2023

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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