Genito-Urinary Medicine


Introduction to Crabs

The crab louse (Pthirus pubis, also pubic louse; pleural: lice) is an insect that lives on coarse human body hair but not on the finer hair of the scalp.

Written by Doctify Team 27/04/2020

What are Crabs?

The crab louse (Pthirus pubis, also pubic louse; pleural: lice) is an insect that lives on coarse human body hair but not on the finer hair of the scalp. They are usually found in the pubic hair, but can also live in other areas of the body that are covered with coarse hair, such as the eyelashes, beard, moustache or underneath the arms. Pubic lice feed exclusively on blood. Humans are the only known hosts of the crab louse. The eggs of the crab louse are laid usually on the coarse hairs of the genital and perianal regions (area around the anus) of the human body. Although the pubic lice and eggs are small and hard to see, they can be visibly seen in the coarse hair.

What causes Crabs?

Crab lice typically infect a new host only by close contact between individuals, normally through sexual intercourse. So, you should be tested for other sexually transmitted infections (STIs) if you have pubic lice through sexual intercourse. The lice can only crawl from hair to hair and need human blood to survive. They can’t fly or jump. Condoms and other barrier contraception do not protect you against crab lice. Close contacts like kissing and hugging have been shown to spread pubic lice. Although rare, it can also be transmitted through sharing towels, beds or clothing.

What are the symptoms of Crabs?

It can take up to several weeks before the symptoms develop, and are the same for both men and women. The main symptom of infestations of crab lice (pediculosis pubis) is itching due to increased sensitivity to louse saliva. The itch can become progressively stronger over a period of time, and is worse at night as the lice are more active. You may also find black powder in your underwear. Do look out for characteristic blue spots on your skin if you suspect yourself to be infested.

How are Crabs treated?

If you think you have pubic lice, seek medical advice from your GP or go to a sexual health clinic. They are usually easy to diagnose, using a magnifying glass, to look for the lice and their pale-coloured eggs.

Insecticide cream, lotion or shampoo can be used to treat pubic lice at home, following advice given by your GP or pharmacist. When applying the product, be careful not to get the product in your eyes. Side effects involve skin irritation, such as redness or burning.

Reapplying the treatment after three to seven days is essential as the first treatment will kill the lice, but not the eggs. This step will ensure that any lice are killed before they mature and lay more eggs. Check for presence of lice or eggs a week after your second treatment.

It is suggested that removing pubic hair can help to reduce crab lice, although there is no evidence supporting this.

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