Crab louse

Crab louse

An infection caused by a lice, a blood-sucking insect, parasites usually found on pubic hair. It's also found around the anus, armpits, body hair such as chest hair, beards, eyebrows, and eyelashes.
The size is about 1 mm, color is white but it becomes brown when it sucks human blood, and because of this it can be easily found with the naked eye.
The female lays 30-40 eggs near the base of the hair.Eggs hatch in about 7 days, become adults in about 13-17 days, and adult females begin to lay eggs after 1-2 days.
It is a rare sexually transmitted disease in Japan with a good sanitary environment, but we still need to remain cautious for possible risk of infection.
In general, symptoms such as strong itchiness appear after the incubation period of 1-2 months from the infection.There is no rash, but dark spots may appear on your underwear due to the dung (blood feces) of the lice.

Route of infection and prevention

Transmitted through sexual activity. Most of the cases are found pubic hair, which cannot be prevented by just wearing a condom. In addition to sexual activity, it can also be transmitted through used or contaminated bedding, towels, etc.
If one of the family members living together is infected, the whole family should be tested to prevent the spread of the infection. Early detection and treatment is important.

If one of the family memebers are infected, the family living with him should be treated at the same time. It can continously spread if everyone is not treated. Refraining from sexual activity is also advised until completely treated. Sharing of clothing, bedding and towels should also be avoided until completely treated. It is basic to wash clothes after use, and heat steritization such as ironing and dry cleaning will also prevent infection.


Testing is done through checking of the blood feces of the lice attached to your underwear. If theres strong itchiness on your pubic area, please consult a doctor and undergo testing at a medical institution.
If you suspect crab louse infestation, use a magnifier to check for adult lice between the hair and for eggs laid at the base of the hairs.
If you find a crab louse, check it under a microscope to distinguish it from another type of lice, such as head lice.
If you are infected with crab louse, you are likely to have a sexual partner or a family member that might be infected as well. They need to be tested too.


Shaving the hair on the area where the lice exists is the most reliable and inexpensive treatment, but incomplete shaving can cause the lice to move to other areas.
Therefore, powder and shampoo containing pyrethroid anthelmintics can be used to disinfect. However, it is not very effective for eggs and timing for the egg hatching should be measured, and must be repeated 3-4 times, once in 3-4 days.
Once treatment is completed, use white underwear for about 2 months, and make sure that blood feces of crab loubse are not found. You may also use a magnifier to check for the presence or absence of adult crab louse or eggs. If not completely cured, another round of treatment should be initiated.
It will recur if not completely treated, and can get infected again if theres direct contact with an infected person through sexual activity.