Crabs

Alternative names
Pediculosis - pubic lice; Lice - pubic; Pubic lice

Definition
Pubic lice are small, six-legged creatures that invade the genital hair region and infest it with their eggs. These lice can also infest armpit hair and eyebrows, but they are drawn to the pubic region.

Causes, incidence, and risk factors

Pubic lice are known scientifically as Phthirus pubis. Lice infestation is a common sexually-transmitted condition and is found mostly in adolescents. Transmission generally occurs during sexual activity, but it can occur through physical contact with contaminated objects such as toilet seats, sheets, and blankets. Some women have acquired it while trying on swimming suits at a store.

The presence of pubic lice is heralded by moderate to severe itching in the area covered by pubic hair. This itch is frequently worse at night. Because the crab louse requires human blood to survive, it buries its head inside a pubic hair follicle. It excretes a substance into the skin that causes the itching.

Sometimes the bite can cause an inflammatory skin reaction that is bluish gray in color. Although the lice do not cause a rash, the constant scratching and digging can cause the skin to become raw, and secondary infections may develop.

On close inspection, adult lice (1.2 mm in diameter) may be seen. A magnifying glass makes the lice more easily visible, and will show the small egg sacs or “nits” that are attached to the pubic hairs, usually at the base.

Although pubic lice appear to prefer the pubic area, they can also thrive on the eyelashes. In young children, pubic lice may be a cause of blepharitis (an eye infection) and their eyelashes should be examined with a high-powered magnifying glass for evidence of lice.

In addition to pubic lice, there is body lice (Pediculus humanis corporis) and head lice (Pediculus humanis capitis).

Risk factors include:

     
  • Being a sexually-active adolescent  
  • Having multiple sexual partners  
  • Having sexual contact with an infected person  
  • Sharing bedding or clothing with an infected person

Symptoms

     
  • Itching in pubic hair region  
  • Eggs (nits) or actual adult lice may be seen  
  • Genital lesions due to bites  
  • Other genital lesions consisting of scratched areas and possibly secondary infections

Signs and tests

An examination of the external genital region typically reveals small gray-white oval eggs (nits) attached to the hair shaft and may reveal adult lice. Scratch marks or signs of secondary infection such as impetigo may also be noted.

Adult lice may be easily identified under the microscope. Their crab-like appearance is the reason that pubic lice are referred to as “the crabs.”

Treatment

Pubic lice are best treated with a prescription wash such as Elimite or Kwell:

     
  • Thoroughly work the shampoo into the pubic hair and surrounding area for at least 5 minutes.  
  • Rinse well.  
  • Comb the pubic hair with a fine-tooth comb to remove eggs (nits). Applying vinegar to pubic hair before combing may help loosen nits, but the hair should be dry when applying the shampoo.

A single treatment is all that is usually needed. If another treatment is recommended, it should be done 4 days to 1 week later.

Over-the-counter medications for the treatment of lice include Rid and Nix.

At the time of treatment, wash all clothing and linens in hot water. Items that cannot be washed may be sprayed with a medicated spray or sealed (suffocated) in plastic bags and not used for 10 - 14 days. It is imperative for all intimate contacts to receive treatment at the same time.

People with pubic lice should be evaluated for other sexually-transmitted disease at the time of diagnosis.

Expectations (prognosis)
Adequate treatment, including environmental cleaning, should eradicate the lice.

Complications
Secondary infections may occur because of skin break-down from frequent scratching.

Calling your health care provider
Call for an appointment with your health care provider if you or your sexual partner have symptoms of pubic lice. If you know that itching is caused by pubic lice, you can try over-the-counter lice treatments. Call the health care provider if these are ineffective. Also call if symptoms persist after treatment.

Prevention

Avoid sexual or intimate contact with infected people. If sexually active, use safe sex behaviors to avoid getting lice.

Good personal hygiene is always recommended. If possible, avoid trying on bathing suits while shopping. However, if you must, be sure to wear your underwear while trying them on. This may prevent transmission.

Johns Hopkins patient information

Last revised: December 3, 2012
by Gevorg A. Poghosian, Ph.D.

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