An Overview of Pubic "Crab" Lice

Sexual contact is the predominant mode of transmission

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Pubic lice (Pthirus pubis)—more commonly known as crabs—are parasitic insects that derive their name from their crab-like appearance. They are notorious for infesting pubic hair but can also be found on facial hair, the hair of armpits, and even on eyebrows and eyelashes.

Typically spread through sexual contact, the insects can only survive for a short period of time away from the warmth and humidity of the human body. The infestation, referred to as pediculosis pubis, can be treated with over-the-counter or prescription topical drugs.

Pubic lice are not the same thing as head lice. Head lice involve a different type of insect (Pediculus humanus capitis) that are more easily transmitted through casual contact.

Symptoms

Itching is the signature symptom of pediculosis pubis. Despite what people think, the itching is not caused by the insect bite per se (although they do feed on human blood) but rather as a result of a hypersensitive reaction to louse saliva. As the population and duration of the lice infestation grow, so, too, will the intensity of the itching.

Some infestations will manifest with bluish or grayish macules (tiny splotches on the skin that are neither raised nor depressed). Adult lice can sometimes be spotted crawling on the skin with the naked eye.

Causes

You get crabs by being in close physical contact with someone who already has them. Sexual contact is by far the most common mode of transmission. Within this context, crabs can be regarded as a sexually transmitted disease (STD), although you can also get them from the towels, bedsheets, or clothing of an infested individual. The lice can only survive for between 24 and 48 hours away from the human body.

The crabs go through various stages in their life cycle. Once they are transmitted, they will begin to lay eggs, called nits, on the shafts of hair. The whitish, oblong-shaped eggs will incubate for six to 10 days before hatching.

The immature lice, known as nymphs, will take between two to three weeks to fully mature, wherein they are capable of reproducing and laying more eggs. Female crabs are usually larger than males and can lay around 60 eggs in their three- to four-week life span. To live, the crabs must feed on blood. They vary in color from a grayish-white to tannish-brown color.

According to the Centers for Disease Control and Prevention (CDC), crabs found on the eyelashes of eyebrows of children may be a sign of sexual abuse.

Unlike head lice, fomites (like bedding or clothes) only play a minor role in the transmission of crabs. Despite popular belief, you cannot get crabs from public toilet seats or pets.

Diagnosis

Pediculosis pubis is diagnosed with a physical inspection of infested hair. Adult crabs are very small (around 2 millimeters or 0.08 inches) but can often be seen with the naked eye. However, because of their coloring, they can blend into the skin tone of certain individuals. A magnifying glass is often needed to positively identify the creatures.

Upon close inspection, you may see faint bluish macules where the crabs have bitten you. Under a magnifying lens, the crabs will have six legs; their two front legs are very large and look like the pinchers of a crab.

If you are unable to spot them or unsure what you are looking at, go to the doctor or STD clinic for an examination immediately. A snipping of your pubic hair may be taken and examined under the microscope to make a definitive diagnosis.

People diagnosed with crabs should be screened for other sexually transmitted infections.

Treatment

There are a number of topical medications used to treat pediculosis pubis. Permethrin 1% lotion or shampoo is an effective over-the-counter drug that can usually resolve the infestation with one treatment.

With that said, lice can sometimes develop a resistance to permethrin, particularly in populations where pediculosis pubis is widespread. In cases like these, lindane 1% lotion may be used, although it is far more likely to cause skin irritation and rash. In certain populations, it has been known to trigger seizures. Lindane is available by prescription only.

The CDC advises against the use of lindane in children, people with extensive dermatitis, and pregnant or lactating women. The U.S. Food and Drug Administration (FDA) similarly warns against the use of lindane in people with uncontrolled seizure disorders and further advises that it should be used with caution in people with skin conditions (such as psoriasis and eczema), the elderly, pregnant or lactating women, and anyone under 50 pounds (110 kilograms).

Ovide (malathion 0.5% lotion) is a prescription medication that can kill head lice but has not been approved by the FDA for the treatment of pubic lice.

How to Use

The application of anti-lice lotions and shampoos involve similar steps:

  1. Wash and dry the affected areas thoroughly.
  2. Put the towel in the washing machine for immediate washing. Do not mix with other pieces of laundry.
  3. Apply the lotion or shampoo to the affected areas and massage in thoroughly.
  4. Leave permethrin on the skin for 10 minutes and up to an hour. Lindane should be left on the skin from eight to 12 hours.
  5. Rinse off thoroughly with water and soap, and dry with a fresh towel.
  6. Use a fine-tooth comb (provided with the medications) to remove dead lice and nits still clinging hair shafts. Use a magnifying glass and tweezers, if needed.
  7. Put on clean underwear and clothing after treatment.

Care should be taken when treating eyelash lice. Some doctors recommend applying petroleum jelly around the perimeter of the eyelashes first, followed by the application of permethrin to the eyelids two hours later. Gently rinse after 10 minutes.

You do not need to shave your pubic hair. As long as you follow the medication instructions completely, you should not have any problem with resolving the infestation. If the first treatment fails to provide relief, a second may be needed in three to seven days. If lice still persist, call your doctor.

All bedding, towels, and used clothing should be thoroughly washed and dried on the hottest temperature setting. Any items that cannot be laundered or drycleaned should be stored in a sealed plastic bag for two weeks to kill any remaining insects.

Until the infestation is fully resolved, avoid intimate contact of any sort.

Crabs are not a reportable STD, but you should advise your sex partners if you've been diagnosed so that they can be treated and screened for other sexually transmitted infections.

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Article Sources

  1. Centers for Disease Control and Prevention. Pubic "Crab" Lice: Frequently Asked Questions. Atlanta, Georgia; updated September 26, 2019.

  2. Centers for Disease Control and Prevention. Lindane. Atlanta, Georgia; updated December 4, 2014.

  3. U.S. Food and Drug Administration. Medication Guide; Lindane (LIHN-dane) Shampoo, USP 1%. Silver Spring, Maryland; August 2007.

  4. Karabela Y, Yardimci G, Yildirim I, et al. Treatment of Phthiriasis Palpebrarum and Crab Louse: Petrolatum Jelly and 1% Permethrin Shampoo. Case Rep Med. 2015;2015:Article ID 287906. doi:10.1155/2015/287906

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