Pediculosis pubis

Signs and symptoms

The main symptom is intense itching in the pubic area and groin, especially at night, due to an allergic reaction to louse saliva. Some people develop grey-blue or slate-colored macules (maculae caeruleae) at feeding sites that may last days. Nits or live lice may be visible on the skin, and black louse droppings can appear in underwear. Scratching may cause crusting, scarring, rust-colored fecal material, blood-stained underwear, and secondary bacterial infection.

Enlarged lymph nodes in the groin or armpits may be felt. Some infested individuals have no symptoms. Complications from persistent scratching include skin thickening, darkened skin, and infections such as impetigo, conjunctivitis, and blepharitis.

Causes

Pubic lice are most often transmitted during vaginal, oral, anal, or manual sex, regardless of condom use, and a single sexual encounter with an infected person carries high risk. Transmission can also occur through kissing, hugging, and less commonly via bedding, clothing, or towels. Crowded conditions facilitate spread as lice crawl from person to person. Infestation on a child's eyebrows or eyelashes may indicate sexual exposure or abuse.

The louse has three stages: egg (nit), nymph, and adult. Nits are oval, yellow to white, and firmly attached to hair shafts; they hatch in 6–10 days. Nymphs resemble smaller adults and must feed on blood; they mature into adults in 2–3 weeks. Adults are tan to grayish-white, 1.5–2.0 mm long, flattened, and broader than head or body lice.

Their four rear legs are large and crab-like, giving the nickname "crabs." Females lay about 30 eggs over a 3–4 week lifespan. Lice die within 1–2 days if they fall off a human host, and humans are the only host.

Diagnosis

To diagnose pediculosis pubis, pubic hair is carefully checked for adult lice, nymphs, or nits. These can be removed with forceps or by cutting the hair, except near the eyes.

Identification may involve a magnifying glass, dermatoscope, or stereo-microscope. Testing for other sexually transmitted infections is advised for those infested. While pubic lice are linked to other STDs, they do not transmit infectious diseases.

Treatment

Treatment can be done at home and typically involves over-the-counter permethrin 1% cream applied to affected areas and washed off after 10 minutes, with a repeat application 7–10 days later to kill newly hatched lice. Alternatives include pyrethrin/piperonyl butoxide products, 0.2% phenothrin (washed off after 2 hours), 0.5% malathion lotion (washed off after 12 hours), or oral ivermectin. Combing pubic hair with a fine-toothed comb dipped in vinegar helps remove nits. Bedding, clothing, and towels should be washed in hot water (50°C or higher) or sealed in a plastic bag for at least three days.

Clean underwear should be worn at the start and after treatment. Shaving is not essential. Sexual partners should be evaluated and treated.

Did You Know?

• Infestation of the eyelashes with pubic lice is called phthiriasis palpebrarum or pediculosis ciliarum.
• The scientific name Pthirus pubis uses an etymologically incorrect spelling officially adopted in 1958.
• A closely related species, Pthirus gorillae, infects gorillas; early humans may have encountered the parasite while sleeping in quarters vacated by gorillas or by eating gorilla carcasses.