Pubic lice (crabs): symptoms and treatment

Pubic lice (Phthirus pubis) are blood-feeding insects with crab-like claws on their middle and hind legs, which is where the nickname comes from. Patient says adults are 1 to 2mm long, and the eggs, called nits, stick firmly to the hair shaft. The eggs hatch after 6 to 10 days, and a female louse can lay up to 300 eggs in a lifetime of 1 to 3 months.

The lice prefer thick, coarse hair, so the pubic area is the typical home. They can also live in hair around the anus, the armpits, the chest, the beard and moustache, and more rarely the eyebrows and eyelashes. Patient says they do not live in scalp hair, which is finer and softer, so they are different from head lice.

The NHS lists itching that is usually worse at night, small red or blue spots where lice have bitten, white or yellow dots attached to the hair (eggs) and dark red or brown spots in underwear (lice droppings). Patient says itching may take one to three weeks to begin after infection and is a reaction to louse saliva.

Scratching can irritate the skin, causing redness or a rash, and eyelash involvement may cause eye inflammation. Some people have no symptoms but can still pass lice on without knowing. The Scottish clinical guideline notes that some people occasionally carry an infestation for a long time without any symptoms.

• Itching in the pubic area, armpits, chest or beard, often worse at night

• Small grey-brown insects or tiny white-yellow eggs attached to hairs

• Faint blue-grey marks or red spots on the skin at feeding sites

• Specks of dark brown or red in underwear

• Itchy, irritated or sore eyelids if lashes are involved

Close body contact is the usual route. Condoms and other forms of contraception do not protect against pubic lice, the NHS notes, although it is still sensible to use condoms because they reduce the risk of other sexually transmitted infections.

The NHS says catching lice from clothes, bedding or towels used by someone with pubic lice is rare. Patient is more reassuring still: the lice cannot survive for long away from a human host, so catching them from bed linen, towels or toilet seats is very unlikely. Non-sexual close contact, such as kissing someone with lice in the beard, can pass them on. Pubic lice only affect humans and cannot be caught from animals.

You can go to a sexual health clinic or a GP. The NHS says a doctor or nurse will check the hair on the pubic area and other possible sites such as armpits, chest or eyelashes, sometimes using a comb and a magnifying lens. Diagnosis rests on finding adult lice or eggs, and the Scottish guideline mentions that a magnifying glass helps.

Because pubic lice are usually caught during sex, the clinician may ask about partners and suggest testing for other sexually transmitted infections. Patient advises that anyone who caught lice from a sexual partner should be tested for other STIs. That conversation is routine and confidential in sexual health clinics.

The main treatments are medicated creams or lotions that kill the lice. The NHS says you usually apply them to the whole body, except the eyebrows and eyelashes, leave them on for 12 hours or overnight, wash them off, and repeat a week later to make sure all lice are dead. You may be asked to return a week after finishing for a check.

Patient and the Scottish clinical guideline describe malathion 0.5% aqueous lotion and permethrin 5% cream as the usual choices, with water-based products preferred over alcohol-based ones because the latter can irritate the skin. Permethrin is not licensed under 18 years of age. Tell the pharmacist or doctor if you are pregnant or breastfeeding, because some products may not be suitable.

Apply to cool, dry skin. If you wash an area during the treatment time, put the product on that area again, according to Patient. Keep insecticide out of the eyes. For eyelash involvement, Patient and the Scottish guideline both describe greasy eye ointment applied to the lashes as an alternative that avoids insecticide near the eye, and say to ask a doctor for advice rather than using the lotion there.

Current and recent sexual partners should be checked, even if they have no symptoms, says the NHS. The Scottish guideline advises avoiding close body contact until you and your partners have completed treatment, and notifying partners from the previous three months.

For laundry, the NHS suggests washing clothes and bedding on a hot wash of 50C or higher, dry cleaning, or sealing items in a plastic bag for at least 2 weeks, and vacuuming the mattress. Do not share clothes, bedding or hygiene products such as razors. Patient notes that there is no good evidence washing is essential, though some people prefer to do it.

Itching can continue for a few days after the lice are dead, because the skin stays sensitive. Dead eggs may remain stuck to hairs, and a fine-toothed comb or a fingernail can remove them. The Scottish guideline recommends a recheck one week after the last treatment.

Go back to a clinic or pharmacist if live lice are still seen after the second application, if the skin becomes very sore, weepy or crusted, or if the eyes are affected. Treatment failures are usually due to missed areas, washing off too early or reinfection from a partner who was not treated, so the first thing to check is how the product was applied.