Sexually Transmitted Diseases (STDs in Men) (cont.)

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Ectoparasitic infections

What are ectoparasitic infections?

Ectoparasitic infections are diseases that are caused by tiny parasitic bugs, such as lice or scabies. They are transmitted by close physical contact, including sexual contact. The parasites affect the skin or hair and cause itching.

What are pubic lice (pediculosis pubis)?

Pubic lice, also called nits, are small bugs that actually are visible to the naked eye. That is, they can be seen without the aid of a magnifying glass or microscope. The scientific term for the responsible organism, the crab louse, is Phthirus pubis. These parasites live within pubic or other hair and are associated with itching.

A lice-killing shampoo (also called a pediculicide) made of 1% permethrin or pyrethrin is recommended to treat pubic lice. These shampoos are available without a prescription.

Malathion lotion 0.5% (Ovide) is another prescription medication that is effective against pubic lice.

None of these treatments should be used for involvement near the eyes because they can be very irritating. The patient's bedding and clothing should be machine-washed with hot water. All sexual partners within the preceding month should be treated for pubic lice and evaluated for other STDs.

Picture of pubic louse (crab)
Picture of pubic louse (crab)

What is scabies?

Scabies is an ectoparasitic infection caused by a small bug that is not visible with the naked eye, but can be seen with a magnifying glass or microscope. The bug is a mite known as Sarcoptes scabiei. The parasites live on the skin and cause itching over the hands, arms, trunk, legs, and buttocks. The itching usually starts several weeks after exposure and is often associated with small bumps over the area of itching. The itching of scabies is usually worse at night.

The standard treatment for scabies is with a 5% cream of permethrin, which is applied to the entire body from the neck down and then washed off after 8 to 14 hours. An alternative treatment is one ounce of a 1% lotion or 30 grams of cream of lindane, applied from the neck down and washed off after approximately 8 hours. Since lindane can cause seizures, it should not be used after a bath or in patients with extensive skin disease or rash. This is because the lindane might be absorbed into the blood stream through the wet or diseased skin. As an additional precaution, this medication should not be used in pregnant or nursing women or children younger than 2 years old.

Ivermectin is a drug taken by mouth that has also been successfully used to treat scabies. The CDC recommends taking this drug at a dosage of 200 micrograms per kilogram body weight as a single dose, followed by a repeat dose two weeks later. Although taking a drug by mouth is more convenient than applying the cream, ivermectin has a greater risk of toxic side effects than permethrin and has not been shown to be superior to permethrin in eradicating scabies.

Bedding and clothing should be machine washed in hot water (as with the treatment of pubic lice). Finally, all sexual and close personal and household contacts within the month before the infection should be examined and treated.

Medically reviewed by a board-certified family practice physician.


"Sexually Transmitted Diseases (STDs)." Centers for Disease Control and Prevention (CDC). 22 Nov. 2010. <>.

Medically Reviewed by a Doctor on 10/16/2014

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