Ectoparasitic Infections

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

Pubic lice (pediculosis pubis)

Pediculosis pubis is an infection of the genital area caused by the crab louse (Phthirus pubis). The lice (commonly called crabs) are small bugs that are visible to the naked eye without the aid of a magnifying glass or microscope. The lice live on pubic hair (or any other hair) and are associated with itching.

The treatment for pubic lice is usually with a 1% cream rinse of permethrin that is applied to the affected area and washed off after 10 minutes. Alternative treatments include pyrethrins with piperonyl butoxide applied for 10 minutes before washing off. 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.

Scabies

Scabies is an ectoparasitic infection caused by a mite (known as Sarcoptes scabiei) that is not visible with the naked eye but can be seen with a magnifying glass or microscope. 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 to a person with scabies and is often associated with small bumps over the area of itching. The itching from scabies is usually worse at night.

The standard treatment for scabies is a 5% cream of permethrin (Elimite), which is applied to the entire body from the neck down and then washed off after 8 to 14 hours. Treatment is repeated in one week.

Ivermectin (Stromectol) 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. While a patient my find that taking a drug by mouth is more convenient than application of 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.

An alternative treatment is 1 ounce of a 1% lotion or 30 grams of cream of lindane, applied from the neck down and washed off after approximately eight hours. Unfortunately, itching may persist for up to two months after successful therapy. Since lindane can cause seizures when it is absorbed through the skin, it should not be used if skin is significantly irritated or wet, such as with extensive skin disease, rash, or after a bath. As an additional precaution, lindane should not be used in pregnant or nursing women or children younger than 2 years old.

As with pubic lice infection, both the bedding and clothing of an infected individual should be machine-washed in hot water. Finally, all sexual and close personal and household contacts within the month before the infection should be examined and treated if infection is found.