scabies

scabies, skin inflammation accompanied by severe itching, particularly at night, caused by the itch mite (Sarcoptes scabiei, variety hominis). The mite passes from person to person by close contact. While scabies occurs worldwide, it is most common in areas that are affected by overcrowding and poverty, particularly in countries with tropical climates. The disease also afflicts many schoolchildren and residents of nursing homes and assisted-living facilities, with periodic epidemics occurring even among persons with strict habits of hygiene and adequate living conditions.

Sarcoptes scabiei attains a length of about 0.35 mm (0.014 inch) and is barely visible to the unaided eye. Despite its small size, it has quite formidable mouthparts and eight very powerful legs. With its hind legs, the female of the species hitches itself to the skin and, with its mouthparts thus closely applied to the skin, cuts its way down into the outer epithelial layer. The mite then tunnels horizontally and gouges out an easily recognizable burrow up to several inches long and usually not very straight. These burrows sometimes are visible as dark wavy lines. The female mite prefers the skin between the fingers and toes, on the wrists and elbows, in the armpits, below the breasts, and on the male genitals. It does most of its moving at night. Sometimes it is visible at the far end of one of its burrows. The female mite lays eggs that hatch into larvae. The larvae emerge from the skin and molt several times through a nymph stage before they become adults and mate. The development from egg to laying adult frequently takes about three weeks.

The initial lesion produced by the burrowing mite becomes intensely itchy after a few days to about a month, and the scratching usually leads to secondary skin lesions consisting of papules (solid elevations), pustules, and crusted skin areas. In addition, a rash breaks out on parts of the body where there are no burrows—on the buttocks, over the shoulder blades, and on the abdomen. The rash is the result of an allergic reaction to mite proteins, feces, and eggs. It is intensely irritating and interferes with sleep.

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Transmission of the mite is by skin-to-skin contact and by contamination of clothing or bedding. Transmission can be prevented by wearing clean underclothes and by frequently changing one’s clothes and washing—conditions not always possible in overcrowded or resource-poor communities. Effective treatment consists of covering the affected person from neck to feet with medicinal lotion that kills the mite. Several medications are available. One percent lindane lotion (gamma-benzene hexachloride) is effective but can be toxic. Permethrin, a synthetic drug based on chemicals derived from the chrysanthemum, is effective and quite safe, even in pregnant women and in very young children. Crotamiton, a prescription anti-itch medication, is another scabicidal lotion that can be applied to the entire body. Sulfur-containing ointments may also work, but they are not well accepted for cosmetic reasons. Ivermectin, taken orally, may be prescribed for individuals who do not respond to lotions or creams or for patients who suffer from crusted scabies. It is desirable to treat an entire family or a group of people living together, for often the early burrowing stage of the disease causes no irritation, and a person can be infested without knowing it.