Scabies is caused by a microscopic insect, or mite (known as Sarcoptes scabiei), which lives in infested humans. (Scabies is technically called an "infestation", rather than an "infection", although the distinction seems to depend on the size of the offending organism.) The mite tunnels or burrows into the patient's skin, usually just below the surface, and then keeps burrowing through the subsurface leaving behind eggs, feces, and dead body parts. (I apologize -- believe me, this is just as bad to talk about with patients and parents in the office as it is to write and read about here...) There may not be any skin reaction at first, but once you become sensitized you will have an allergic reaction to the mites and their leavings, which consists mainly of a rash with an intense itch. Sensitization can take up to 1 or 2 months once a person has been infested with the mites. The burrows are visible on the skin surface as looped-thread-like lines, but often they are scratched away by the patient long before a doctor sees them.
The burrows are usually found in warm, moist parts of the body. In older children and adults, the mites usually hang out between fingers and toes, in the folds of the wrists, elbows, and shoulders, around the waist (since we all wear clothes that are at least a little tight at the waist), and in the groin and genital areas. The mites can be transmitted from person to person. Usually it takes a lot of contact between two people for the mites to infest someone new, but there is a variant form in which there are large numbers of mites in the lesions: this causes crusted or "Norwegian" scabies and can infest a new victim with minimal contact. The mites can live away from a person -- on clothes and linens -- for up to 3 or 4 days, and can infest a new victim that way also.
"Norwegian scabies" does not refer to a different kind of scabies mite, but to a particular form of infestation. Also known as "crusted scabies', the rash is heavily crusted with flakes of skin falling off, with thickening of skin on protruding parts of the body such as the elbows, knees, palms, and soles. The scales that fall off contain tremendous numbers of mites, and this form of scabies is more contagious than usual mainly because there are so many mites in each flake of skin. Since even casual contact with a patient with crusted scabies may result in infestation, many doctors will treat casual contacts just as they do household contacts.
Scabies is NOT a disease of poverty: it can and does affect people of all social, economic, and ethnic categories. (I know of doctors who have picked up Norwegian scabies from patients and brought it home to their own families.)
Treatment for scabies is pretty straightforward, although it's time-consuming too. Since the mite is transferred from one person to another by personal contact or by linens and clothes, we assume that everyone in the same house with a scabies patient has scabies themselves whether or not they are yet itching. The most common medicine used for scabies is permethrin, a cream which your doctor must prescribe for you. Other medicines such as lindane and crotamiton are used as well, but have more side effects and don't work quite as well in my experience. The treatment I prescribe runs like this -- and everyone in the house has to do this at the same time:
This should dispose of the mites in most cases. Sometimes a few unhatched mite eggs will survive somewhere; in these cases a second round with the cream usually takes care of the problem. There have also been a few reports of scabies that have become immune to permethrin; although rare, it's quite possible for scabies mites to become immune, especially if permethrin is not used exactly as directed. The itching will probably last for a while, though, since it takes a while for your body to eliminate the remains of the mites. A good antihistamine will help reduce the itching; your doctor can recommend one for you. In rare cases we may also prescribe a (very) few days of steroids to help get rid of the skin reaction, but this has to be done quite carefully, since the skin inflammation can increase the amount of topical steroids that are absorbed by your body.