Scabies is a relatively contagious infection caused by a tiny mite called Sarcoptes scabiei .
Scabies is caused by a tiny insect about 0.3 mm long called a mite. When a human comes in contact with the female mite, the mite burrows under the skin, laying eggs along the line of its burrow. These eggs hatch, and the resulting offspring rise to the surface of the skin, mate, and repeat the cycle either within the skin of the original host or within the skin of its next victim.
The intense itching almost always caused by scabies is due to a reaction within the skin to the feces of the mite. The first time someone is infected with scabies, he or she may not notice any itching for a number of weeks (four to six weeks). With subsequent infections, the itchiness begins within hours of picking up the first mite.
Prevalence rates are not clear; some studies suggest that between 6 and 27 percent of the population have scabies at any one time. Scabies is more common among schoolchildren and individuals living in crowded conditions.
Causes and symptoms
Scabies is most common among people who live in overcrowded conditions and whose ability to practice good hygiene is limited. Scabies can be passed between people by close skin contact. Although the mites can only live away from human skin for about three days, sharing clothing or bedclothes can pass scabies among family members or close contacts. In May 2002, the Centers for Disease Control (CDC) included scabies in its updated guidelines for the treatment of sexually transmitted diseases .
The itching (pruritus) from scabies is worse after a hot shower and at night. Burrows are seen as winding, slightly raised gray lines along the skin. The female mite may be seen at one end of the burrow, as a tiny pearl-like bump underneath the skin. Because of the intense itching, burrows may be obscured by scratch marks left by the patient. The most common locations for burrows are the sides of the fingers, between the fingers, the top of the wrists, around the elbows and armpits, around the nipples of the breasts in women, in the genitalia of men, around the waist (beltline), and on the lower part of the buttocks. Babies may have burrows on the soles of their feet, palms of their hands, and faces.
Scratching seems to serve some purpose in scabies, as the mites are apparently often inadvertently removed. Most infestations with scabies are caused by no more than 15 mites altogether.
Infestation with huge numbers of mites (on the order of thousands to millions) occurs when an individual does not scratch or when an individual has a weakened immune system. These patients include the elderly; those who live in institutions; the mentally retarded or physically infirm; those who have other diseases which affect the amount of sensation they have in their skin (leprosy or syringomyelia); leukemia or diabetes sufferers; those taking medications which lower their immune response ( cancer chemotherapy or immunosuppressant drugs given after organ transplantation); or people with other diseases which lower their immune response (such as acquired immunodeficiency syndrome or AIDS ). This form of scabies, with its major infestation, is referred to as crusted scabies or Norwegian scabies. Infected patients have thickened, crusty areas all over their bodies, including over the scalp. Their skin is scaly. Their fingernails may be thickened and horny.
Diagnosis can be made simply by observing the characteristic burrows of the mites causing scabies. A sterilized needle can be used to explore the pearly bump at the end of a burrow, remove its contents, and place it on a slide to be examined. The mite itself may then be identified under a microscope.
Occasionally, a type of mite carried on dogs ( Sarcoptes scabiei var. canis ) may infect humans. These mites cannot survive for very long on humans, and so the infection is very light.
Several types of lotions (usually containing 5% permethrin) can be applied to the body and left on for 12 to 24 hours. One topical application is usually sufficient, although the scabicide may be reapplied after a week if mites remain. Preparations containing lindane are no longer recommended for treating scabies because of the potential for damage to the nervous system. Itching can be lessened by the use of calamine lotion or antihistamine medications.
In addition to topical medications, the doctor may prescribe oral ivermectin, a drug that was originally developed for veterinary practice as a broad-spectrum antiparasite agent. Studies done in humans, however, have found that ivermectin is as safe and effective as topical medications for treating scabies. A study published in 2003 reported that ivermectin is safe for people in high-risk categories, including those with compromised immune systems.
The prognosis for complete recovery from scabies infestation is excellent. In patients with weak immune systems, the biggest danger is that the areas of skin involved with scabies will become secondarily infected with bacteria.
Good hygiene is essential in the prevention of scabies. When a member of a household is diagnosed with scabies, all that person's recently worn clothing and bedding should be washed in very hot water.
One of the biggest concerns among family members of an individual with scabies is its ready transmissibility. Care should be taken to avoid sharing bedding, towels, and clothing with an infected family member. Some healthcare providers recommend that all family members be treated with a scabicide, whether or not scabies is evident. Linens of all family members should be washed in the hottest water possible to avoid cross-contamination.
Mite —An insect parasite belonging to the order Acarina. The organism that causes scabies is a mite.
Pruritus —The symptom of itching or an uncontrollable sensation leading to the urge to scratch.
Topical —Not ingested; applied to the outside of the body, for example to the skin, eye, or mouth.
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Rosalyn Carson-DeWitt, MD Rebecca J. Frey, PhD