Scabies is caused by a tiny, 0.3 mm-long, parasitic insect called a mite. When a human comes into contact with the female mite, the mite burrows under the skin, laying eggs along the lines 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, causing red lesions.
The intense itching, or pruritus, that is 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 four to six weeks. With subsequent infections, the itchiness will begin within hours of picking up the first mite.
Causes & 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.
Mite burrows within the skin are seen as winding, slightly raised gray lines along a person's skin. The female mite may be found 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 include 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. The itching from scabies becomes worse after a hot shower and at night. Scratching, however, 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 those who live in institutions; are mentally retarded, or physically infirm; have other diseases which affect the amount of sensation they have in their skin (leprosy or syringomyelia); have leukemia or diabetes; are taking medications that lower their immune response (cancer chemotherapy, drugs given after organ transplantation); or have 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
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.
Several types of lotions (usually containing 5% permethrin) can be applied to the body and left on for 12–24 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 as of 2003 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. Ivermectin is 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 a 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. Extensive cleaning of the household, however, is not necessary because the mite does not live long away from the human body.
Darmstadt, Gary L., and Al Lane. "Arthropod Bites and Infestations." In Nelson Textbook of Pediatrics, edited by Richard Behrman. Philadelphia: W.B. Saunders Co., 1996.
Maguire, James H. "Ectoparasite Infestations and Arthropod Bites and Stings." In Harrison's Principles of Internal Medicine, edited by Anthony S. Fauci, et al. New York: McGraw–Hill, 1998.
"Scabies (The Itch)." Section 10, Chapter 114 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2002.
Stoffman, Phyllis. The Family Guide to Preventing and Treating 100 Infectious Diseases. New York: John Wiley and Sons, Inc., 1995.
Burroughs, R. F., and D. M. Elston. "What's Eating You? Canine Scabies." Cutis 72 (August 2003): 107–109.
Burstein, G. R., and K. A. Workowski. "Sexually Transmitted Diseases Treatment Guidelines." Current Opinion in Pediatrics 15 (August 2003): 391–397.
Fawcett, R. S. "Ivermectin Use in Scabies." American Family Physician 68 (September 15, 2003): 1089–1092.
Santoro, A. F., M. A. Rezac, and J. B. Lee. "Current Trend in Ivermectin Usage for Scabies." Journal of Drugs in Dermatology 2 (August 2003): 397–401.
American Academy of Dermatology (AAD). 930 East Woodfield Road, Schaumburg, IL 60173. (847) 330-0230. <http://www.aad.org>.
Kathleen D. Wright
Rebecca J. Frey, PhD