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What is scabies?

Scabies is an itchy, highly contagious skin condition caused by an infestation by the itch mite Sarcoptes scabiei. Mites are small, eight-legged parasites (in contrast to insects, which have six legs). They are tiny, just 1/3 millimeter long, and burrow into the skin to produce intense itching, which tends to be worse at night. The mites that infest humans are female and are 0.3 mm to 0.4 mm long; the males are about half this size. Scabies mites can be seen with a magnifying glass or microscope. The scabies mites crawl but are unable to fly or jump. They are immobile at temperatures below 20º C, although they may survive for prolonged periods at these temperatures.

Scabies infestation occurs worldwide and is very common. It has been estimated that worldwide, about 300 million cases occur each year. Human scabies has been reported for over 2,500 years. Scabies has been reported to occur in epidemics in nursing homes, hospitals, long-term care facilities, and other institutions. In the U.S., it is seen frequently in the homeless population, but occurs episodically in other populations of all socioeconomic groups as well.

How do you get scabies?

Direct skin-to-skin contact is the mode of transmission. Scabies mites are very sensitive to their environment. They can only live off of a host body for 24 to 36 hours under most conditions. Transmission of the mites involves close person-to-person contact of the skin-to-skin variety. It is hard, if not impossible, to catch scabies by shaking hands, hanging your coat next to someone who has it, or even sharing bedclothes that had mites in them the night before. Sexual physical contact, however, can transmit the disease. In fact, sexual contact is the most common form of transmission among sexually active young people, and scabies has been considered by many to be a sexually transmitted disease (STD). However, other forms of physical contact, such as mothers hugging their children, are sufficient to spread the mites. Over time, close friends and relatives can contract it this way, too. School settings typically do not provide the level of prolonged personal contact necessary for transmission of the mites.

Can you catch scabies from a dog or cat?

Dogs and cats are infected by different types of mites than those which infect humans. Animals are not a source of spread of human scabies. Scabies on dogs is called mange. When canine or feline mites land on human skin, they fail to thrive and produce only a mild itch that goes away on its own. This is unlike human scabies, which gets worse and worse unless the condition is treated.

What does scabies look like? What are the signs and symptoms of scabies?

Scabies produces a skin rash composed of small red bumps and blisters and affects specific areas of the body. Scabies may involve the webs between the fingers, the wrists and the backs of the elbows, the knees, around the waist and umbilicus, the axillary folds, the areas around the nipples, the sides and backs of the feet, the genital area, and the buttocks. The bumps (medically termed papules) may contain blood crusts. It is helpful to know that not every bump is a bug. In most cases of scabies affecting otherwise healthy adults, there are no more than ten to fifteen live mites, even if there are hundreds of bumps and pimples.

The scabies rash is often apparent on the head, face, neck, palms, and soles of the feet in infants and very young children, but usually not in adults and older children.

Textbook descriptions of scabies always mention "burrows" or "tunnels." These are tiny threadlike projections, ranging from 2 mm to 15 mm long, which appear as thin gray, brown, or red lines in affected areas. The burrows can be very difficult to see. Often mistaken for burrows are linear scratch marks that are large and dramatic and appear in people with any itchy skin condition. Scratching actually destroys burrows.

What does scabies feel like?

It is important to note that symptoms may not appear for up to two months after being infested with the scabies mite. Even though symptoms do not occur, the infested person is still able to spread scabies during this time. When symptoms develop, itching is the most common symptom of scabies. The itch of scabies is insidious and relentless. The itch is typically worse at night. For the first weeks, the itch is subtle. It then gradually becomes more intense until, after a month or two, sleep becomes almost impossible.

What makes the itch of scabies distinctive is its relentless quality, at least after several weeks. Other itchy skin conditions—eczema, hives, and so forth—tend to produce symptoms that wax and wane. These types of itch may keep people from falling asleep at night for a little while, but they rarely prevent sleep or awaken the sufferer in the middle of the night.

What is the treatment for a scabies infestation?

Curing scabies is rather easy with the administration of prescription scabicide drugs. There are no approved over-the-counter preparations that have been proved to be effective in eliminating scabies. The following steps should be included in the treatment of scabies:

1. Apply a mite-killer like permethrin (Elimite). These creams are applied from the neck down, left on overnight, then washed off. This application is usually repeated in seven days. Permethrin is approved for use in people two months of age and older.
2. An alternative treatment is one ounce of a 1% lotion or thirty grams of cream of lindane, applied from the neck down and washed off after approximately eight hours. 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, the elderly, people with skin sores at the site of the application, children younger than two years of age, or people who weigh less than 110 pounds. Lindane is not a first-line treatment, and is only recommended if patients cannot tolerate other therapies or if other therapies have not been effective.
3. Ivermectin, an oral medication, is an antiparasitic medication that has also been shown to be an effective scabicide, although it is not FDA-approved for this use. 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. Although 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. It is typically used only when topical medications have failed or when the patient cannot tolerate them.
4. Crotamiton lotion 10% and cream 10% (Eurax, Crotan) is another drug that has been approved for the treatment of scabies in adults, but it is not approved for use in children. However, treatment failures have been documented with the use of crotamiton.
5. Sulfur in petrolatum applied as a cream or ointment is one of the earliest known treatments for scabies. It has not been approved by the FDA for this use, and sulfur should only be used when permethrin, lindane, or ivermectin cannot be tolerated. However, sulfur is safe for use in pregnant women and infants.
6. Antihistamines, such as diphenhydramine (Benadryl), can be useful in helping provide relief from itching. Sometimes, a short course of topical or oral steroids is prescribed to help control the itching.
7. Wash linens and bedclothes in hot water. Because mites don't live long away from the body, it is not necessary to dry clean the whole wardrobe, spray furniture and rugs, and so forth.
8. Treat sexual contacts or relevant family members (who either have either symptoms or have the kind of relationship that makes transmission likely).

Just as the itch of scabies takes a while to reach a crescendo, it takes a few days to subside after treatment. After a week or two, relief is dramatic. If that doesn't happen, the diagnosis of scabies must be questioned.

Are cases of scabies often misdiagnosed?

Scabies is very easy to misdiagnose because early subtle cases may look like small pimples or mosquito bites. Over a few weeks, however, mistakes like this become evident as patients feel worse and worse with symptoms they can't ignore.

What are possible complications of scabies?

The intense itching of scabies leads to prolonged and often intense scratching of the skin. When the skin is broken or injured due to scratching, secondary bacterial infections of the skin can develop from bacteria normally present on the skin, such as Staphylococcus aureus or beta-hemolytic streptococci.

In what special situations can scabies be more easily spread?

Elderly and weakened people in nursing homes and similar institutional settings may harbor scabies without showing significant itching or visible signs. In such cases, there can be widespread epidemics among patients and healthcare workers. Such cases are dramatic but, fortunately, uncommon.

Scabies At A Glance

• Scabies is an itchy, highly contagious skin condition caused by an infestation by the itch mite Sarcoptes scabiei.
• Direct skin-to-skin contact is the mode of transmission.
• A severe and relentless itch is the predominant symptom of scabies.
• Sexual contact is the most common form of transmission among sexually active young people, and scabies has been considered by many to be a sexually transmitted disease (STD).
• Scabies produces a skin rash composed of small red bumps and blisters and affects specific areas of the body.
• Treatment includes oral or topical scabicidal drugs.

Medically reviewed by Norman Levine, M.D., Board Certified - American Board of Dermatology

References:

Chosidow, O. "Clinical Practices. Scabies." N Engl J Med 354.16 Apr. 2006: 1718-1727.

Dourmishev, A.L., L.A. Dourmishev, and R.A. Schwartz. "Ivermectin: Pharmacology and Application in Dermatology." Int J Dermatol 44.12 Dec. 2005: 981-988.

McCroskey, Amy L., and Adam J. Rosh. "." eMedicine.com. Apr. 5, 2010.

United States. Centers for Disease Control and Prevention. "." Nov. 10, 2008.