Author(s): Diana Martin

Infectious Agent

Scabies is caused by the human itch mite, Sarcoptes scabiei var. hominis.

Transmission

Direct transmission of conventional scabies occurs after prolonged skin-to-skin contact with a person infested with the mite. Indirect transmission of conventional scabies through contact with contaminated objects is rare. Animals are not a source of scabies.

Crusted scabies, by contrast, is more contagious than conventional scabies. Although <20 mites typically are found on a host with conventional scabies, a person with crusted scabies, formerly called Norwegian scabies, can harbor thousands of mites in just a small area of skin. The large number of mites present in crusted scabies greatly increases the chances that a person with crusted scabies will pass mites to others by both direct and indirect routes of transmission.

Epidemiology

Scabies occurs worldwide and is transmitted most easily in settings where skin-to-skin contact is common. Scabies also can be associated with sexual activity due to prolonged skin-to-skin contact. Scabies accounted for 1.5% of dermatologic complaints and <0.5% of all complaints in returning travelers presenting at GeoSentinel clinics. Scabies is more common in travelers with longer travel (>8 weeks) than in those who travel for shorter periods. Scabies is more common in tourists or volunteers than in business travelers or travelers visiting friends or family. Scabies is common in refugees and asylum seekers.

Crusted scabies most commonly occurs among debilitated, disabled, elderly, or immunosuppressed people, often in institutional settings. No reports of crusted scabies in travelers returning to the United States have been published.

Clinical Presentation

The most common signs and symptoms of scabies are intense itching (pruritus), especially at night, and a papular itchy rash. The itching and rash each can affect much of the body or be limited to common sites (e.g., armpits, elbows, wrists, webbing between the fingers, nipples, the beltline or waist, penis, buttocks). The rash also can include small vesicles and scales.

Burrows, caused by the female scabies mite tunneling just beneath the surface of the skin, are sometimes seen. Burrows appear as tiny raised and crooked (serpiginous) grayish-white or skin-colored lines on the skin surface. Because infected people often only have a total of 10–15 mites, these burrows can be difficult to find; they are often in the webbing between the fingers, in the skin folds on the wrist, elbow, or knee, and on the breast, penis, or shoulder blades. In infants and very young children (but not usually in older children or adults), the head, face, neck, palms, and soles often are involved.

Symptoms occur 2–6 weeks after an initial infestation. For people who previously had scabies, symptoms appear much sooner, typically 1–4 days after exposure. Conventional scabies is characterized by intense itching, particularly at night, and by a papular or papulovesicular erythematous rash. Characteristic features of crusted scabies include widespread crusting and scales containing large numbers of mites; itching might be less prominent than in conventional scabies.

Diagnosis

Scabies is diagnosed clinically. Telltale signs include burrows, typically found in skin folds and intertriginous areas in a patient with itching, and the characteristic rash. Although finding mites, mite eggs, or scybala (mite feces) under the microscope can confirm the diagnosis of scabies, microscopic identification of mites is far less sensitive than clinical diagnosis. Clinically, crusted scabies often is mistaken for psoriasis, but can be accurately diagnosed by using skin scrapings because of the high number of mites in the sores. The Centers for Disease Control and Prevention (CDC) Parasitic Diseases Branch provides consultations to health care providers at parasites@cdc.gov or 404-718-4745.

Treatment

Recommended treatments for conventional scabies include permethrin (5%) cream, which is approved by the US Food and Drug Administration (FDA), and ivermectin, which is not FDA-approved for scabies, but is indicated for scabies in the World Health Organization essential medicines list. Permethrin cream should be applied over the body from the neck down, left on for 8–12 hours or overnight, then washed off; patients will need a second application 1 week later. Treat household members and close contacts along with the index case. Oral ivermectin is reported to be safe and effective to treat conventional scabies at a single dose of 200 µg/kg, repeated after 1–2 weeks. Oral ivermectin should not be used in children weighing <15 kg or in pregnant people.

Treat crusted scabies more aggressively by using a combination of permethrin and ivermectin. Daily full-body application of permethrin for 7 days and ≤7 doses of oral ivermectin might be required. Details of the treatment regimen are found at the CDC’s Parasitic Diseases Branch website. No over-the-counter treatments are available for scabies.

Prevention

Avoidance is the best way to prevent scabies; no chemoprophylaxis is known. Prolonged skin-to-skin contact with people with conventional scabies and even brief skin-to-skin contact with people with crusted scabies are the primary routes of transmission. Travelers should avoid sharing or handling clothing or bed linens used by an infected person, especially if the person has crusted scabies.

CDC website: Scabies