The human itch mite, Sarcoptes scabiei var. hominis.
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 highly contagious than conventional scabies. Although fewer than 20 mites typically are found on a host with conventional scabies, a person with crusted scabies (formerly called Norwegian scabies) may harbor thousands of mites in just a small area of skin. This large number greatly increases the chances that a person with crusted scabies will pass mites to others by both direct and indirect routes of transmission.
Scabies occurs worldwide and is transmitted most easily in settings where skin contact is common. Crusted scabies most commonly occurs among elderly, disabled, debilitated, or immunosuppressed hosts, often in institutional settings. Scabies is more common in travelers with longer travel (>8 weeks) than in those who travel for shorter periods.
Symptoms occur 2–6 weeks after an initial infestation. If someone has had scabies previously, symptoms appear much sooner (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 may be less prominent than in conventional scabies.
Scabies is a clinical diagnosis. 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. Often misdiagnosed as psoriasis, crusted scabies can be diagnosed accurately by using skin scrapings because of the high number of mites in the sores.
Permethrin (5%) cream is the first-line treatment for conventional scabies. Apply the cream over the entire body (from the neck down), leaving it on for 8–12 hours or overnight. Then wash the cream off and reapply 1 week later. Treat household members and close contacts similarly.
Oral ivermectin is reported to be safe and effective to treat conventional scabies. Although not approved by the US Food and Drug Administration to treat scabies, consider off-label ivermectin use where topical treatment has failed or in patients who cannot tolerate other approved medications. The recommended oral dose is 200 µg/kg, repeated in 2 weeks.
Treat crusted scabies more aggressively by using a combination of permethrin and ivermectin. Daily full-body application of permethrin for 7 days and up to 7 doses of oral ivermectin may be required. Treatment of crusted scabies is best managed by a physician.
Avoidance is the best form of prevention. 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. Do not share or handle clothing or bed linens used by an infested person, especially if the person has crusted scabies.