The human itch mite, Sarcoptes scabiei var. hominis.
Transmission of scabies occurs through prolonged skin-to-skin contact with a person with conventional scabies. Crusted scabies (formerly called Norwegian scabies) is a more severe form of scabies in which a person is infested with a large number of mites and is more highly contagious. Indirect transmission may occur through contact with objects contaminated by a person with crusted scabies but is rare if the person has conventional scabies.
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 people, often in institutional settings. Skin conditions are the most common complaint among European travelers to tropical countries, and scabies is a more common skin infection among travelers with longer travel (>8 weeks).
Symptoms occur 2–6 weeks after a person is first infested. However, if someone has had scabies before, 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. Crusted scabies is characterized by widespread crusts and scales that contain large numbers of mites, although itching may be less than in conventional scabies.
Scabies is generally diagnosed by identifying burrows in a patient with itching and by observing the characteristic rash. Diagnosis can be confirmed by microscopically identifying mites, mite eggs, or scybala (mite feces); however, microscopic identification of mites is far less sensitive than clinical diagnosis. Crusted scabies is often misdiagnosed as psoriasis but can be accurately diagnosed using skin scrapings due to the high number of mites in the sores.
Permethrin (5%) cream is considered by many to be the drug of choice. Ivermectin is reported to be safe and effective to treat scabies. Ivermectin is not currently FDA-approved for scabies but can be considered for off-label use in patients in whom treatment has failed or who cannot tolerate other approved medications. Crusted scabies must be treated more aggressively; a combination of permethrin and ivermectin is recommended.
Avoid prolonged skin-to-skin contact with people who have conventional scabies and even brief skin-to-skin contact with people who have crusted scabies. Contact with items such as clothing and bed linens that have been used by an infested person should be avoided, especially if the person has crusted scabies.