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Scabies, an infection by the itch or scabies mite, Sarcoptes scabiei var. hominis, remains a major public health problem throughout the developing world ( Fig. 293.1 ). Scabies in its most severe form, crusted or Norwegian scabies ( Fig. 293.2 ), has now become a significant reemerging ectoparasitosis in the developed world, especially among homeless people, institutionalized older adults, individuals with intellectual disability, and immunocompromised individuals.
Micrograph of a cleared and slide-mounted scabies mite, Sarcoptes scabiei (ventral view).
(From Centers for Disease Control and Prevention [CDC], Atlanta, GA. CDC Public Health Image Library, image 6301.)
Crusted (Norwegian) scabies on the extensor surface of the elbow secondarily infected with Staphylococcus aureus.
Note the confluence of the crusts and pustules and the similarity of the lesion to psoriasis, both in its hyperkeratotic appearance and in its location on an extensor surface. Risk factors for crusted scabies include immunocompromise secondary to advanced age, prolonged glucocorticoid therapy, cancer chemotherapy, and human immunodeficiency virus or human T-cell lymphotropic virus type 1 infection.
(From Fitzpatrick TB, Johnson RA, Wolff K, Suurmond D. Color Atlas and Synopsis of Clinical Dermatology . 4th ed. New York: McGraw-Hill; 2001:841.)
Epidemiology
The worldwide annual prevalence of scabies has been estimated to be about 300 million cases. Although more often associated with crowding, homelessness, and institutionalization, scabies occurs worldwide in both sexes, at all ages, and among all ethnic and socioeconomic groups. In the United Kingdom, scabies is more prevalent in women and children living in urban areas and occurs more often in winter than summer. In a prospective survey in Belgium, Lapeere and colleagues reported a crude incidence for scabies of 28 cases/100,000 inhabitants per year. The highest annual incidence of scabies was noted in immigrants (88/100,000) and in people older than age 75 years (51/100,000). Scabies is hyperendemic throughout the developing world, especially in sub-Saharan Africa (13% annual prevalence rate), India, the Aboriginal regions of northern Australia, and the South Pacific Islands, especially the Solomon Islands.
Scabies infections with crusted (Norwegian) scabies are more prevalent among several specific high-risk groups including men who have sex with men, patients treated in sexually transmitted disease clinics, homeless individuals with the acquired immunodeficiency syndrome (AIDS), and patients with human T-cell lymphotropic virus type 1 (HTLV-1) infection. Many experts now recommend evaluating all high-risk patients with crusted scabies for human immunodeficiency virus (HIV) and HTLV-1 infection. In a prospective study of 23 patients with crusted scabies in Peru, HTLV-1 infection was diagnosed in 16 patients (69.6%) by enzyme-linked immunosorbent assay and confirmed by Western immunoblot analysis. In addition to HTLV-1 infection, other significant comorbid features for crusted scabies in the Peruvian study included corticosteroid therapy (8.6%), malnutrition (8.6%), and Down syndrome (4.3%).
Transmission
In contrast to ectoparasitic fleas and flies, scabies mites cannot jump or fly, but they can crawl at a rate of 2.5 cm/min on warm, moist skin (see Fig. 293.1 ). They can survive for 24 to 36 hours at room temperature and average humidity and remain capable of infesting humans. Scabies is most easily transmitted by skin-to-skin contact, as with sex partners and children playing, as well as health care providers examining highly infectious patients with crusted scabies. High-risk sexual behaviors for contracting scabies include sporadic sexual contacts and men who have sex with men. Scabies mites have not been demonstrated to transmit HIV, HTLV-1, or any other infectious agent. The more mites there are on a human host, the greater is the risk for transmission by close direct contact, more so than by indirect contact with fomites, such as shared bedding and clothing. Although rare, indirect transmission of scabies occurs and is more common in immunocompromised hosts with AIDS, in family members of an index atypical (crusted) case, and within the institutional settings described.
Several nonhuman species of sarcoptic mites can cause animal scabies with itching, inflammation, and hair loss. Animal scabies occurs commonly in domestic pets and animals, especially in cats, dogs, pigs, horses, and camels. Immunocompromised individuals may also contract animal scabies from domestic animals, usually dogs, with sarcoptic mange. Animal scabies mites are facultative ectoparasites in humans and cannot effectively complete their life cycles in human (dead-end) hosts. Infections are usually self-limited in humans but can be treated successfully, if indicated, with 5% permethrin lotion, 10% crotamiton cream or lotion, or oral ivermectin.
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