Scabies is a parasitic infestation of the epidermis. It is caused by the obligate human parasite Sarcoptes scabiei var hominis and is transmitted via direct skin-to-skin contact (usually 10-20 minutes' contact) and rarely by fomites such as bedding and clothing. Scabies affects all ethnic groups and socioeconomic levels. Scabies is most common in young children, and the highest prevalence is seen in children younger than 2 years. It is extremely contagious, spreading between individuals who share close contact or living spaces. Frequent outbreaks occur in schools, group homes, and orphanages where direct contact with infested individuals is common. The most common predisposing factors are overcrowding, poverty, poor nutrition, and being undomiciled. Other predisposing conditions include immunocompromised status, HIV infection, and severe intellectual or physical disability. The highest prevalence of infestation is seen in tropical regions such as Central America, the Northern Territory of Australia, and the Pacific Islands.
Infestation begins with the female mite, which burrows within the stratum corneum of the host and lays its eggs. The eggs develop into larvae, nymphs, and adults. The average number of adult female mites in an infested individual is 10-15, but this number can be much larger in those who are immunocompromised. If separated from human hosts, the mite can survive at most a few days. Symptoms and signs typically develop approximately 3 weeks after the primary infestation.
Clinically, scabies infestation manifests in 3 ways: classic, nodular, or crusted (previously called Norwegian scabies).
- Classic scabies in children presents with pruritic papules affecting flexural areas, including the axillary folds, wrists, and dorsal ankles; the interdigital web spaces of the hands and feet; anogenital area; and truncal areas, especially around the nipples and periumbilical areas (the circle of Hebra). It is accompanied by itch, which is classically worse at night, especially just after getting into bed.
- Nodular scabies, a clinical variant of classic scabies, is less common in children than in adults. It is considered a hypersensitivity reaction to the mite and manifests with erythematous nodules.
- The crusted variant of scabies is most common in individuals who are immunocompromised. It presents with widespread scaly, crusted, or hyperkeratotic papules and plaques. Scales may have a powdery texture. Pruritus may be severe but is usually minimal or absent. Nail dystrophy can be present. It is extremely contagious due to the high mite burden; there may be up to a million mites on a single individual.