This summary discusses adult patients. Scabies in children is addressed separately.

Scabies is an intensely pruritic eruption on the skin caused by infestation of the mite Sarcoptes scabiei var hominis. It is transmitted most often via direct person-to-person contact and less frequently by fomites. It is extremely contagious, spreading between individuals who share close contact or living spaces. Prevalence rates are higher in children, residents of long-term care facilities, and marginalized populations, although scabies can appear in individuals of all ages and socioeconomic groups.

Scabies is designated as one of the neglected tropical diseases (NTDs) by the World Health Organization (WHO) because of the disease burden and its complications, particularly in areas with limited health care access, and its potential public health control strategies. Factors that contribute to the persistence and spread of scabies are overcrowding, delays in diagnosis, and poor public health awareness. Outbreaks in health care facilities, such as nursing homes, can result in dozens of patients and staff becoming infected.

In a typical infestation, there are 10-20 mites. The mite burrows into the stratum corneum of the epidermis. Most patients mount an intense hypersensitivity reaction to the mites, their eggs, or their feces. This results in a widespread and highly pruritic eruption that can result in disruption of the cutaneous barrier through scratching and superinfection. For those who are acquiring scabies for the first time, the hypersensitivity reaction usually develops 2-6 weeks after initial infestation, but for those with previous history of infestation, the itching may start in 24-72 hours. Without medical treatment, the condition persists because the mites lay eggs, causing continued infestation.

Nodular scabies, a less common variant, refers to the presence of pink-to-red or hyperpigmented nodules that represent a localized, intense hypersensitivity reaction to retained mite parts or antigen. The presence of mites within these nodules has also been reported, but nodular scabies may be seen even after the infestation has been successfully treated. It may involve the genitals. Typical sites also include the lower abdomen and buttocks.

Scabies can cause a generalized eruption resembling erythroderma or exfoliative dermatitis in patients who are elderly, institutionalized individuals, and patients with immunosuppression or neurologic dysfunction (crusted scabies). In these patients, the mite burden is much higher, with thousands to millions of mites present on affected skin.