The worldwide prevalence has been estimated at about 300 million cases annually.¹ Mites cannot fly or jump, but crawl at the rate of 2.5 cm/min on warm skin. Transmission of scabies therefore usually requires spread via direct contact from person to person. Transmission from parents to children, and especially from mother to infant, is common. In adolescents, infection is often associated with sexual encounters. Institutional transmission from patient to patient or patient to caregiver is common. Spread via fomites (bedding, clothing) is less frequent, but well documented and is facilitated by the fact that mites can survive off a host for 24 to 36 hours at room temperature and average humidity, but longer in colder, more humid conditions. Probability of transmission is determined by parasite density and the duration of close contact.

The skin eruption of classic scabies is considered a consequence of both infestation and a type IV hypersensitivity reaction to the mite. The prominent clinical feature of scabies is itching. It is often severe and usually worse at night. Incubation period between infection and development of symptoms is 3 to 6 weeks for primary infestation, but may be as short as 1 to 3 days in cases of reinfestation. Scabies is generally a nuisance on account of itching, rash, and its ability to spread among people, but bacterial superinfection or severe infestation may occur. The risk of severe outbreaks and complicated scabies is particularly high in institutions (including nursing homes and hospitals) and among socially disadvantaged populations and immunocompromised hosts.

The initial lesion is a small, nondescript erythematous papule, which is often excoriated through manipulation by the patient before presentation. Not always present,
but essentially pathognomonic is the burrow, a thin, grayish, reddish, or brownish line that is 2 to 15 mm long. Burrowing is facilitated by secretion of proteolytic enzymes, which can trigger allergic reactions. Allergic miniature wheals, vesicles, pustules, and rarely urticaria or bullae may also be present. All of these are more common in recurrent infection.

Predilection sites for scabies include the sides and webs of the fingers, the flexor aspects of the wrists, elbows, axilla, breast, the periumbilical areas, waist, male genitalia, the lower buttocks and upper thighs, the extensor surface of the knees, and the lateral and posterior aspects of the feet. Head and back are usually spared except in very young children or in crusted scabies. Nonspecific secondary lesions, including eczematization and impetiginization, may occur.

Patients with an impaired immune system, such as patients with HIV, with cancer, or on immunosuppressive medications may develop widespread infestation, usually referred to as crusted scabies (formerly “Norwegian scabies”). Affected patients experience only mild itching, but exhibit thick crusts of skin that contain thousands of mites resulting in high infectivity.