Scabies at a Glance
- Human infestation caused by host-specific itch mite that lives its entire life cycle within the epidermis.
- Causes a diffuse, pruritic eruption after an incubation period of 4–6 weeks.
- Is transmitted by close physical contact or by fomites.
- Topical therapy is most popular, but oral ivermectin is effective.
- Because of the common occurrence of asymptomatic mite carriers in the household, all family members and close contacts should be treated simultaneously.
Scabies is worldwide and affects all ages, races, and socioeconomic levels. Prevalence varies considerably with some underdeveloped countries having rates from 4% to 100% of the general population.1 An infested host usually harbors between 3 and 50 oviparous female mites,2 but the number may vary considerably among individuals. For example, patients with crusted formerly “Norwegian” scabies (Fig. 208-1) who have a defective immunologic or sensory response (i.e., leprosy, paraplegic, or HIV-infected patients) harbor millions of mites on their skin surface, with minimal pruritus.
Crusted scabies. Hyperkeratotic plaques populated with thousands of mites.
It has been well established that close personal contact is a prime route of transmission. Although sometimes considered a sexually transmitted disease, the equally high prevalence in children attests that casual contact or sharing of objects among children and other family members is also sufficient to transmit the disease. Transmission via inanimate objects has been best demonstrated with crusted scabies. This condition is notoriously contagious, and anyone roaming within the general vicinity of these patients risks acquiring the infestation. Indeed, 6,000 mites/g of debris from sheets, floor, screening curtains, and nearby chairs have been detected.3 Mites are also prevalent in the personal environment of normal scabies patients.4,5 In one study, live mites were recovered from dust samples taken from bedroom floors, overstuffed chairs, and couches in every patient's dwelling.5
Etiology and Pathogenesis
Scabies is an infestation by the highly host-specific mite, Sarcoptes scabiei var. homini, family Sarcoptidae, class Arachnida. The mite is pearl-like, translucent, white, eyeless, and oval in shape with four pairs of short stubby legs. The adult female mite is 0.4 × 0.3 mm with the male being slightly smaller—just slightly too small to be seen by the naked eye. The scabies mite is able to live for 3 days away from the host in a sterile test tube, and for 7 days if placed in mineral oil mounts.4,6 Mites cannot fly or jump.
The life cycle of mites is completed entirely on human skin. The female mite by a combination of chewing and body motions is able to excavate a sloping burrow in the stratum corneum to the boundary of the stratum granulosum.7,8 Along this path, which can be 1 cm long, ...