Scabies

**Definition:** A highly contagious infestation of the skin by the mite Sarcoptes scabiei var. hominis. It may result in epidemics when introduced into institutions such as schools and nursing homes. The mites burrow into the skin, favouring the extremities, genitalia and, in infants, the axillae. The characteristic widespread intensely pruritic papulovesicular rash results largely from the host response rather than directly to burrowing by mites. Where such a response is absent as in immunosuppressed or debilitated patients, unchecked proliferation of mites results in crusted scabies. Sarcoptic mites from other mammals such as dogs may cause a transient pruritic eruption.

**Long definition:** An infestation of the skin by the human scabies mite, Sarcoptes scabiei var. hominis, which is passed from human to human by close skin contact. The mites burrow into the epidermis and thereby excite a host immune response which limits the extent of the infestation but results in a widespread inflammatory eruption of intensely pruritic papules. These frequently become secondarily infected as a result of excoriation. In individuals who fail to mount an immune response, the mites may proliferate unchecked with the formation of extensive crusts containing enormous numbers of mites (crusted scabies).