Scabies

Scabies is caused by the mite Sarcoptes scabiei. Spread is by direct person-to-person contact. The adult female mite burrows into the stratum corneum where she lays her eggs. The eggs hatch after a few days in the stratum corneum where the larvae mature into adult mites within two weeks and the female mites lay their eggs to continue the cycle.

The symptoms and signs occur about 4 weeks after infestation, because of a hypersensitivity reaction to the mites: intensely itchy skin with irregular slightly scaly burrows (seen between the finger webs, wrists, ankles, medial and lateral borders of feet) and papules (seen on the penis). In infants, the face is often affected with red itchy papules and hands and feet with vesicles. A secondary eczematous rash is often seen a few weeks after treatment.

Crusted scabies (previously called Norwegian scabies) is severe infestation with scabies resulting in hyperkeratosis of the skin including the subungual skin (Figure 21.1). This is seen in cases of untreated scabies (e.g. elderly individuals with dementia) or those with immune deficiency (e.g. immunosuppression following organ transplantation or in HIV infection).

Diagnosis is based on the clinical features described above. With a magnifying glass, the mite may be visible as a white dot at the end of the burrow. Close contacts should also be examined for scabies.

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