2.4.2 Scabies

Approximately 300 million individuals are affected worldwide each year. Overcrowding (due to poverty, wars, or natural disasters) promotes the spread of the disease. The infestation commonly spreads among family members.

Sarcoptes scabiei var. hominis is a human ectoparasite with marked host specificity. Human-to-human transmission occurs with close personal contact, sometimes via clothing or bedding. Female mites (0.3-0.5 mm long) dig a burrow in the stratum corneum of the epidermis to lay their eggs and its feces (scybala). Sexually mature mites develop in 3 weeks. In immunocompetent adults, the number of mites that live on skin surface is usually limited to 10-12 in total. Immunologic reaction to mite antigens causes inflammation and pruritus.

Symptoms usually develop 3-6 weeks after first infestation, or within 24 hours after re-infestation. Severe pruritus, especially at night, is present. Initially the mites burrow and develop elongated inflamed papules; in infants there can be evidence of vesicles. An exanthema with tiny papules and fine crusts may be found symmetrically over the flanks and proximal extremities, indicating a cellular immunologic reaction against scabies antigens. Face and interscapular regions are spared. Later complications are excoriations, eczematisation and impetiginisation. Occasionally there are minimal findings on examination, secondary to fastidious habits or use of topical corticosteroids (scabies incognito).

A special form of scabies is Norwegian (crusted) scabies, which results from massive infestation in immunosuppressed individuals. It clinically presents with generalized hyperkeratotic papules and/or plaques with thick scales. Norwegian scabies is highly contagious.

Microscopic examination demonstrates the mite, its eggs, or scybala (feces). Dermatoscopy can also help prediagnosing and guiding the site of scraping (see Chapter 9.2.1).

Histopathology is usually not necessary, except in cases when other dermatoses should be ruled out.

Recurrences occur in considerable number of patients, especially immunosuppressed ones.

Complications include impetiginisation, eczematisation, post scabietic papules (nodular scabies) and persistent inflammatory reaction without presence of mites. Itching often persists after successful eradication of the mites.

Differential diagnosis includes pruritic dermatoses, prurigo, dermatitis herpetiformis, urticaria, and psoriasis (in crusted scabies). Scabies burrows are useful for differentiation.

The treatment of choice is topical permethrin cream (5%) applied once for 8-12 hours. If there are still signs of active scabies after 14 days, the treatment should be repeated. Second line treatments include benzyl benzoate used on 3 consecutive days or crotamiton. Lindane (gamma-hexachlorocyclohexane) is comparatively toxic and in some countries no longer on the market. Ivermectin orally (single 400 mg dose for adults) is used in crusted scabies or if topical treatment is difficult.

Additional general measures should be taken to prevent reinfections. All family members and other contact persons should be treated. Clothing and bedding should be changed and washed. All non- washable items should be aired for 4 days (mites can live for 2-3 days at room temperature).