[Histopathology of the more common viral skin infections]

We describe the histopathological characteristics of viral skin infections. Herpes simplex virus and varicella-zoster virus produce an intraepidermal vesicle with variable degrees of epithelial necrosis. Typical findings include keratinocytes with ballooned nuclei with a ground-glass appearance and giant multinucleated keratinocytes. In the endothelial cells of the dermal blood vessels, cytomegalovirus produces large eosinophilic nuclear inclusions surrounded by a clear halo. Human herpes virus 8 is etiologically associated with Kaposi sarcoma. In its early stages, this tumor contains blood vessels with a fine endothelium passing through the dermal collagen bundles. In the plaque and nodular stages, the vessel lumens are more clearly visible and there is a progressive increase in the number of neoplastic spindle cells with a low degree of pleomorphism and atypia, and occasional mitoses. The infiltrate is made up of lymphocytes and plasma cells. Contagious ecthyma and milker's nodule give rise to an acanthotic epidermis with ballooned keratinocytes containing eosinophilic cytoplasmic viral inclusions. Molluscum contagiosum shows lobules of epithelium that open onto the epidermal surface and characteristic inclusion bodies. Acanthosis, papillomatosis, and hyperkeratosis are observed in common warts, with confluence of the epidermal ridges in the centre of the lesion and koilocytes.