Diagnostic exercise: Multiple skin nodules in a cat

A 10-year-old, neutered, male, domestic short-haired cat had numerous, small, firm, round, red, nonpruritic, nonpainful, dermal nodules 5-16 mm in diameter that ruptured within 48 hours of their appearance and subsequently crusted over. The masses were located in all regions of the body. One mass was excised from the dorsal right carpus and examined histologically, and 2 masses from the interscapular region were cultured for bacteria. The excised dermal mass from the carpus effaced normal dermal architecture, pressed tightly against the epidermis, and was composed of tightly packed round to polyhedral cells that extended to the deep margins of the sections. The overlying epidermis was extensively ulcerated and vesiculated with intraepidermal nests of cells identical to those in the dermis. There was marked anisokaryosis in the deeper regions of the mass with numerous multinucleated cells and cells with giant bizarre nuclei. The histological appearance and CD18 immunocytochemical staining of this mass are consistent with a diagnosis of feline progressive epitheliotropic dendritic cell histiocytosis.