Caudal peduncle mineralized lesion resembling calcinosis circumscripta in a wels catfish Silurus glanis

: This report presents an intriguing case of a mineralized lesion resembling calcinosis circumscripta observed in the caudal peduncle of a wels catfish, housed in a public aquarium in northern Italy. The investigation encompasses a comprehensive analysis of various aspects, including clinical presentation, diagnostic imaging, as well as gross and microscopic pathology, and immunohistochemistry. Histopathology, in particular, highlighted a severe focally extensive granulomatous reaction, confirmed by dense histiocytic inflammatory infiltrates and the massive presence of multinucleated foreign body type giant cells localized around lakes and aggregates of mineralized material. Moreover, the usefulness and limitations of immunohistochemistry and special stains in characterizing fish tissues and cell types are highlighted. Although reported in elasmobranchs and sturgeons, to our knowledge, this is the first description of calcinosis circumscripta in teleost fish and it offers valuable insights into the understanding of similar pathologies in aquatic organisms.