Epidermal Tumors on Rainbow Smelt and Atlantic Salmon from Nova Scotia

Abstract

Epidermal tumors on rainbow smelt Osmerus mordax are described in detail for the first time. These tumors apparently progress from small preneoplastic lesions to papillomas, and to carcinomas on some males. These tumors and similar tumors on Atlantic salmon Salmo salar have not been previously reported from Canada. Tumors were found on rainbow smelt mainly in freshwater during the spring spawning run and on Atlantic salmon grilse in freshwater in October, These tumors varied in size and number on each fish. The smaller growths were usually translucent and whitish, with a smooth surface. Some larger tumors were papillomatous and appeared reddened because of increased vascularization. Preneoplastic lesions and papillomas had a thickened epidermis with an intact basement membrane, and most of the cells were enlarged. fn papillomas of rainbow smelt, tumor cells that were not close to papillae were vacuolated, and necrotic regions were present. Carcinomas on the heads of male rainbow smelt had regions of pleomorphic cells which invaded the subepithelial tissues and penetrated the cartilage of the mandible of two specimens found in brackish and seawater. Intranuclear basophilic inclusions suggestive of a viral agent were present in the tumors of both rainbow smelt and Atlantic salmon.