A Study of Epithelioma in the Atlantic Salmon (S. Salar)
A benign proliferative epidermal lesion in juvenile Atlantic salmon, previously reported from Sweden, was investigated from four premises in Scotland and one in Norway. Two-year parr and two to three-year smolts and grisle were affected. The condition appeared during the summer in parr and regressed by early winter. In smolts the condition presisted for up to a year. The lesions were plaque-like or papillomatous, and single or multiple, covering up to onethrid of the body surface in small parr. Sloughing of the epitheliiomas frequently became infected with Saprolegnia sp. fungi. Microscopically, the lesions consisted of proliferation of cells most resembling those of the middle epidermis. They were a few to many times the thickness of the normal epidermis; mitotic figures, irregular arrangement and single cell necrosis were observed. Mucous cells were infrequent. The lesions were frequently infiltrated with a mixed population of inflammatory cells. This was felt to be indicative of a graft rejection type reaction and may explain the sloughing of the tumors. At the ultrastructural level the cells resembled malpighian epidermal cells, including intracellular bridges. Intracytiplasmic and extracellular virus-like particles were seen. Fish with tumors were demonstrated by the migration inhibition factor (MIF) test to have cell mediated immunity against homologous and heterologous tumor suspension. No precipitating antibodies were demonstrated by the agar gel diffusion test. A proportion of tumor cells in cryostat sections showed fluorescence in a double indirect fluorescent antibody test using homologous serum. In artificial media the tumor cells grew primarily as suspended clumps, rather than as a monolayer. Virus isolation was attempted, using nine established cell lines.