Epidermal hyperplasia of walleye, Stizostedion vitreum vitreum (Mitchill), associated with retrovirus-like type-C particles: prevalence, histologic and electron microscopic observations

Epidermal hyperplasia consisting of discrete translucent raised outgrowths of cells were observed on the skin of walleye, Stizostedion vitreum vitreum (Mitchill), during their spawning period in the spring. The cells constituting the hyperplastic growths were limited to the epidermal layer, and were associated with surface budded, 120-nm-diameter, retrovirus-like particles located in the expanded intercellular spaces. These tumour-like growths were distinct from the other virus-associated skin lesions of walleye including dermal sarcoma, lymphocystis disease and herpesvirus-associated hyperplasia. Lesions could be differentiated by careful observation in the field and comparison of portions of each growth by histologic and electron microscopic observations.