Morphological Differentiation of Virus-associated Skin Tumors of Walleye (Stizostedion vitreum vitreum)

Abstract

Walleye populations in the central region of Canada have been observed to have at least four skin growths that are associated with different virus particles. These include lymphocystis disease, dermal sarcoma, and two different forms of dermal hyperplasia. All of these growths have been observed in walleye populations in a particular spawning run with some individual fish having more than one type of infection. Observations over a period of years indicate that these lesions can be found in the same locality from year to year although the observed incidences may vary.
The histopathology and ultrastructure of lymphocystis is typical of other lymphocystis in freshwater and marine fishes consisting of grossly hypertrophied cells containing many 260 nm diameter lymphocystis virus particles.
The dermal sarcoma tumors on gross observation may be confused with lymphocystis since both tumors are large, opaque, white growths often tinged with red. The dermal sarcoma can be readily differentiated on close observation by their smooth appearance in contrast to the typical granular appearance of the enlarged cells of lymphocystis.
A third skin tumor identified as an epidermal hyperplasia has a clear slime-like appearance with a bluish tint and consists of cells associated with 135 nm retrovirus particles that bud from the cell membrane into the intercellular spaces. These virus particles in contrast to those of dermal sarcoma virus are fewer in number and not found within cytoplasmic vacuoles.
A fourth dermal lesion, which in appearance is not unlike that of the retrovirus associated dermal hyperplasia, has a more diffuse character often with swelling of the underlying tissue. This growth is associated with a herpesvirus and is the only virus which has been routinely isolated and propagated in cell culture.