Gross examination of a spawning run of walleye (Stizostedion vitreum vitreum) showed a large proportion of fish to have tumors on the body and fins that appeared to be characteristic of lymphocystis disease. Light and electron microscopic examination revealed the presence of two distinct tumor types. One was characteristic of lymphocystis, consisting of typical enlarged nonneoplastic cells surrounded by hyaline layers and containing many 260 nm diameter lymphocystis virus particles in the cytoplasm. The other tumor, referred to as a dermal sarcoma, consisted of a solid mass of normal-sized cells and contained in the cytoplasm large numbers of 135 nm diameter virus particles referred to as walleye dermal sarcoma (WDS) virus. The WDS virus was similar in appearance to the leukoviruses and, with its outer layer sectioned tangentially, exhibited symmetry like a member of a leukovirus group designated by Fenner as subgenus C.

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 lympho cystis 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.

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SUMMARYThe RNA from infectious pancreatic necrosis virus has been purified and had a sedimentation velocity of I4S on sucrose gradients, a buoyant density of 1.6o g/ml in Cs2SO 4 and pyrimidine to purine ratios near unity. The RNA had the appearance of a linear double stranded molecule with an average length of 0"92 #m and a standard deviation of 0.07 #m when observed under the electron microscope using the Kleinschmidt protein film technique. This would correspond to a mol. wt. of 2. 4 + 0.2 × IO 6. The RNase A resistance of IPN virus RNA exhibited a marked salt dependence ; it was 92 % resistant in o. I M-NaC1, but only 9 % resistant, or less, in o.oI M-NaC1. The RNA was resistant to denaturation by boiling at NaC1 concentrations of o'04 M or higher, but did denature at lower concentrations.Polyacrylamide gel electrophoresis of the RNA indicated that two RNA species were present and the standard deviation of lengths in the electron microscope indicated that they could not differ by more than 4 × IO5 in mol. wt.

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