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.

Northern pike from several locations in central Canada were observed to have two different types of hyperplastic epidermal lesions on the body and fins. One type of epidermal hyperplasia consisted of flat lesions, bluish-white in color with a granular or "gritty" appearance. Histological examination of this granular lesion showed the presence of grossly hypertrophied cells surrounded by normal sized epidermal cells. The enlarged nuclei in thin section contained many typical herpesvirus capsids measuring 100 nm in diameter. A large proportion of the cytoplasmic mass consisted of many dark staining inclusions in which were embedded numerous herpes-like virions. For this herpesvirus of pike the name esocid herpesvirus 1 is proposed in keeping with the herpesvirus nomenclature of Roizman et al. (19). The second type of epidermal hyperplasia was a smooth convex whitish translucent tissue mass consisting largely of a population of randomly oriented normal sized undifferentiated cells. Electron microscopic examination showed them to be associated with clusters of C-type retrovirus measuring 150 nm in diameter. The formation of these virus particles was by budding from the cytoplasmic membrane into the inter-cellular spaces. Neither the herpesvirus nor the C-type particles could be isolated in fish cell cultures.

A new virus, provisionally named Herpesvirus vitreum, was isolated from hyperplastic epidermal tissue from a walleye, Stizostedion vitreum vitreum (Mitchill), taken in Saskatchewan, Canada. The virus, which was isolated in the walleye ovarian (WO) cell line, was identified as a herpesvirus on the basis of size (190-230 nm), morphology and apparent pattern of replication. The virus, which passes polycarbonate membranes of 200 nm mean pore diameter, was ether-labile. Virus replicated in WC-1 cells at 4 and 15X, but not at 20 C. Although walleye cell lines (WO, WC-1, We-2) were susceptible to infection at 15 C, non-percid cell lines were refractory. Syncytial formation and lysis occurred in susceptible cell lines. Virus was quantified by plaque assay at 13 to 15 C for two weeks. Replication was inhibited by IO'^-'^M phosphonoacetate and by lO"^-*^ M 5-bromodeoxyuridine (BUDR), but addition of excess thymidine reversed the inhibition by BUDR. Viral replication in WO cells, but not in WC-1 cells, was inhibited by the antiherpetic drug acyclovir (10"^-" M). The relationship of the herpesvirus isolate and epithelial neoplasms was not determined.

Disease-free rainbow trout (Salmo gairdneri) in Europe have a natural (6 S) serum protein capable of inactivating a tissue culture-adapted isolate of infectious pancreatic necrosis virus, IPNV strain Sp (M.

Epidermal hyperplasia consisting of discrete translucent raised outgrowths of eells 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 microseopie observations.

An electron microscopical and biochemical examination of the properties of infectious pancreatic necrosis virus (IPN) and of its ribonucleic acid (RNA) was made. The buoyant density of IPN in CsCI was found to be 1.33 g/cm3. Electron microscopical examination of the banded virus revealed structures similar in size (74 nm) and shape to reoviruses but lacking a characteristic inner capsid structure. Polyacrylamide gel electrophoretic analysis of IPN-RNA revealed a single nonsegmented component of molecular weight 3.2 X 106. Its susceptibility to ribonuclease, base composition, and resistance to thermal denaturation indicated a single-stranded RNA structure. However, its sedimentation behavior (16S) independent of ionic strength in sucrose gradients, partial solubility in 2 M LiCl, and ribonuclease resistance in the presence of Mg2+ suggest an unusual secondary structure of unknown nature. The accumulated data indicate that IPN virus does not belong to either the picornavirus or reovirus groups and may represent a new group of viruses.

The rainbow trout, Salmo gairdneri, gonad (RTG-2) cell line was more sensitive to lytic infection by infectious pancreatic necrosis virus (IPNV) than was the fathead minnow, Pimephales promelas, (FHM) cell line in laboratory and field studies. Laboratory investigations with known strains of IPNV (VR 299 and Buhl) also indicated that the cytopathic response in IPNV-infected chinook salmon, Oncorhynchus tshawytscha, embryo (CHSE-214) cells was greater than in infected FHM cells. In contrast, titration experiments with standard infectious hematopoietic necrosis virus (IHNV) isolates (Oregon and Cedar River) showed that the FHM cells were more sensitive to destructive infection by IHNV than was either of the salmonid cell lines (RTG-2, CHSE-214). Key words: fish cell culture, infectious pancreatic necrosis virus, infectious hematopoietic necrosis virus, chinook salmon embryo cells, fathead minnow cells, rainbow trout cells, salmonid viruses