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.

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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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The exact cellular site of replication of infectious pancreatic necrosis virus (IPNV) in carrier fish is unknown. In order to determine if IPNV replicates in trout leucocytes, we purified leucocytes from normal (non-carrier) trout and separated the cells into an adherent and a non-adherent population. IPNV replicated in less than 0-01 % of the adherent leucocytes with a yield of about 400 p.f.u./cell. IPNV also became associated with less than 0-07% of the non-adherent leucocytes; either IPNV did not replicate in these cells or the yield was, at best, only a few p.f.u./cell. Trout persistently infected with IPNV (carrier fish) were tested for the presence of IPNV in leucocytes by co-cultivating with a sensitive fish cell line; this same population of trout was also tested for IPNV by organ sampling using standard methods. Ninety-eight per cent of the trout were positive for IPNV by organ sampling, but only 75 % yielded IPNV from leucocytes. Thus a blood sample from a living fish can be used to detect the presence of IPNV.