Oncorhynchus masou virus (OMV) : Incidence of tumor development among experimentally infected representative salmonid species.

Abstract

Coho (Oncorhynchus kisutch), chum (O. keta), masu salmon (O. masou) and rainbow trout (Salmo gairdneri), which survived experimental infection with Oncorhynchus masou virus (OMV), were observed for the development of tumors induced by OMV.Tumors in coho and chum salmon were first observed 120 days post-infection and after 200 days, 35 % of the coho and 40 to 60 % of the chum salmon were affected. The rate of tumor induction was not influenced by the age of the fish at the time of infection. Tumors of rainbow trout and masu salmon were not present at 200 days post-infection but appeared after 240 and 270 days, and the rate of tumor inducement reached 12 % for rainbow trout and almost 100% for masu salmon after 365 days.Tumors occurred mainly around the mouth, but were also observed on the fins, opercula, body surfaces and corneas of the eyes. Histopathologically, the tumors were composed of abundantly proliferative, well differentiated epithelial cells supported by fine connective tissue stroma. OMV was recovered from the culture medium of one passage of the transplanted tumor cells by primary culture series.