During 1978, we isolated a previously undescrived pathogenic virus from ovarian fluids of normal appearing adult landlocked masu salmon (Oncorhynchu masou) at the Otobe Salmon Hatchery, Hokkaido, Japan. Characteristics of the newly recognized virus conform to those of the Herpesvirus group, and the agent is provisionally named Oncorhynchus masou virus (OMV). The OMV has proven to be lethal for chum salmon (O. keta) by artificial immersion infection. Following immersion, 80 to 150-day-old fry began to die at 11 to 12 days later, and 35 to 60% of them succumbed in the ensuing 50 days. However, no death occurred among 240-day-old chum salmon that were similarly infected. Marked histopathologic changes were observed in liver sections. These were multiple foci of severe necrosis and syncytia formation. Further repeated experiments using other species of salmonids revealed that coho salmon (O. kisutch), kokanee salmon (O. nerka), and rainbow trout (Salmo gairdneri) were also susceptible to OMV, although some variations in susceptibility were noted. From the evidence thus far obtained, OMV is clearly a new pathogen of salmonids
