Histopathological Study on Oncorhynchus masou Virus Disease (OMVD) of Cultured Rainbow Trout in Natural Outbreaks and Artificial Infection

Oncorhynchus masou virus (OMV) was isolated from naturally diseased rainbow trout Oncorhynchus mykiss cultured in Nagano Prefecture. In the present study, histopathological and electron microscopic features were performed on spontaneously and experimentally infected fish with OMV. The definite change was necrosis of OMV-infected cells, which were observed in the spleen, hematopoietic tissues in the kidney, liver, intestine, heart, gill filaments, epidermis and lateral musculature. In particular, the intestine showed severe necrosis and hemorrhage in the epithelium and underlying tissues, which is the first description of OMV disease.