Tetsuichi Nomura

An epidemiological study of furunculosis in salmon propagation in Japanese rivers

Abstract The distribution and incidence of Aeromonas salmonicida in mature chum salmon (Oncorhynchus keta), pink salmon (Oncorhynchus gorbuscha), and masu salmon (Oncorhynchus masou) in Japan, all of which showed no apparent clinical signs of furunculosis, was investigated from September 1979 to November 1991. A total of 17 909 chum, pink and masu salmon was collected from 37 rivers in the northern part of Japan. Aeromonas salmonicida was distributed widely in these populations of salmonids. A changing pattern of the annual incidence of A. salmonicida in salmon was observed: the incidence of A. salmonicida increased until the middle of October and decreased thereafter. The agent could not be isolated from immature fish. Aeromonas salmonicida was also isolated from the ovarian fluids of fish showing no apparent clinical signs of furunculosis. We concluded that the A. salmonicida carrier state in mature fish may pose a serious problem in the prevention of furunculosis.

Surveillance and control of infectious hematopoietic necrosis virus (IHNV) and Oncorhynchus masou virus (OMV) of wild salmonid fish returning to the northern part of Japan 1976–1991

Abstract From September 1976 to December 1991, 15 432 females of five species of mature salmonid fishes, including masu salmon ( Oncorhynchus masou ), chum salmon ( Oncorhynchus ketra ), pink salmon ( Oncorhynchus gorbuscha ), sockeye salmon ( Oncorhynchus nerka ), kokanee salmon ( Oncorhynchus nerka ), and rainbow trout ( Oncorhynchus mykiss ) were collected from 47 catching stations located in Hokkaido and the northern part of Honshu, mainland Japan. Ovarian fluid and epithelial tumour tissues were inspected for viruses. Three viruses, Oncorhynchus masou virus (OMV), infectious hematopoietic necrosis virus (IHNV) and chum salmon virus (CSV) were isolated during the course of this investigation. IHNV was found in the ovarian fluid of chum and masu salmon, and has been detected on 11 occasions from fish taken at seven sampling localities. In five cases these isolates came from masu salmon. In 1980, OMV was first isolated from wild masu salmon on the Okhotsk coast. Subsequently, this virus has been isolated from the ovarian fluid and epithelial tumour tissue of masu salmon and has been found to be distributed widely in the northern part of Japan. CSV was isolated from the ovarian fluids of wild masu salmon in 1987 and again isolated in 1988 and 1991 at four localities. In Hokkaido, all eggs of masu salmon and facilities had been disinfected by iodophore before the early-eyed stage, thus avoiding an outbreak of infectious hematopoietic necrosis (IHN), OMV infection and also CSV infection until now, and the number of places where OMV had been isolated has decreased.