Bacterial kidney disease (BKD) has

Bacterial kidney disease (BKD) has been reported as the cause of serious mortality among salmonid fish cultured in fresh water (7). The causative agent of BKD is a grampositive, nonmotile, non-spore-forming diplobacillus and belonged to the genus Corynebactenum (9). Research by Sanders and Fryer (13) on the guanosine-cytosine (GC) content, peptidoglycan, and cell wall composition of the bacterium prompted them to place it in a new genus, Renibactenium, and give it the species name salmoninarum. The first appearance of BKD in Japan was reported from Hokkaido in 1973 (8), and it spread rapidly among Japanese salmonid fish farms. Clinical signs of this disease are graywhite necrotic abscesses in the kidney, exophthalmia, and abdominal distension. The infection progresses slowly, and a long period is required between initial infection of fish and the appearance of clinical signs (7). It is generally accepted that horizontal transmission is an important means of spreading BKD, although vertical transmission also occurs. Austin and Rayment (2) reported that this bacterium can be isolated from feces and sediment in freshwater tanks in which BKD-infected rainbow trout, Oncorhynchus mykiss, were cultured. BKD also occurs in salmon after they are transferred from fresh water to seawater. The possibility of horizontal transmission in seawater was suspected (6); however, it has not been demonstrated. In Japan, the production of cultured coho salmon, Oncorhynchus kisutch, reached 14,000 metric tons in 1989. Coho salmon culture begins with eggs imported from the United States, and after hatching, the fry are reared in fresh water for 10 months and then transferred to net cages in seawater. BKD is the most prevalent bacterial disease in these cultured coho salmon, and the loss attributable to BKD in seawater is larger than that in fresh water (10). In spite of the loss of coho salmon in seawater, no epizootiology of R. salmoninarum released from the dead fish into the net cage has been investigated. In this study, we attempted to detect R. salmoninarum antigen in the aquatic environment of cultured coho salmon.