n from six samples of flathead and one sample of Japanese sculpin; however, bacterial isolation was not accomplished, and these fishes did not show any signs of BKD. The pathogenicity of R. salmoninarum to flathead or Japanese sculpin is still unknown. Although flathead and Japanese sculpin are not important in the Japanese fishing industry, they may be carriers of BKD. Thus, these is a need to examine the pathogenicity of R. salmoninarum to these fishes. The isolation of R. salmoninarum from 20 ml of seawater in which coho salmon were cultured failed because of the inability of SKDM to support growth. Austin and Rayment (2) attempted the isolation of R. salmoninarum from fresh water of an aquarium in which rainbow trout were artificially infected with BKD. Because R. salmoninarum was not detected in any water samples, they concluded that the bacterium may have had an affinity for organic, particulate matter. In this experiment, organic particulates contained in 20 ml of seawater were collected with filters and the culture ofR. salmoninarum was attempted. However, the bacterium identified as R. salmoninarum was not isolated. To support the suggestion ofAustin and Rayment (2), the detection ofR. salmoninarum from a lot of organic particulates, at least with more than 20 ml of seawater, might be necessary. It is well known that shellfish take up nutrients by filtration of bacteria, organic matter, or particles in the environment. We speculated that some R. salmoninarum cells might be taken up by shellfish if the release of bacteria occurred in the coho salmon net pen culture. R. salmoninarum antigen was detected in 31 of 86 scallops hung from the net pen of cultured coho salmon, and no R. salmoninarum antigen was detected in the same lot of scallop before hanging. This fact indicates that R. salmoninarum cells were released from coho salmon infected by BKD to the seawater environment and were taken up by these scallops. This study indicates that R. salmoninarum is released from BKD-infected coho salmon cultured in net pens and that other animals, such as other fish species or shellfish, inhabiting the environment take in this bacterium by filtration or eating. This suggests that horizontal transmission of BKD occurs in seawater. Evelyn (6) reported that R. salmoninarum-free sockeye salmon (Oncorhynchus nerka) became infected while they were held for several months in seawater cages adjacent to cages containing BKD-infected salmon. These animals did not show any signs of BKD; however, these findings suggest that R. salmoninarum antigen released from coho salmon infected by BKD was taken up by fish and shellfish inhabiting seawater environments and that these animals play an important role in transmission of BKD. In the present investigation, chum salmon fingerlings were released in close proximity to the coho salmon net pen culture in Okkirai Bay, and no loss of chum salmon caused by BKD has been observed. However, it appears possible for R. salmoninarum to infect chum salmon populations. Therefore, there is a need to eliminate BKD-infected or carrier coho salmon from the net pen populations.
n from six samples of flathead and one sample of Japanese sculpin; however, bacterial isolation was not accomplished, and these fishes did not show any signs of BKD. The pathogenicity of R. salmoninarum to flathead or Japanese sculpin is still unknown. Although flathead and Japanese sculpin are not important in the Japanese fishing industry, they may be carriers of BKD. Thus, these is a need to examine the pathogenicity of R. salmoninarum to these fishes. The isolation of R. salmoninarum from 20 ml of seawater in which coho salmon were cultured failed because of the inability of SKDM to support growth. Austin and Rayment (2) attempted the isolation of R. salmoninarum from fresh water of an aquarium in which rainbow trout were artificially infected with BKD. Because R. salmoninarum was not detected in any water samples, they concluded that the bacterium may have had an affinity for organic, particulate matter. In this experiment, organic particulates contained in 20 ml of seawater were collected with filters and the culture ofR. salmoninarum was attempted. However, the bacterium identified as R. salmoninarum was not isolated. To support the suggestion ofAustin and Rayment (2), the detection ofR. salmoninarum from a lot of organic particulates, at least with more than 20 ml of seawater, might be necessary. It is well known that shellfish take up nutrients by filtration of bacteria, organic matter, or particles in the environment. We speculated that some R. salmoninarum cells might be taken up by shellfish if the release of bacteria occurred in the coho salmon net pen culture. R. salmoninarum antigen was detected in 31 of 86 scallops hung from the net pen of cultured coho salmon, and no R. salmoninarum antigen was detected in the same lot of scallop before hanging. This fact indicates that R. salmoninarum cells were released from coho salmon infected by BKD to the seawater environment and were taken up by these scallops. This study indicates that R. salmoninarum is released from BKD-infected coho salmon cultured in net pens and that other animals, such as other fish species or shellfish, inhabiting the environment take in this bacterium by filtration or eating. This suggests that horizontal transmission of BKD occurs in seawater. Evelyn (6) reported that R. salmoninarum-free sockeye salmon (Oncorhynchus nerka) became infected while they were held for several months in seawater cages adjacent to cages containing BKD-infected salmon. These animals did not show any signs of BKD; however, these findings suggest that R. salmoninarum antigen released from coho salmon infected by BKD was taken up by fish and shellfish inhabiting seawater environments and that these animals play an important role in transmission of BKD. In the present investigation, chum salmon fingerlings were released in close proximity to the coho salmon net pen culture in Okkirai Bay, and no loss of chum salmon caused by BKD has been observed. However, it appears possible for R. salmoninarum to infect chum salmon populations. Therefore, there is a need to eliminate BKD-infected or carrier coho salmon from the net pen populations.
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