魚病研究

市販のJ-O-3型ビブリオ病ワクチンを投与した養殖カンパチで眼球突出，眼球白濁，頭部上皮組織の剥離および鰓蓋発赤の症状を呈する疾病が発生した。死亡魚から分離した細菌はAPE20E，抗Vibrio anguillarum J-O-1 ウサギ血清への凝集反応，ビブリオ属細菌同定用マルチプレックスPCRおよび16S rRNA配列の解析からV. anguillarum J-O-1型に同定された。分離細菌は感染実験によりカンパチに病原性を示すことが確認された。また，分離細菌を用いて作製したホルマリン不活化ワクチンでカンパチを免疫して攻撃実験を行った結果，対照区と比べ免疫区は有意に死亡率が低かった。カンパチで確認されたV. anguillarum J-O-1型感染症はホルマリン不活化菌体の免疫で予防できる可能性が示された。

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Yersinia ruckeri is the causative agent for enteric redmouth disease of salmonids. Although the bacterium (NBRC102019) was isolated from the sediment of a river before 2007 in Japan, the disease first occurred in a different region in March 2015 in a chum salmon Oncorhynchus keta hatchery. In the present study, we characterized the isolate from the river sediment and 9 isolates from the disease outbreak. The serotype of NBRC102019 was classified as O-6, whereas the serotypes of the 9 isolates from the outbreak were all O-7. Multilocus sequence typing revealed that the 9 isolates from the outbreak belong to the same genotype and that the genotype of NBRC102019 was distinct from that of the 9 isolates. In challenge tests using chum salmon with one of the isolates from the outbreak and NBRC102019, substantial mortality was observed (the average cumulative mortality of the duplicated test was 43.3%) with the outbreak isolate but not with NBRC102019, although both of these isolates did not cause any deaths in rainbow trout O. mykiss. These results suggest that the Y. ruckeri that caused the outbreak in chum salmon belongs to a distinct lineage from that of the bacterium isolated from the river sediment.

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