Fish Pathology

Mortalities were observed in farmed greater amberjack Seriola dumerili immunized with a commercial Vibrio anguillarum serotype J-O-3 vaccine. Symptoms of infection included extruded eye, opacity of cornea, exfoliation of head epithelium and operculum redding. Bacteria isolated from infected fish were identified as V. anguillarum serotype J-O-1 based on APE20E, anti V. anguillarum J-O-1 serum, species-specific multiplex PCR assays, and 16S rRNA sequence analysis. Experimental infection revealed that the isolate was pathogenic to greater amberjack. In a vaccine trial, mortality of fish immunized with formalin-killed cells (FKC) of the isolate was significantly lower than that of control fish, suggesting that FKC of the isolate prevents greater amberjack from V. anguillarum serotype J-O-1 infection.

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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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