A Case of Mortality Caused by <i>Bacillus</i> sp. Infection in Farmed, Red‐Spotted Masu Trout ( <i>Oncorhynchus masou ishikawae</i> ) in Japan

Red-spotted masu trout (Oncorhynchus masou ishikawae; hereinafter, referred as amago trout), is a freshwater salmonid fish species endemic to Japan. From September to October 2023, chronic mortality was observed on an amago trout farm in eastern Japan. To identify the cause of death, bacterial isolation, predominant bacterial species identification, histopathology and experimental challenge test were performed. As a result, bacterial colonies with characteristic features were isolated from all moribund fish. The 16S rRNA and rpoB gene sequences of the isolates showed high similarity with those of Bacillus sp. Identical 16S rRNA and rpoB gene sequences were also detected in moribund fish tissues. The histopathology revealed bacteria in multiple organs, including the spleen, in five of six fish examined. In the experimental challenge test, the fish that were infected with a high dose of bacteria (1.26E+07 cfu per fish) started to die at 3 days post-infection (dpi) and all fish died by 7 dpi. The cumulative mortality rate was lower in the group infected with a lower number of bacteria (1.26E+06 cfu per fish). Our findings identify Bacillus sp. as the disease agent. This is the first report of Bacillus infection in salmonids.