Abstract

In Liaoning Province, China, mass mortality among turbot ( Scophthalmus maximus ) was observed in multiple cultured farms in 2021. Aside from hemorrhages in the fins and infrabranchial muscles, no visible damage in the infected fish was observed upon visual inspection. Histopathological examination revealed swollen branchial lamellae ends and severe necrosis in the liver, kidney , and spleen. On the one hand, chinook salmon bafinivirus (CSBV) was isolated and identified in the affected turbot using cell isolation, transmission electron microscopy (TEM) observation, and whole-genome sequencing technology. On the other, based on the 16S rRNA sequence and the analysis of the biochemical characteristics, the dominant strains isolated from the diseased fish were identified as Vibrio scophthalmi, Aeromonas salmonicida , and Vibrio splendidus . Phylogenetic analysis revealed that CSBV strain in this study (CSBV Bces-Sm22) had a high identity homology with CSBV strain Bces-Po19, isolated from Japanese flounder Paralichthys olivaceus in 2019. Furthermore, Vibrio scophthalmi, Vibrio splendidus, and Aeromonas salmonicida were close to China, Spain, and Turkey strains, respectively. In this study, we report the first case of turbot that was naturally co-infected with Chinook salmon bafinivirus (CSBV) and bacteria ( Vibrio scophthalmi, Aeromonas salmonicida, and Vibrio splendidus ) with high mortality rates.