LAUSR

Abstract Recently, diseases of koi carp Cyprinus carpio var. koi have resulted in the mass mortality of stock at a koi carp breeding farm in Tianjin. The diseased fish exhibited obvious symptoms including skin ulceration, hemorrhages on body surface, and abdominal and intestinal dropsy. Seven Gram-negative bacterial isolates were isolated from the liver and spleen of diseased koi carp. Based on the phenotypic characteristics and sequence analysis of the 16S rRNA and gyrB genes, four isolates were identified as Aeromonas veronii and named as KCL-1 to KCL-4; three isolates were identified as Vibrio cholerae and named as KCS-1 to KCS-3. Two isolates A. veronii KCL-1 and V. cholerae KCS-1 were representatively used for the artificial experimental infection in the koi carp. Infection with single bacterial isolate or co-infection with these two isolates showed that both A. veronii KCL-1 and V. cholerae KCS-1 were virulent for the koi carp, while the former was more virulent than the latter. Meanwhile, the fish infected with the A. veronii KCL-1 and co-infected with the two isolates exhibited similar external clinical signs and histopathological changes with the naturally diseased ones. The positive result of the fluorescence in situ hybridization proved the presence of these two isolates in the spleen and liver of the co-infected fish. These results indicated that both A. veronii and V. cholerae participated in the disease outbreaks of the koi carp.