Isolation and identification of pathogens causing haemorrhagic septicaemia in cultured Chinese sturgeon (Acipenser sinensis )

Chinese sturgeon (Acipenser sinensis) is an endemic fish species of China listed as a critically endangered (CR) species on the IUCN Red List (since 2010). There was a recent outbreak of a novel disease characterized by haemorrhagic ascites and intestinal and renal haemorrhaging in cultured Chinese sturgeon, which clearly posed a serious threat to this species. Two kinds of pathogenic bacteria were isolated from moribund diseased Chinese sturgeon. Two isolates, AHth-1 and AVth-2, were selected as representative strains and were identified as Aeromonas hydrophila and Aeromonas veronii based on their morphology, biochemical characteristics, and a phylogenetic analysis derived from 16S rRNA, gyrB, and rpoD gene sequencing. Experimental infection tests showed that both identified isolates were pathogenic to juvenile Dabry's sturgeon (A. dabryanus) via intraperitoneal injection. Histopathology of Dabry's sturgeon infected with A. hydrophila or A. veronii revealed liver hyperaemia, hepatic, and splenic necrosis, and enteritis. Antimicrobial susceptibility tests showed that both bacterial strains were equally sensitive to fluoroquinolones antibiotics, and to third- and fourth-generation cephalosporins. These results provide a scientific reference for the diagnosis, prevention, and treatment of haemorrhagic septicaemia caused by A. hydrophila or A. veronii in cultured sturgeon.