Pathogenicity Analysis of Co‐Infection by Vibrio anguillarum and Rahnella aquatilis Causing Ulcerative Fin Disease in Tawny Puffer ( Takifugu flavidus ): Pathogen Isolation, Histopathology and Apoptosis Analysis

ABSTRACT Takifugu flavidus has high edible and medicinal value, and it is one of the pufferfish with high price and good quality in the market. With the expansion of pufferfish farming, the economic losses caused by bacterial diseases are becoming increasingly serious. Among them, ulcerative fin disease is one of the typical diseases. In this study, dominant strains were isolated from T. flavidus suffering from ulcerative fin disease, and artificial infection experiments were conducted using these strains. The dominant strains were identified by colony morphology, physiological and biochemical identification, drug sensitivity test and genome sequencing. The pathological damage and apoptosis of fins, liver, spleen, intestine, stomach and gills were systematically analysed by microscopic observation, transmission electron microscope observation and TUNEL detection. Two dominant strains were identified as Vibrio anguillarum (16E1‐4) and Rahnella aquatilis (LB1‐1). Artificial infection tests showed that V. anguillarum was more pathogenic than R. aquatilis . The median lethal dose (LD 50 ) for 16E1‐4 was 1.11 × 10 5 CFU/g (body weight), and the LD 50 for LB1‐1 was 4.58 × 10 7 CFU/g. The LD 50 of co‐infection with strains 16E1‐4 and LB1‐1 was 5.27 × 10 5 CFU/g. Histopathology showed that co‐infection caused serious damage to the fins, liver, spleen, intestines, stomach and gills of the T. flavidus . TUNEL assay showed that co‐infection induced apoptosis in these organs. The results not only provide a theoretical basis for the prevention and control of bacterial diseases in T. flavidus culture, but also provide a reference for the prevention and control of similar diseases in other fish.