Isolation and Characterisation of Aeromonas hydrophila , Aeromonas veronii and Vibrio cholerae O139 From Diseased Bester Sturgeon ( Huso huso × Acipenser ruthenus ): A Case Report

This study investigated bacterial pathogens associated with disease and mortality in farmed bester sturgeon (Huso huso × Acipenser ruthenus) reared in freshwater cages in the Vacha Reservoir, Bulgaria. Affected fish exhibited lethargy, anorexia, haemorrhagic skin lesions and multifocal internal pathology. Histological examination revealed hepatic degeneration and vacuolar change, consistent with systemic bacterial infection. Bacterial isolates recovered from blood and internal organs were identified as Aeromonas hydrophila, A. veronii and Vibrio cholerae O139 using culture-based methods, MALDI-TOF MS, species-specific PCR and 16S rRNA gene sequencing. Virulence profiling showed that A. hydrophila carried multiple virulence-associated and enterotoxin genes, whereas A. veronii possessed only aerA. The V. cholerae O139 isolate lacked ctxA and rfbO1, confirming a non-toxigenic profile. In vivo infection trials with clinical sturgeon isolates A. hydrophila HGM24 and V. cholerae O139 GM24 produced dose-dependent mortality, demonstrating the pathogenic potential of both isolates. This case represents the first concurrent detection and characterisation of A. hydrophila, A. veronii and V. cholerae O139 in bester sturgeon in Bulgaria. It underscores the importance of integrated diagnostic approaches combining pathogen identification, virulence assessment, histopathology and experimental infection in freshwater aquaculture disease investigations.