Characteristics and pathogenicity of a virulent Aeromonas hydrophila associated with ulcerative syndrome in farmed rainbow trout, Oncorhynchus mykiss (Walbaum)

Raceway-cultured rainbow trout, Oncorhynchus mykiss (Walbaum), suffered skin hemorrhagic ulcers in the caudal regions of the body in March-June 2010 in Uttarakhand State, India. The advanced stage of the disease was characterized by deep ulcers and hemorrhagic frayed fins. The causative agent was identified as Aeromonas hydrophila (designated RTMCX1), a highly pathogenic fast-growing bacterium. Amplification and sequencing of the 16S rRNA gene (1304 base pair) revealed that the sequence was 99% similar to A. hydrophila subsp. hydrophila. Our strain exhibited strong ß-hemolytic activity against sheep blood and rainbow trout erythrocytes, and was capable of growing in rainbow trout serum and 0.5-10% NaCl at 4.0-35°C. Mortality was 100% in experimentally-infected fish, with a median lethal dose (LD₅₀) of 1.9 × 10⁴ colony forming units/g body weight. Infiltration of inflammatory cells in the kidney and gall bladder, rhabdomyolysis in the muscle, and interstitial nephritis were the dominant histopathological changes. RTMCX1 was sensitive to a majority of 27 tested antibiotics, indicating the possibility of controlling this bacterium by antimicrobial compounds.