Abstract
Koi (Cyprinus rubrofuscus) is a world-famous ornamental fish known for its unique characteristics and economic value. This species is susceptible to various infections, leading to high mortality and morbidity rates. In this study, bacterial pathogens isolated from naturally diseased koi were identified as belonging mainly to the genera Aeromonas and Proteus through comprehensive characterisation and molecular identification. Experimental infections with Proteus sp. closely related to Proteus vulgaris (vulgaris/hauseri complex) showed some clinical signs very similar to those in naturally infected koi, suggesting that Proteus sp. in this study is a possible pathogen in addition to Aeromonas, indicating a potential co-infection. The median lethal dose (LD50) for Proteus sp. was determined to be 1.7 × 10(8) CFU/mL, with histopathological analysis showing changes such as fusions, lymphocyte aggregation, and necrosis in the kidney and liver. The bacterium was found to be sensitive to various antibiotics, suggesting that broad-spectrum antibiotics may be effective in treating the pathogen. This study represents the isolation of Proteus sp. closely related to P. vulgaris (vulgaris/hauseri complex) from koi and provides valuable insights for the prevention and management of related disease outbreaks. The study on bacterial infections in koi fish, particularly the isolation and characterization of Aeromonas species alongside Proteus sp., highlights significant implications for koi health management. The findings underscore the pathogenicity, potential for coinfection, and the susceptibility of Proteus sp. to broad-spectrum antibiotics, providing key understanding for disease outbreak management in ornamental koi populations.