Pathological findings of experimental Aeromonas hydrophila infection in Golden Mahseer (Tor putitora)

a) Introduction: Mahseers, belong to the family Cyprinidae, are well renowned for the excellent game as well as food i¬shes. Various aspects simultaneously considered towards the domestication of this fish species also include the study of fish health issues, preventive and remedial programmes for sustainable aquaculture. Experimental studies can be conducted in the control condition to study the pathological symptoms during bacterial infection. Considering the bacterial pathogens as a major constraint for aquaculture, the present study was carried out to standardize the bacterial concentration of A. hydrophila for the experimental challenge of Golden Mahseer for the first time, and various clinical changes were also monitored during the infection. Materials and Methods: A total of 140 live fishes were acclimatized to the laboratory conditions for experimental study. A constant water quality was monitored during the acclimation period and throughout the experiment. The test group was injected with A. hydrophila and various physiological, biochemical and tissue level changes were observed during the experiment in comparison to phosphate buffer saline (PBS) injected control group. b) Results: LD50 value of A. hydrophila for Golden Mahseer was successfully standardized. Clinical signs including weakness, slower movement, swimming closer to the surface, fin haemorrhages and red patches at the gut region were observed. Enlargement of spleen followed by tissue necrosis along with signs of haemorrhagic septicaemia was also seen in infected fishes. The necrosis of hepatocytes was observed in the histological section of liver tissue. The bacterial infection increases the superoxide dismutase activity and cortisol level in Golden Mahseer. c) Conclusion: The LD50 value of A. hydrophila for experimental challenge studies on Golden Mahseer is 1.74 × 105 cfu per 100 g of body weight. The symptoms of severe bacterial haemorrhage were observed. Necrosis of hepatocytes was observed in infected fishes. The superoxide dismutase activity and cortisol level also increased in infected fishes. A. hydrophila was confirmed to be the etiological agent which was re-isolated using spread plate method and confirmed by 16s rRNA sequencing.