Pathology of Aeromonas hydrophila infection in rainbow trout (Oncorhynchus mykiss) of Himachal Pradesh

Trout fish farming is playing an important role in providing livelihood and nutritional security to the people of hilly state like Himachal Pradesh. The present investigation was conducted during a natural outbreak of Aeromonas hydrophila infection in rainbow trout (Oncorhynchus mykiss) in a commercial trout fish farm located in the Mandi district of Himachal Pradesh, India. The overall mortality was 30%. A total of twenty-one moribund rainbow trout of around 5 months age were investigated. The clinical signs exhibited by the fish before death were lethargy, slow movement and gasping. The gross lesions at necropsy were distended abdomen (dropsy), cutaneous haemorrhages and/or ulcerative lesions at the base of fins and anal area, ascites and mild to severe degree of congestion and haemorrhage in the visceral organs. The samples collected at necropsy included swabs from the skin lesions, liver and kidney for bacteriological studies and representative tissues for histopathological evaluation. Bacteriological studies revealed the presence of Aeromonas hydrophila in the samples of moribund trout. Microscopically, gills, liver, kidneys and skeletal muscles revealed predominant vascular changes such as congestion, haemorrhage and oedema, vacuolar or Zenker’s degeneration, necrosis and an inflammatory reaction. In addition, decreased haematopoiesis in the anterior kidney and melano-macrophage center infiltration in the liver and kidney were also observed. In conclusion, the outbreak was diagnosed as acute severe septicaemic Aeromonas hydrophila infection associated with significant mortality in rainbow trout leading to economic losses.