Examination of virulence and the effects of environmental conditions on the growth of Aeromonas hydrophila in farmed tilapia and histopathological changes in infected fish

Abstract

The study aims to examine the pathogenicity, the effects of environmental conditions on the growth of Aeromonas hydrophila in tilapia, and histopathological changes in infected fish. A. hydrophila isolates, which were recovered from diseased tilapia samples collected at farming cages/ponds in several northern Vietnam provinces, were identified by biochemical tests, PCR confirmation, and sequencing of 16S rRNA and housekeeping genes (gyrB). Three representative strains after identification were subjected to evaluate the pathogenicity via challenge experiments using the intraperitoneal injection method and to determine the impacts of environmental factors on their growth. The present study showed an average LD50 value (lethal dose 50%) of A. hydrophila to tilapia at 4.6×10⁵ CFU/fish, the infected fish in the challenge tests presented the clinical and gross lesions similar to the natural diseased fish, including haemorrhaging at the base of fins, skin, anal opening and in visceral organs, especially in liver and intestine. Histopathological examination of the diseased fish showed the hyperplasia of epithelial cells and haemorrhage of the gill filaments. The tissues of the liver, kidney, and spleen exhibited the lesion of haemorrhage, congestion, and degeneration, while the brain tissue appeared the colonisation by the bacteria. The A. hydrophila strains from tilapia showed a high tolerance to environmental conditions with the capacity to survive and multiply at a temperature of 15-45°C, a salinity of 0-60‰ and a pH of 5-10. The present results provide useful information to establish the strategies in prevention of A. hydrophila infection in tilapia and other freshwater fish.