The Amazon is likely the most diverse ecosystem in the world with at least 3000 fish species. In Peru, despite the outbreaks reported by the farmers there are almost no reports of bacteria in fish species. The aim of this study was to isolate, identify and characterize bacterial pathogens in Amazonian fish species. Finally, they were tested some vaccine protocols in Colossoma macropomum, the main native fish in South America against Aeromonas hydrophila, the most important bacteria in Amazonian fish.
Forty-four isolates retrieved from diseased Amazonian fish were evaluated: Aeromonas sp (34 - Aeromonas hydrophila, A. dhakensis, A. veronii, A. jandaei, and A. caviae), Pseudomonas sp (3), Edwardsiella sp (2), Klebsiella sp (2), and Plesiomonas sp (3). The species identifications were confirmed biochemically and molecularly. Aeromonas hydrophila was the most common isolate with almost 25% of the cases. Virulence genes analyses, DL50 trials, and histopathology assays were also performed to characterize the disease in C. macropomum.
Vaccines were administered by intraperitoneal injection (0.1 mL= equivalent to 8 × 107 CFU per fish). For vaccination, the bacterium was inactivated with formaldehyde and fish (300) were placed in 250L-tanks (25 fish per tank). Finally, there were used six protocols of vaccination in C. macropomum. G1: Immersion; G2: Immersion + immersion; G3: Immersion + injection; G4: Injection; G5: Control immersion; G6: Control injection. After 35 days, fish from each treatment were intraperitoneally infected with 0.1 mL of bacterial inoculum and returned to the previous tanks.
Finally, this study shows the virulence characteristics of A. hydrophila in the most important farmed native fish in the Amazon and broadens the geographic range of the presence of this pathogen worldwide. Then, lethal dose tests (LD10, 50, 90, 99) were stablished at 4.6 x 106; 6.4 x 106; 8.9 x 106; and 1.11 x 107 CFU.mL-1.Only injected vaccines showed protection against the infection (Figure 1).
Histopathologically, unvaccinated fish showed necrosis with karyolysis, loss of the cytoplasmic delimitation of the renal tubules cells and hepatocytes, hemorrhage, cellular edema, and presence of bacterial colonies (Figure 2).
Intraperitoneal vaccines are a good strategy to improve the resistance of C. macropomum against A. hydrophila.