Aeromonas jandaei and Aeromonas veronii caused disease and mortality in Nile tilapia, Oreochromis niloticus (L.)

Diseases caused by motile aeromonads in freshwater fish have been generally assumed to be linked with mainly Aeromonas hydrophila while other species were probably overlooked. Here, we identified two isolates of non-A.hydrophila recovered from Nile tilapia exhibiting disease and mortality after exposed to transport-induced stress and subsequently confirmed their virulence in artificial infection. The bacterial isolates were identified as Aeromonas jandaei and Aeromonasveronii based on phenotypic features and homology of 16S rDNA. Experimental infection revealed that the high dose of A.jandaei (3.7x10(6) CFU fish(-1)) and A.veronii (8.9x10(6) CFU fish(-1)) killed 100% of experimental fish within 24h, while a 10-fold reduction dose killed 70% and 50% of fish, respectively. When the challenge dose was reduced 100-fold, mortality of the fish exposed to A.jandaei and A.veronii decreased to 20% and 10%, respectively. The survivors from the latter dose administration were rechallenged with respective bacterial species. Lower mortality of rechallenged fish (0%-12.5%) compared to the control groups receiving a primary infection (37.5%) suggested that the survivors after primary infection were able to resist secondary infection. Fish exposed to either A.jandaei or A.veronii exhibited similar clinical signs and histological manifestation.