ABSTRACT
The high mortality rate among Nile tilapia (Oreochromis niloticus) and African catfish (Clarias gariepinus) polycultured in earthen ponds in Manzala, Egypt, was investigated. Mortality has been linked to poor water quality parameters accompanied with bacterial infections. Moribund farmed fishes exhibited general septicemic signs. Fish from both species (45 each) were sampled and analyzed bacteriologically. Vibrio alginolyticus (32.3%), Enterococcus faecalis (29.4%), Aeromonas caviae (23.5%), and A. veronii (14.7%) were isolated from moribund fishes using selective media and further identified by biochemical tests. 16S rRNA gene sequencing and phylogenetic analysis confirmed the identity of these isolates. Experimental infection of O. niloticus with different bacterial isolates resulted in clinical signs of hemorrhagic septicemia and mortality rates of 80%, 60%, 40%, and 30%, respectively, for E. faecalis, A. veronii, V. alginolyticus, and A. caviae. Water parameter analysis revealed marked divergence from typical values. In addition, different bacterial isolates (including Staphylococcus sciuri, S. aureus, E. faecalis, A. veronii, A. caviae, and V. alginolyticus) were identified and isolated from water samples. BLAST analysis of water bacterial isolates displayed a 100% similarity score with relevant fish isolates, indicating that the water was the likely source of infections. Histopathological examination revealed signs of bacterial infection in both fish species. In addition, common circulatory and degenerative changes with melanophore aggregation, and lymphocytic depletion in hematopoietic organs were recorded.
