Large scale mortality in cultured Nile tilapia (Oreochromis niloticus): natural co-infection with Aeromonas hydrophila and Streptococcus iniae

BACKGROUND: Nile tilapia is a highly valuable fish in the aquaculture sector. A culture farm has reported heavy mortalities of tilapia. AIMS: The present study aimed to identify the etiological agent responsible for the heavy mortality in cage cultured tilapia. METHODS: The moribund and freshly dead fishes were analyzed for clinical signs. Biochemical and molecular characterizations were performed to identify the etiological agents of the disease. Also, polymerase chain reaction (PCR) assay was used to detect the presence of the virulence genes. The susceptibility of the isolates to various antibiotics was tested by the disk diffusion method. RESULTS: The results of the biochemical tests and PCR assay confirmed that co-infection with Aeromonas hydrophila, and Streptococcus iniae was responsible for the disease severity. Phylogenetic analysis of the 16S rRNA gene showed that A. hydrophila and S. iniae isolates shared 99% and 98% sequence homology with A. hydrophila and S. iniae previously deposited in the Genbank database. The multiple antibiotic resistance (MAR) index of A. hydrophila was 0.16 and that of S. iniae was 0.71. The PCR test revealed that both pathogens harbored numerous virulence factors. The experimental infection study confirmed that the synergistic action of A. hydrophila and S. iniae led to increased mortality in tilapia. Histopathological changes were observed in the liver and spleen tissues of the co-infected fishes. CONCLUSION: These findings indicate that the disease outbreak in the tilapia culture farm occurred as a result of co-infection by A. hydrophila and S. iniae.