OCCURRENCE OF MULTIPLE INFECTIONS OF BACTERIA AND MEGALOCYTIVIRUS DURING DISEASE OUTBREAKS IN NILE TILAPIA (Oreochromis niloticus) FARMS  

The reality of dead-loss in cultured fish farms due to multiple pathogen infections probably outweighs single infection. Here, we describe the isolation and characterization of the potential pathogens concurrently infected in natural disease outbreaks of Nile tilapia (Oreochromis niloticus) and their pathogenicity in the red tilapia fingerling model. Concurrent infections of bacteria and Megalocytivirus were found in two affected farms. Based on conventional phenotypic tests and sequence analysis of 16S rDNA fragment, most predominant bacteria were identified as Flavobacterium columnare and Aeromonas veronii, and remainders were Streptococcus agalactiae, Plesiomonas shigeloides and Vibrio cholerae. Experimental infection with selected single bacterial isolates demonstrated that both alpha- and beta-hemolytic A. veronii isolates were highly pathogenic to tilapia fingerling, whereas F. columnare and S. agalactiae were less virulent and P. shigeloides was avirulent. The fish exposed to A. veronii or F. columnare mimicked major internal and external clinical signs of naturally infected fish respectively. This suggests that A. veronii and F. columnare are two main pathogens co-responsible for the dead-loss of cultured tilapia farms in the present study, whereas remaining pathogens might serve as opportunistic pathogens in the disease outbreaks.