Streptococcosis in freshwater farmed seabass Lates calcarifer and its virulence in Nile tilapia Oreochromis niloticus

This study investigated disease outbreaks caused by Streptococcus iniae in freshwater pond-cultured seabass Lates calcarifer in Thailand. The affected fish exhibited typical signs of streptococcal infection except for the ophthalmological lesions. Four putative S. iniae isolates (CU/KSC1, CU/KSC2, CU/KSC3 and CU/KSC4) were identified based on phenotypic characteristics and PCR amplification of the 16S rRNA gene. Phylogenetic analysis of the partial (1234 bp) 16S rRNA nucleotide sequences revealed that CU/KSC1-3 clustered within the group of S. iniae previously reported from diseased Nile tilapia in Thailand, while CU/KSC4 was more distant. Infectivity trials in seabass fingerlings (approximately 7 g bodyweight) via intraperitoneal injection (IP) showed > 60% accumulative mortality (63.3-83.3%) in the fish that received 10(6) colony forming units (CFU) fish(-1) of either isolate at 10 d post infection (dpi). A similar IP challenge protocol using the most virulent isolate (CU/KSC4) was conducted to examine the pathogenicity of the seabass isolate in Nile tilapia Oreochromis niloticus. A lower mortality, compared to that of the laboratory infected seabass, was observed in Nile tilapia, where the mortality at 10 dpi was 26.7, 30 and 33.3% in Nile tilapia receiving S. iniae at 10 (Bromage and Owens 2020), 10(6) and 10(7) CFU fish(-1), respectively. Moreover, S. iniae was re-isolated from the kidney of both affected fish species, but not from the surviving fish. However, the intestines obtained from these survivors were PCR positive for S. iniae, giving the possibility that the bacteria may be excreted into the rearing water via feces. To our knowledge, this is the first report of S. iniae isolated from disease occurrences in freshwater-inland cultured seabass in Thailand.