Characterization of streptococcus-like bacteria from diseased Oreochromis niloticus farmed on Lake Kariba in Zambia

Tilapia (Oreochromis niloticus) farming on Lake Kariba, Zambia has been practiced since early 2000s. In recent years, the industry has experienced disease outbreaks with clinical signs consistent with streptococcus-like infections. Affected fish were growers weighing approximately 200 g or above from farms designated A and B, reared in cages on the lake. The objective of this study was to identify and characterize the causative bacteria of these outbreaks. A total of eighty-six diseased fish were sampled between 2014 and 2016. Bacteria were cultured from internal organs and were characterized by biochemical tests, PCR and sequencing of the 16S rRNA gene or 16S-23S intergenic spacer region. The results showed the prevalence of Lactococcus garvieae at farm A in 2015 and 2016 outbreaks (15/20 and 23/40, respectively) and at farm B (6/20) in 2016. Infection of naive fish with one isolate of L. garvieae recovered from the outbreaks resulted in clinical signs and mortalities akin of what had been observed during the outbreaks, confirming Koch's postulates. In the 2014 outbreaks, the low prevalence of Streptococcus agalactiae (4/15 and 2/11 respectively at farms A and B) and S. iniae (2/15) at farm A suggests that other pathogens were likely involved in the disease outbreak observed.