Coinfection by Group C Streptococcus dysgalactiae subsp. equisimilis and Flavobacterium davisii in Nile Tilapia from the United States

Streptococcal disease and columnaris disease are common in tilapia aquaculture across the globe. Coinfections are also common but often overlooked or not reported. In the present study, we describe the clinical presentation of streptococcosis and columnaris coinfection in diseased Nile tilapia (Oreochromis niloticus), and the phenotypic (Lancefield grouping, hemolytic pattern, biochemical characteristics, antibiotic susceptibility) and genotypic (multilocus sequence analysis for streptococci and multiplex PCR for flavobacteria) characterisation of its etiological agents, namely Group C Streptococcus dysgalactiae subsp. equisimilis and Flavobacterium davisii. We further fulfilled Koch’s postulates with the S. dysgalactiae subsp. equisimilis isolate in genetically improved Nile tilapia.