Efficacy of an Inactivated Bacterin Vaccine Against Emerging Streptococcus dysgalactiae in Channel Catfish (Ictalurus punctatus).

Lancefield Group C Streptococcus dysgalactiae recently emerged as a catfish pathogen in the US. Reports from hatcheries suggest S. dysgalactiae causes substantial broodfish loss. We aimed to develop a challenge model and evaluate vaccine efficacy against S. dysgalactiae in juvenile channel catfish (Ictalurus punctatus). Three S. dysgalactiae (14-1, 14-24 and 14-25) isolates were tested for virulence. Intracoelomic (IC) injection demonstrated S. dysgalactiae 14-1 was the most virulent. Reisolated 14-1 was used to develop a formalin-killed cell (FKC) vaccine. Catfish (200) were immunized via IC injection of 100 μL FKC with adjuvant (1:1 v/v), and another 200 catfish were sham vaccinated. Five fish from each group were bled for antibody titre every 14 days. At 28 and 56 days post-vaccination (dpv), subsets of both groups were challenged with S. dysgalactiae via IC injection (200 μL/fish) in triplicate (20 fish/tank). Cumulative percent mortality was significantly (p < 0.05) higher in sham (46.67% and 23.33%) than vaccinated fish (5.00% and 5.00%) at 28 and 56 dpv, respectively. Relative percent survival in vaccinated groups was 89.3% at 28 dpv and 78.6% at 56 dpv. Conclusively, the adjuvanted S. dysgalactiae bacterin induced humoral immunity and protected juvenile catfish. Future studies are planned to evaluate this bacterin in broodfish.