Effectiveness of a divalent Streptococcus dysgalactiae inactivated vaccine in cobia (Rachycentron canadum L.)

Streptococcus dysgalactiae is a major concern in cobia (Rachycentron canadum) cultured in Taiwan; however, there have been no reports on vaccines against this pathogen. This study considered the immune response and protective efficacies of a S. dysgalactiae-derived inactivated vaccine in this fish species. Streptococcus dysgalactiae isolate SD49, belonging to the predominant PFGE pulsotype S1A1, in which 86% of isolates have been classified and SD12 being the highest virulent strain were two vaccine candidates chosen for this study. The bacterin vaccine using a mixture of these two isolates was safe and immunized fish induced greater lysozyme, phagocytosis activities, and antibody responses. Notably, the vaccine protected cobia from the highest virulent S. dysgalactiae strain with a 100% relative percentage survival when challenged 6 weeks post primary vaccination. Together, the divalent vaccine could stimulate immune responses and effectively protect cobia against S. dysgalactiae infection.