Development of formalin killed vaccine candidate against streptococcosis caused by Enterococcus sp. in Nile tilapia

Multiple bacteria under Streptococcus, Enterococcus and Lactococcus genera are considered to be associated with Streptococcosis. This study was conducted to develop and evaluate the effectiveness of whole cell formalin killed vaccine prepared from E. faecalis, E. hirae and E. faecium against the streptococcosis in Nile tilapia. To develop candidate vaccine, efficacy of whole cell killed E. faecalis (strains BFF1B1 & BFTS22), E. hirae (strain BFTS29) and E. faecium (strain BFTS31) were evaluated. The vaccine prepared by mixing 3 % formalin and left for 24 h kill the bacterial cells. Physiological saline (0.85 %) was mixed to the harvested formalin free killed cells to make final suspension for vaccine. This vaccine was administered to the fingerling tilapia (average weight 30 +/- 5 g) interperitoneal injection (IP) and bath immersion (BI) methods. The vaccinated group was monitored for 28 days to assess the developed immunity and vaccine safety into the fish. Post vaccination in vivo challenge test were performed against the pathogens. The hematological parameters (RBC, WBC, platelets and hemoglobin) and specific IgM were found significantly higher (p < 0.001) in the vaccinated fish compared to those in the non vaccinated fish. The mean mortality in IP and BI method ranged from 10.0 +/- 0.00 % to 23.3 +/- 5.77 % and 18.33 +/- 5.30 % to 40.0 +/- 0.10 %, respectively. The relative percentage survival (RPS) in IP and BI varied from 68.78 % to 88.57 % and 59.52 % to 69.05%, respectively. Mortality was significantly lower in the immunized fish compared to those in non-immunized fish. The RPS were not different (p > 0.05) between IP and vaccination method. Formalin killed vaccine showed an excellent efficacy against Enterococcus sp. infection Nile tilapia by IP method although bath immersion could also be an alternative.