Evaluation of the protective efficacy of inactivated vaccines from wild-type Streptococcus agalactiae (GBS) on red Tilapia (Oreochromis sp.)

Abstract

Streptococcus agalactiae causing eye edema and hemorrhagic disease on red tilapia is a serious problem for the aquaculture industry. The experiment was conducted to evaluate the protective efficacy of formalin-inactivated S. agalactiae AG5 (Group B Streptococcus, GBS) vaccine on red tilapia (Oreochromis sp.) by feeding method. The experiment was arranged in a completely randomized design and fish were fed vaccine-mixed feeds with different concentrations of 104; 105; 106; 107; 108 CFU/g of feed, respectively. At 3 weeks after being fed vaccines, fish were infected with wild-type S. agalactiae AG5 strain by injecting 100 µL of medium containing a dose of LD50 = 6.87 × 103 CFU/mL into the abdomen. The effectiveness of relative percentage survival (RPS) protection was determined within 1 week after infection. Samples of dead fish were recorded with symptoms and brain samples were cultured on TSA medium and incubated at 28oC for 24 h. The colonies were examined using PCR test with F1/IMOD-specific primer pairs. The results showed that the inactivated S. agalactiae AG5 (GBS) vaccine had the highest protective effect on red tilapia of 50% at the vaccine dose of 107 CFU/g of feed. The study also showed that red tilapia had an immune response with the mean antibody titers in the vaccine treatments, ranging from 2.24 ± 0.20 to 3.59 ± 0.42 (P < 0.05).