Immune responses and immunoglobulin gene expression of Streptococcus agalactiae infected Nile Tilapia and the effective dose of inactivated Streptococcus agalactiae vaccine for Nile tilapia (chitralada 3 strain) in Thailand

Abstract

Abstract Streptococcus agalactiae is the primary pathogen in the Nile tilapia (Oreochromis niloticus) culture and creates an impact through economic damage. the immune system functioned to eliminate the pathogen in infected fish. This study demonstrated the effect of different bacterial concentrations on tilapia immunity and optimal vaccine concentration to induce immunity in Nile tilapia. The experiment was performed at 102, 104, 106, 108, and 1010 CFU/fish of S. agalactiae compared with the control (PBS) through intraperitoneal injection for 72 h. Immunoglobulin gene expression, antibody titers, and blood smeared to measure the survival rate. The vaccine experiment investigated formalin-inactivated S. agalactiae vaccination and administered S. agalactiae injections for 14 days. The statistic revealed a significant difference (p < 0.05) in the 108 and 1010 CFU/fish injections with high survival rates (62.22% and 53.33%, respectively). Immunoglobulin gene expression was highly represented in the 1010 CFU/fish injection; antibody titers were significantly improved from the control treatment, and antibody levels were high in the 1010 CFU/fish injection. The comparison of the blood measurement from the blood smear technique indicated gradual leucocyte enhancement, especially of lymphocytes. In addition, the erythrocyte/leucocyte ratio was reduced in the highly bacterial injection, in which the experiment disclosed that the leucocytes increased. Conversely, the erythrocytes stayed at the same number. The lymphocytes were almost two-fold in 1010 CFU/fish compared to 108 CFU/fish. As depicted in the lowest concentration of 106 CFU/fish, the vaccine performance had a high relative percent survival (RPS) at 86.67%. This research suggested that the tilapia infected with high S. agalactiae concentrations did not affect the mortality of the tilapia, and vaccine concentration was effective in 106 CFU/fish.