The main pathogen in the Nile tilapia (Oreochromis niloticus) culture, Streptococcus agalactiae, causes economic harm. Infected fish's immune systems worked to eliminate of the infection. 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. Fish that survived employed to gather blood, and immune responses were assessed through measures of the survival rate include blood smears, antibody titers, and immunoglobulin gene expression. 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 group, and antibody levels were high in the 10¹⁰ CFU/fish injection. The analysis of blood cell types using the blood smear method revealed a progressive increase in leucocytes, particularly lymphocytes, neutrophils, and monocytes, in the treatment group compared to the control group. Moreover, the erythrocyte/leucocyte ratio decreased significantly in response to the high bacterial injection, indicating an increase in leucocytes. Conversely, the erythrocyte level stayed ed within at the 7.03-9.70 × 10² cell/ml and shown no significant difference (p > 0.05). 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.

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