Immunogenicity and seasonal effect of formalin-killed bivalent vaccines against Streptococcus iniae and Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss) under farm conditions.

Streptococcosis and Yersiniosis are prominent diseases affecting rainbow trout farming, frequently leading to large-scale fish deaths and substantial financial setbacks for producers. This study aimed to evaluate the immunogenicity of formalin-killed vaccines (injection form) and assess the seasonal influences on streptococcosis and yersiniosis in rainbow trout. In this study, 1500 fish were divided into two groups: a vaccinated group (VG, vaccinated against both streptococcosis and yersiniosis) and an unvaccinated control group, with three replicates per group in each season. The average fish weights (mean ± SD) for each season were as follows: spring, 52.90 ± 2.02; summer, 54.84 ± 2.90; fall, 55.45 ± 3.57; and winter, 56.12 ± 2.90. No notable changes were observed in blood indices such as hematocrit, red blood cells, and hemoglobin, except for white blood cells during spring (P = 0.0127) and fall (P = 0.0202), between the vaccinated and non-vaccinated groups (NVG) (P > 0.05). Serum biochemical indices, including total protein, cortisol, and glucose, were notably higher in the VG relative to the NVG in all seasons except summer (P < 0.05). Additionally, immune criteria, including serum lysozyme, bactericidal activity against S. iniae (in all seasons), complement activity, mucus lysozyme (in all seasons), total mucus Ig, and blood NBT, were notably elevated in the VG versus the NVG in all seasons except summer (P < 0.05). In addition, serum antioxidant enzymes such as catalase, superoxide dismutase, and glutathione peroxidase were notably higher in the VG relative to the NVG in all seasons except summer (P < 0.05). Antibody titers against streptococcosis and yersiniosis were notably higher in the VG against the NVG in all seasons except summer (P < 0.05). The VG revealed notably higher expression of immune-antioxidant genes, including Nuclear Factor Erythroid 2-Related Factor 2, Interleukin-1 beta, Tumor Necrosis Factor alpha, and Insulin-like Growth Factor 1 relative to the NVG in all seasons except summer (P < 0.05). Overall, injection vaccination in spring and fall, when the weather is cooler, provides the most effective immune response, antioxidant activity, gene expression, and antibody production against streptococcosis and yersiniosis in rainbow trout. Winter also has some beneficial effects, but summer is unsuitable for injection vaccination due to high temperatures.