Streptococcosis is widely recognized as a major infectious disease hampering tilapia production globally. Vaccines have been proven to be a sustainable and effective tool to prevent disease outbreaks, allowing farmers to achieve more stable crops. However, optimal fish protection can be affected by the type of vaccine, delivery method and due to multifactorial interaction between fish intrinsic characteristics and environmental factors.
This study evaluated the effects of tilapia feed supplementation with a yeast-based immunostimulant rich in β-1,3/1,6-Glucans (Safglucan®) on the antibody titers and survival after S. agalactiae challenge. Juveniles (20g ±4.7) were fed on control or supplemented diets for two weeks at 28°C ± 2.85 water temperature, prior to intraperitoneal vaccination (Aquavac Strep SA - MSD). Non vaccinated groups were injected with 0.9% saline solution. Fish were challenged by intraperitoneal injection with S. agalactiae (strain SA583-19) at 7- or 21-day post vaccination (DPV). Negative control group was injected with 0.1 mL of sterile BHI broth. All animals were fed on control diet with no β-glucans supplementation after challenge.
At 7-DPV, anti-S. agalactiae IgM ELISA results from surviving animals demonstrated an increase in antibody titers in the vaccinated groups. A more pronounced increase was observed from the 14-DPV onwards. No clear effect of the immunostimulant in the anti-body titers were observed. At 21-DPV, significant higher relative percentage survival (RPS) was observed in the vaccinated groups, with the best response on the β-glucans supplemented group. At 7-DPV, significant RPS was observed only in the group vaccinated and supplemented with β-glucans.
The supplementation of tilapia feed with Safglucan® at 1g/Kg of feed significantly reduced the time required for the initial induction of a protective immune response and increased the level of protection conferred by the vaccine against streptococcosis in the experimental infection.