Effect of selenium nano-vaccine on hematological biomarkers and immune biochemical activity of nile tilapia (Oreochromis niloticus) challenged with Streptococcus pyogenes

Streptococcus pyogenes infection in Nile tilapia causes high mortality and economic loss. This study evaluated the efficacy of a formalin-killed S. pyogenes bacterium loaded on nano-selenium (nano-vaccination) to prevent streptococcal disease outbreaks on farmed tilapia. 120 Nile tilapia fingerlings were allocated into 4 groups: control group: was injected with 0.1mL of normal saline without any challenges, infected group: was injected with 0.1mL of 1 × 10⁷ (CFU)mL^{- 1}, nano-vaccine group: was injected with two doses of nano-vaccine with an interval of 21 days until day 32nd and nano-vaccine bacteria group: was injected with two doses of nano-vaccine then challenge with bacteria after day 32nd. Hematological biomarkers, inflammatory, oxidative stress, antioxidant enzymes, and histology were estimated in the liver. The selenium nano-vaccinated group showed significant improvement in hematological profiles and elevated antioxidant enzyme activity compared with the control. Compared to the unvaccinated fish, the vaccinated fish showed an enhancement in antioxidant capacity and hematological stability after bacterial challenge. In contrast, infected fish showed higher oxidative stress and inflammatory indicators. Selenium nano-vaccination enhanced hematological performance, antioxidant defense, and immune response in Nile tilapia, allowing effective protection against S. pyogenes infection with minimal inflammation and oxidative damage.