Survival, serum biochemical parameters, hepatic antioxidant status, and gene expression of three Nile tilapia strains under pathogenic Streptococcus agalactiae challenge.

Streptococcosis is the leading bacterial disease impacting Nile tilapia (Oreochromis niloticus) and causes substantial economic losses in China. This study assessed the resistance and tolerance of three Nile tilapia (88, 99, and NG) to Streptococcus agalactiae infection. Survival rates were monitored, and samples were collected from blood, liver, and spleen at 0, 1, 2, 5, 7, and 14 days post-infection. Serum biochemical parameters, hepatic antioxidant enzymes, and the expression of related antioxidant and immune genes were measured. Results showed that strain 88 had superior resistance, with the highest survival rate, reduced liver damage, and lower alanine aminotransferase and alkaline phosphatase levels at 1 and 5 days post-infection. This strain also had higher serum cholesterol and triglyceride levels between days 5 and 14. Antioxidant activities increased in all strains post-infection, with strain 88 showing significantly higher superoxide dismutase (SOD) activity and elevated catalase (CAT) and SOD gene expressions; this indicated enhanced control of reactive oxygen species and reduced tissue damage. Additionally, strain 88 exhibited lower levels of inflammatory markers (TNF-α, IL-1β, IL-10) with higher IgM levels and superior MHC-II expression during early infection, which demonstrated a strong immune response. These results indicate that strain 88 is a promising candidate for selective breeding to improve streptococcosis resistance in tilapia.