Studies on blood biochemical indices and expression of hepatic HSP70 mRNA of different tilapia strains artificially challenged with Streptococcus iniae

In order to establish the superiority, four different tilapia strains (GIFT tilapia, New GIFT tilapia, Egypt Nile tilapia and Red tilapia) were compared in the study. At day 100 tilapias in the treatment group were intraperitoneally injected with Streptococcus iniae suspension (2.95×108 /mL) to evaluate disease resistance of four different tilapia strains against S. iniae in terms of the serum biochemical parameters and hepatic HSP70 mRNA expression before and after infection. Another set of twenty fish taken from each tank was challenged with S. iniae using the same method as above, and the cumulative mortality of 4 strains was recorded at different times post infection. The results showed that GIFT tilapia and New GIFT tilapia were more sensitive to S. iniae at 96 h after infection, cumulative mortalities were 36.67% and 38.33% respectively. Egypt Nile tilapia was less sensitive to pathogen and no death occurred during the experiment. The levels of serum cortisol (COR) and glucose (GLU) and expression levels of HSP70 mRNA in GIFT tilapia, New GIFT tilapia and Red tilapia strains were highly significant(P0.05), and the activities of glutamic-pyruvic transaminase (ALT), glutamic-oxaloacetic transaminase (AST)and lysozyme (LSZ) also increased, while serum alkaline phosphatase (AKP), triglyceride and cholesterol levels were significantly lower than those prior to infection (P0.05). By utilizing the energy from resolved hepatin and lipid, the synthesis of hepatic HSP70 mRNA and levels of LSZ and globulin in Egypt Nile tilapia were improved, and thus the non-specific immunity strengthened. It is suggested that in the course of tilapia breeding, effective combination of the disease resistance with growth be conducted so as to guarantee sustainable development of the tilapia industry.