The changes in cortisol and expression of immune genes of GIFT tilapia Oreochromis niloticus (L.) at different rearing densities under Streptococcus iniae infection.

The present study was conducted to investigate the effects of stocking density on serum cortisol (COR) levels and expression of immune genes in the head kidney of juvenile GIFT tilapia ( Oreochromis niloticus) after Streptococcus iniae ( S. iniae) infection. Juveniles (2700) were distributed into 30 tanks at five stocking densities (150, 300, 450, 600 and 750 g/m), and each treatment had six replicates. After a 45-day feeding trial, a S. iniae challenge study was conducted for 96 h. Differences in mortality, serum COR and gene expression of C-type lysozyme, heat-shock protein 70 (HSP70), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) were analyzed. The levels of serum COR at all infected treatments were significantly higher compared with the levels of pre-infection ( P < 0.05). At 96 h post-infection, serum COR levels of high stocking densities (600 and 750 g/m) were significantly higher than those of low-density groups ( P < 0.05); the mRNA levels of C-type lysozyme, HSP70, IL-1β, TNF-α and IFN-γ were significantly lower in tilapia reared at 600 g/m group than in those reared at 300 g/m group ( P < 0.05). Fish reared in high-density treatments (600 and 750 g/m) exhibited significantly higher post-challenge mortality (43.3 and 40.0 %) at 96 h following S. iniae infection, and it was significantly higher than other groups ( P < 0.05). The data suggest that when living in high stocking density, GIFT tilapia showed decrease in immune capability, together with increased serum COR and susceptibility to S. iniae. [ABSTRACT FROM AUTHOR]