The present study was conducted to evaluate the growth, immunity, haematology, antioxidant components and resistances to Aeromonas hydrophila infection in the common carp, Cyprinus carpio fed quercetin (QUR) enriched diet. A number of 360 fish (30.5 ± 1.5 g) were distributed into 12 tanks (30 fish/tank) in three replicates and fed experimental diets containing 0% (control), 0.1% (1 g/kg diet), 0.5% (5 g/kg diet) and 1% (10 g/kg diet) QUR for 60 days. Based on the results, the survival and growth performance (weight gain [%], specific growth rate and feed conversion ratio) improved in response to dietary QUR (p < 0.01). The analysis of fish body composition indicated higher content of protein in 0.5% and 1% QUR-supplemented fish compared to others (p < 0.01). The plasma (complement activity, IgM content, lysozyme activity, bactericidal activity, total protein, total albumin) and mucus (IgM content, lysozyme activity) immune components significantly increased almost all in the supplemented fish compared to those in the control group; however, the highest values were observed in group of 1% QUR (p < 0.01). The mucosal protease and alkaline phosphatase increased only in fish fed 1% QUR (p < 0.01). The expression of the immune-related genes (C3, Lyz and IgM) significantly increased in response to 0.5 and 1% QUR compared to control (p < 0.01). The expression of the inflammatory genes, tumour necrosis factor-α (TNF-α) and interleukin-8 (IL-8) significantly declined in fish fed 1% QUR compared to supplemented fish. The activity of liver antioxidant enzymes [superoxide dismutase (SOD), glutathione peroxidase (GPx), catalase (CAT)] was also increased by QUR supplementation. The highest liver antioxidant enzyme activity was found in group of 1% QUR (p < 0.01). The QUR supplementation also affected haematology of the fish. In this regard, the values of RBC, Hct and WBC significantly elevated in response to dietary QUR (p < 0.01). The MCHC and MCH indices significantly increased in fish fed 0.5 and 1% QUR compared to other diets (p < 0.01). As an indicator of oxidative stress, the levels of malondialdehyde (MDA) showed no significant differences between the experimental groups (p > 0.01). The fish supplemented with 0.5 and 1% QUR significantly showed higher survival rate after challenge with A. hydrophila compared to other experimental groups (p < 0.01). In conclusion, QUR at optimal dietary level improved the growth, antioxidant and immune responses and resistance to Aeromonas hydrophila infection in the common carp.
