The optimum thymol requirement in diets of Channa argus : effects on growth, antioxidant capability, immune response and disease resistance

In the present study, we aimed to evaluate the effects of dietary thymol on growth, antioxidant status, liver health, immune response and disease resistance of snakehead fish ( Channa argus ) (initial weight, 11.86 ± 0.15 g). The fish were fed for 56 days with a basal diet supplemented with thymol at 0, 150, 300, 450, 600 and 750 mg/kg, leading to six experimental diets. The final body weight (FBW), weight gain (WG), protein efficiency ratio (PER) and specific growth rate (SGR) were increased as dietary thymol level increased, arriving a summit value at 450 mg/kg dietary thymol level, and then, values of these indices started to decrease when dietary thymol level further rose, and feed conversion ratio (FCR) is the opposite state. Our results showed a significant increase ( p  < .05) in the superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH‐Px) activities in liver, spleen, head kidney and intestine with diets supplemented with 300 and 450 mg/kg thymol compared to the control and significantly decrease ( p  < .05) in the malondialdehyde (MDA) activity. Results showed that dietary supplementation with 300 and 450 mg/kg thymol significantly increased ( p  < .05) the serum acid phosphatase (ACP), alkaline phosphatase (AKP), lysozyme (LZM) activities and immunoglobulin M (IgM), complement 3 (C3), complement 4 (C4) concentrations of C. argus compared to the control, and decreased ( p  < .05) the serum glutamic‐pyruvic transaminase (ALT) and glutamic‐oxaloacetic transaminase (AST) activities. Moreover, the expression of HSP70 , TNF‐α , IL‐1β and IL‐8 were down‐regulated in liver, head kidney and intestine by dietary supplementation with 300 and 450 mg/kg thymol, and the expression of IL‐10 and TGF‐β was up‐regulated. After challenge with Aeromonas veronii , survival rates in all thymol treatment groups were significantly increased ( p  < .05). Consequently, our results indicate that adequate dietary supplementation thymol can effectively enhance the growth, antioxidant status, immune response and disease resistance of C. argus , and the optimum dietary thymol requirement for maximum WG of C. argus was estimated to be 356 mg/kg.