Effects of dietary vitamin C, thyme essential oil, and quercetin on the immunological and antioxidant status of common carp (Cyprinus carpio)

Nowadays, natural bioactive substances are a used in aquaculture feeds to improve fish welfare and therefore increase the quality and sustainability of fish production. The present study was conducted to compare the effects of dietary vitamin C (VC), thyme essential oil (TE), and quercetin (QR) levels on the hematology, and antioxidant and immune parameters of common carp, Cyprinus carpio. Four hundred twenty fish (20.46 +/- 0.07 g) were distributed randomly into seven experimental groups (assayed in triplicates). The experimental diets were formulated as follows: CON (0, control); VCL (500 mg VC/kg); VCH (1000 mg VC/kg); TEL (10 g TE/kg); TEH (20 g TE/kg); QRL (200 mg QR/kg); and QRH (800 mg QR/kg). At the end of the 60-day feeding trial, an increase in white blood cells (WBC) was recorded in the treatment groups compared to that of the control (P < 0.05). Hemoglobin (Hb) and mean corpuscular hemoglobin concentration (MCHC) were significantly affected by the TE treatments (P < 0.05). Fish fed the QR diets presented higher levels of hematocrit (Hct) (P < 0.05); as well as enhanced serum and liver antioxidant capacities; such as catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione reductase (GR) (P < 0.05). Malondialdehyde (MDA) activity was significantly lower in fish fed dietary additives (P < 0.05). The treated groups showed significant improvements in serum and intestine immune parameters lysozyme (LYZ), alternative complement pathway (ACH50), total immunoglobulin (Ig), respiratory burst activity (RBA), and myeloperoxidase (MPO) (P < 0.05). Additionally, serum and liver enzyme parameters such Alanine transaminase (ALT), alkaline phosphatase (ALP), and aspartate transaminase (AST) in fish fed the experimental diets were significantly lower than that of fish fed the control diet (P < 0.05). Dietary supplementations had progressive effects on skin mucus LYZ, protease, ALP, Ig, ACH50, and total protein (TP) levels than the untreated group (P < 0.05). The experiment herein successfully demonstrated that dietary VC, TE, and QR stimulate fish antioxidant capacity, as well as humoral and mucosal immune responses, and can be recommended as beneficial feed additives for common carp.