In the present study, the effect of Dracocephalum kotschyi essential oil (DKE) was investigated on growth, haematology, immune and antioxidant defence system and resistance of rainbow trout juveniles to bacterial infection (Aeromonas hydrophila). For this purpose, the fish were fed a diet containing different concentrations of DKE including 0 (control), 0.2, 0.25 and 0.3 mg/kg diet in three replicates for 60 days. After feeding trial, the fish were challenged with a pathogenic dose of A. hydrophila. Based on the results, immune components in plasma (alternative complement activity [ACH50], IgM content, lysozyme activity, total protein and total albumin) and mucus (protease activity, IgM content and lysozyme activity) significantly elevated in fish fed diet containing 0.2 and 0.25 mg DKE/kg diet compared with other groups (p < 0.01). The plasma bactericidal activity increased in all DKE-supplemented fish. Supplementation of fish with 0.3 mg DKE/kg diet depressed the mucosal lysozyme activity and IgM content. The liver antioxidant enzymes, glutathione peroxidase (GPx), catalase (CAT) and superoxide dismutase (SOD) showed increased activity in response to 0.25 and 0.3 mg DKE/kg diet compared with other experimental diets (p < 0.01). Dietary DKE changed the haematology of the fish. The haematocrit, red blood cell count (RBC), white blood cell (WBC), haemoglobin content and mean erythrocyte cell haemoglobin content (MCHC) increased in response to DKE. In contrast, the mean corpuscular haemoglobin (MCH) decreased in fish fed 0.2 and 0.25 mg DKE/kg diet compared with other diets (p < 0.01). The supplementation of fish with 0.2 and 0.25 mg DKE/kg diet decreased the expression of cytokine-related genes, TNF-α and IL-8. In contrast, the DKE up-regulated the expression of the immune-related genes, C3 and lysozyme. DKE at concentration of 0.3 mg/kg diet depressed the C3 and lysozyme gene expressions. The DKE supplementation decreased the mortality rate of the fish after bacterial challenge compared with non-supplemented ones (p < 0.01). In conclusion, the findings of this study revealed the immune-stimulating effects of DKE at optimized dietary concentrations. In addition, DKE decreased the mortality induced by A. hydrophila infection.
