Tarragon (Artemisia dracunculus) Essential Oil at Optimized Dietary Levels Prompted Growth, Immunity, and Resistance to Enteric Red-Mouth Disease in the Rainbow Trout (Oncorhynchus mykiss)

Fingerlings of the rainbow trout, Oncorhynchus mykiss (n = 300, 10.63 ± 0.6 g), were fed tarragon (Artemisia dracunculus) essential oil (TGO) for 2 months to examine its effects on growth properties, immunity, and resistance to Yersinia ruckeri infection. The treatments were control or TG1, TG2 (fed 0.5% TGO), TG3 (1% TGO), and TG4 (2% TGO). According to the results, an improvement was observed in growth parameters in all TGO-treated groups compared to the control (P < 0.05). The digestive enzyme activities (protease and lipase) were significantly elevated in response to dietary TGO (P < 0.05). The immune system of the fish was enhanced by TGO, as it stimulated the immune parameters in serum (lysozyme, myeloperoxidase (MPO), alternative complement (ACH₅₀), Ig) and mucus (lysozyme, protease, ACH₅₀, Ig) (P < 0.05). The treatments, TG3 and TG4, showed more immune performance in response to TGO (P < 0.05). The fish in TG2 treatment had a higher levels of serum total protein than other groups (P < 0.05). The concentration of triglycerides (TRIG) and cholesterol (CHOL) in serum significantly decreased (P < 0.05) in response to TGO, as the lowest levels were observed in the treatment, TG3. The antioxidant enzymes (superoxide dismutase (SOD) and catalase (CAT)) of serum elevated in TGO-treated fish, with the maximum values for the TG4 group (P < 0.05). TGO reduced (P < 0.05) alanine aminotransferase (ALT) and alkaline phosphatase (ALP) levels in serum. After bacterial challenge, the TGO-treated fish showed lower mortality compared to the control, where the lowest mortality was observed in TG4 (P < 0.05). In conclusion, TGO improved growth, immunity, and survival after bacterial challenge in the rainbow trout, with more performance in fish fed 1%–2% TGO.