| Summary: | An eight-week feeding experiment was performed to determine the potential effects of Blue Mint Bush, Ziziphora clinopodioides, extract (ZE) on growth, digestive enzymes activity, immune response (serum and mucus), and disease resistance in rainbow trout. Accordingly, 450 fish (21.19 ± 1.09 g) were divided into five groups and fed with the experimental diets including: ZE 0% (control diet), ZE 0.5 %, ZE 1.5 %, ZE 3% and ZE 4.5 %. The administration of ZE, except ZE 0.5 %, significantly increased weight gain and specific growth rate compared to the control group (p < 0.05). The highest and lowest digestive enzymes activity (lipase, protease, and amylase) were observed in ZE 3% and the control group, respectively. The results indicated a significant improvement in the number of leucocytes as well as neutrophils and lymphocytes in fish fed with ZE 3 % diet (p < 0.05). The highest serum lysozyme, ACH50, Ig, levels were recorded in ZE 3 %, while the lowest values were obtained in the control group (p < 0.05). Also, serum bactericidal activity in fish receiving ZE 1.5, 3, and 4.5 % showed a significant difference with the control group (p < 0.05). Mucus lysozyme, bactericidal activity, and Ig values in ZE 3 % were higher than the control group (p < 0.05); while there was no significant difference in alkaline phosphatase level (p> 0.05). The highest survival rate (56 %) was recorded in the fish fed with ZE 3 % when the fish were exposed to Yersinia ruckeri on 15 days. Collectively, our results demonstrated that ZE-enriched diets, especially at the level of 3 %, supported growth performance, digestive enzyme activity, non-specific immune system, and resistance to Y. ruckeri infection in fry rainbow trout.
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