在这项研究中，使用食物中不同水平的百里香酚（TYM）（0, 1, 1.5, 2和2.5 g/kg食物）来评估其对虹鳟（Oncorhynchus mykiss）的生长、消化性能、免疫力以及对Streptococcus iniae引起的感染的抵抗力的影响。共有450条鱼（35.8 ± 4.4 g；平均值±标准差）分配到15个水槽中（每个水槽30条鱼），并被喂养TYM 60天。饲喂期结束后，与其它饲料组相比，饲喂1.5-2.5 g TYM的鱼显示出更好的生长、更高的消化酶活性和身体蛋白质含量（P < 0.05）。回归分析表明，生长参数与饲料TYM水平之间存在多项式关系。基于不同的生长参数，FCR的最佳饲料TYM水平为1.89％。与其它饲料组相比，饲喂1.5-2.5 g TYM的鱼，肝脏抗氧化酶活性（超氧化物歧化酶（SOD）、谷胱甘肽过氧化物酶（GPx）和过氧化氢酶（CAT））和血液中的免疫成分（替代补体活性（C3）、总免疫球蛋白（Ig）、溶菌酶活性、细菌杀伤活性和总蛋白质）以及粘液中的乳碱性磷酸酶活性（ALP）、蛋白酶活性、溶菌酶活性、细菌杀伤活性和总蛋白质）明显提高（P < 0.05）。与其它实验组相比，饲喂2-2.5 g TYM显著降低鱼体中的丙二醛（MDA）水平（P < 0.05）。此外，使用1.5-2.5 g TYM饲料水平升调了免疫相关基因（C3，Lyz和Ig）的表达（P < 0.05）。相反，炎症基因肿瘤坏死因子（TNF-α）和白细胞介素-8（IL-8）的表达在回应2-2.5 g TYM时明显下调（P < 0.05）。饲喂TYM对鱼的血液学也产生了影响，在饲喂2-2.5 g TYM的鱼中，红细胞平均血红蛋白浓度（MCHC）、血红蛋白（Hb）、红细胞（RBC）、血细胞比容（Hct）和白细胞（WBC）的值显著增加（P < 0.05）。此外，随着TYM饲喂量的增加，平均红细胞体积（MCV）也显著下降（P < 0.05）。在受挑战的Streptococcus iniae感染后，与其它饲料组相比，饲喂2-2.5 g TYM的鱼存活率显著提高（P < 0.05）。本研究的结果表明，虹鳟饲料中的TYM可以改善鱼类的生长和免疫能力，并增加鱼类对Streptococcus iniae感染的抵抗力。本研究的结果推荐饲喂鱼类2-2.5 g TYM水平的最佳方法。Copyright © 2022 Hafsan Hafsan等。

In this study, thymol (TYM) at dietary levels of 0, 1, 1.5, 2, and 2.5 g/kg diet was used to evaluate its effects on growth, digestive performance, immunity, and resistances to the infection induced by Streptococcus iniae in the rainbow trout, Oncorhynchus mykiss. A number of 450 fish (35.8 ± 4.4 g; Mean ± SD) were distributed to 15 tanks (30 fish/tank) in three replicates and fed TYM for 60 days. After feeding period, Fish fed 1.5-2.5 g TYM showed better growth, higher digestive enzyme activity, and body protein content compared to other diets (P < 0.05). Regression analysis indicated a polynomial relationship between growth parameters and dietary TYM levels. Based upon the varied growth parameters, the optimum dietary TYM level was 1.89% for FCR. TYM at dietary levels of 1.5-2.5 g significantly enhanced liver antioxidant enzyme activity [superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT)], immune components in blood [alternative complement activity (C3), total immunoglobulin (Ig), lysozyme activity, bactericidal activity, and total protein], and in mucus [alkaline phosphatase (ALP), protease activity, lysozyme activity, bactericidal activity, and total protein] compared to other diets (P < 0.05). TYM at dietary levels of 2-2.5 g significantly decreased malondialdehyde (MDA) levels compared to other experimental groups (P < 0.05). In addition, use of TYM at dietary levels of 1.5-2.5 g upregulated the expression of the immune-related genes (C3, Lyz, and Ig) (P < 0.05). In contrast, the expression of inflammatory genes, tumor necrosis factor (TNF-α) and Interleukin-8 (IL-8) significantly were downregulated in response to 2-2.5 g TYM (P < 0.05). The hematology of the fish also altered in response to dietary TYM, where the values of corpuscular hemoglobin concentration (MCHC), hemoglobin (Hb), red blood cell (RBC), hematocrit (Hct), and white blood cell (WBC) significantly increased in fish fed 2-2.5 g TYM compared to other diets (P < 0.05). In addition, MCV significantly decreased in response to 2-2.5 g TYM (P < 0.05). After challenge with Streptococcus iniae, the survival rate was significantly higher in fish fed 2-2.5 g TYM compared to other diets (P < 0.05). The results of the present study concluded that TYM in the diet of rainbow trout can improve the fish growth and immunity and increase the resistance of the fish to Streptococcus iniae infection. The results of this study recommend an optimized dietary level of 2-2.5 g TYM for the fish.Copyright © 2022 Hafsan Hafsan et al.