Curcumin Supplementation Mitigates Oxytetracycline-Induced Hematological, Immunological, and Oxidative Stress in Rainbow Trout (Oncorhynchus mykiss)
Oxytetracycline (OTC) is a broad-spectrum antibiotic widely used in aquaculture for the treatment of bacterial diseases caused by important fish pathogens, including Aeromonas, Pseudomonas, Vibrio, and Lactococcus spp. Although OTC remains an important therapeutic agent in fish farming, its use may also induce undesirable physiological effects, including hematological disturbances, immunosuppression, and oxidative stress. This study investigated the protective effects of dietary curcumin against OTC-induced physiological alterations in rainbow trout (Oncorhynchus mykiss). Fish were fed diets supplemented with 0, 10, 50, or 100 mg kg−1 curcumin for eight weeks, followed by exposure to 0, 50, and 75 mg L−1 OTC for 48 h. Hematological variables, innate immune responses, and oxidative stress biomarkers were comprehensively evaluated. OTC exposure significantly disrupted physiological homeostasis by reducing red blood cell (RBC) count, hemoglobin (Hb), hematocrit (Ht), mean corpuscular volume (MCV), and mean corpuscular hemoglobin (MCH). Furthermore, white blood cell (WBC) count, nitroblue tetrazolium (NBT) activity, phagocytic activity (PA), phagocytic index (PI), total protein (TP), immunoglobulin M (IgM), bactericidal activity (BA), lysozyme activity (LYZ), and myeloperoxidase (MPO) activity were significantly decreased, indicating marked suppression of both cellular and humoral innate immune responses. Oxidative stress was characterized by increased malondialdehyde (MDA) concentrations together with reduced superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and reduced glutathione (GSH) levels, whereas glutathione S-transferase (GST) activity increased following OTC exposure. Dietary curcumin supplementation generally improved hematological and immune indices and was associated with reduced lipid peroxidation and improved antioxidant status. However, the magnitude and statistical pattern of these responses varied among parameters and tissues, and significant CUR × OTC interactions were not observed for all endpoints. Overall, dietary curcumin supplementation progressively attenuated these alterations, although the response pattern varied among parameters. Increasing dietary curcumin supplementation generally led to greater physiological improvement, although the response varied among endpoints. Notably, at 75 mg/L OTC, supplementation with 100 mg/kg curcumin, compared with 0 mg/kg curcumin, increased RBC count by approximately 14.5%, Ht by 47.8%, WBC count by 32.2%, and NBT activity by 38.9%. In the liver, the same treatment reduced MDA by approximately 14.6% and increased SOD activity and GSH levels by 48.7% and 47.2%, respectively (p < 0.001 for all). The highest dietary curcumin level generally produced the greatest physiological improvement. In conclusion, dietary curcumin effectively mitigated OTC-induced hematological, immunological, and oxidative disturbances in rainbow trout, highlighting its considerable potential as a functional feed additive to improve fish health, enhance physiological resilience under antibiotic-associated stress, and support more sustainable aquaculture practices.