Dietary Ellagic Acid Protects and Ameliorates Copper Sulfate-Induced Toxicity in Common Carp (Cyprinus carpio)

Copper sulfate (CuSO4) is one of the most widely used chemotherapeutic agents in aquaculture and aquatic veterinary medicine because of its broad-spectrum antiparasitic, antifungal, and antimicrobial activities. However, repeated exposure may induce oxidative stress and impair hematological and immune functions, thereby compromising fish health. This study evaluated the effects of pre-exposure and post-exposure dietary ellagic acid (EA) supplementation on CuSO4-induced hematological, immunological, and oxidative stress alterations in common carp (Cyprinus carpio). A total of 150 fish were randomly allocated to five experimental groups: Control, EA, CuSO4, pre-exposure EA + CuSO4, and post-exposure CuSO4 + EA. Fish were fed a diet supplemented with 100 mg EA kg−1 feed for 14 days either before or after repeated CuSO4 exposure (1 mg L−1, 30 min day−1 for seven consecutive days). Hematological variables, innate immune responses, and oxidative stress biomarkers were subsequently evaluated. CuSO4 exposure significantly decreased red blood cell (RBC) count, hemoglobin (Hb), hematocrit (Ht), phagocytic activity (PA), phagocytic index (PI), total plasma protein (TP), immunoglobulin M (IgM), bactericidal activity (BA), lysozyme (LYZ), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), and reduced glutathione (GSH), while significantly increasing mean corpuscular hemoglobin concentration (MCHC), white blood cell (WBC) count, respiratory burst activity (NBT), myeloperoxidase (MPO), and malondialdehyde (MDA) (p < 0.05). Dietary EA supplementation markedly alleviated these alterations by restoring hematological homeostasis, improving innate immune responses, reducing lipid peroxidation, and reinforcing antioxidant defense systems in the liver, kidney, spleen, and gill. Both pre-exposure and post-exposure EA supplementation were associated with improvements in the evaluated biomarkers under the experimental conditions of the present study. However, because the two supplementation strategies were not evaluated against a time-matched CuSO4 recovery control group, the present study does not permit a direct comparison of their relative efficacy. These findings demonstrate that dietary EA was associated with improvements in hematological, immunological, and oxidative stress biomarkers following experimental CuSO4 exposure, potentially through antioxidant and immunomodulatory mechanisms. Therefore, dietary EA may serve as a promising functional feed additive to support antioxidant and immune status during experimental CuSO4 exposure in common carp. Further studies including pathogen challenge models are required before its use as an adjunct to CuSO4 treatments can be recommended.