Physiological and structural impacts of oxytetracycline exposure in rainbow trout (Oncorhynchus mykiss): implications for fish health and welfare.

The present study evaluated the pathophysiological and tissue alterations induced by dietary oxytetracycline (OTC) in rainbow trout (Oncorhynchus mykiss). Fish were exposed to three different doses (T1 = 40 mg OTC/kg body weight, T2 = 80 mg OTC/kg body weight, T3 = 120 mg OTC/kg body weight) alongside a control group (T0) for a period of 30 days. Hematological, immunological, and biochemical parameters were monitored on days 0, 15, and 30 days of OTC exposure. Higher OTC doses caused a significant reduction in total erythrocyte count (TEC), total leucocyte count (TLC), hemoglobin, packed cell volume (PCV), and mean corpuscular hemoglobin concentration (MCHC) while mean corpuscular volume (MCV) levels increased. Dose-dependent reduction was also observed in immune and biochemical markers such as phagocytic activity, NBT, total Ig, protein, albumin, and globulin, but an increase was noticed in the glucose levels in OTC-fed groups. Histological examination revealed liver alterations including sinusoidal dilation, hepatocyte hypertrophy, pyknosis, and congestion, whereas gills showed lamellar fusion, epithelial lifting, and necrosis, with the most severe changes in the T3 group. SEM analysis confirmed mucus cell proliferation, secondary lamellar thickening, fusion and degeneration. In liver, micro- and macro-steatosis were also evident in OTC-fed groups. Overall, prolonged exposure to high OTC doses markedly compromised physiological integrity, tissue architecture, and immune function, highlighting potential risks to fish health and welfare.