Effects of short-term dietary oxytetracycline treatment in the farmed fish <i>Piaractus mesopotamicus</i>

Oxytetracycline (OTC), a key antibiotic used in global aquaculture, has still unclear ecotoxicological effects. In this study, freshwater fish Piaractus mesopotamicus were fed diets containing 750 mg kg-1 of either pure OTC (ATB1) or commercial OTC (ATB2) for 10 days (treatment period-TP), followed by a 21-day withdrawal period (depuration period-DP). Fish fed with ATB2 showed decreased hematocrit (at DP) and increased glucose levels (TP and DP). In general, catalase activity increased in the liver, gills, and muscle of OTC-treated individuals at both TP and DP, particularly with ATB2. Similarly, glutathione S-transferase activity rose in the brain, gills, and muscle (TP and DP). Conversely, alkaline phosphatase activity in the liver decreased in both treated groups (TP and DP). Additionally, only ATB2 induced lactate dehydrogenase in fish muscle after 1-day depuration. Principal component analysis identified most antioxidant enzymes, hematocrit, weight, length, and mean corpuscular hemoglobin concentration as key biomarkers, distinguishing ATB2 from control fish. These results indicate that the dietary therapeutic dose of OTC caused adverse effects in P. mesopotamicus. Differences in biomarker responses between ATB1 and ATB2 might be linked to unknown compounds in the commercial formulation, potentially influencing biological responses or altering OTC bioavailability. Further research on the toxicity of antimicrobial impurities and degradation compounds should accompany enhanced quality control measures in aquaculture to guarantee sustainable and safe products.