Acute effect of tetracycline on systemic stress markers and organ histopathology in juvenile Nile tilapia, Oreochromis niloticus

Antibiotic contamination in aquaculture systems frequently encounter severe threat to aquatic life including fish. This study aimed to evaluate acute toxicological effects of tetracycline in juvenile Nile tilapia, <i>Oreochromis niloticus.</i> We exposed ten fish in each tank with nominal tetracycline concentration of 0 μg/L, 10 μg/L, 100 μg/L, 500 μg/L, 1000 μg/L, and 5000 μg/L, under semi-static condition for 96 h. After the exposure periods, six fish per treatment (n = 6) were sampled to evaluate blood glucose, hemoglobin, frequencies of erythrocyte cellular and nuclear abnormalities, and histopathology of gills, intestine, liver, and kidney. Tetracycline induced clear dose-dependent effects in studied fish. Blood glucose increased significantly (p < 0.05) from 100 μg/L and onward, indicating acute metabolic stress, whereas hemoglobin decreased significantly at 500 and 5000 μg/L indicating compromised oxygen transport. Erythrocyte cellular and nuclear abnormalities rose in a threshold manner; twin, tear-drop, elongated, binucleated cells and nuclear buds became significantly more frequent at ≥ 100 μg/L and intensified at higher doses. Gill pathology such as congestion, lamellar degeneration and fusion, epithelial hypertrophy/hyperplasia was appeared from medium doses and was most pronounced at 5000 μg/L. Significant reduction in villus length, vacuolation of enterocyte and fusion of brush-border of intestine was observed at 500 μg/L and higher doses. Liver lesions such as vacuolation, hemorrhage, melanomacrophase centers, and necrosis were intensified with doses. Besides, renal alterations such as mild glomerular expansion at 100 μg/L, dilation of Bowman's space and tubular changes at 500 μg/L and above concentrations were observed. Our findings highlight sensitive early-warning biomarkers and underscore the need for chronic exposure studies and residual analysis for risk assessments of antibiotic usage in aquaculture.