Tilapia is one of the most consumed farmed sh, which requires the use of antibiotics in certain phases of its production. This study assessed the safety of 30 days of oral orfenicol (FFC)-dosing at 0-10 times the therapeutic dose (1X: 10 mg/kg biomass/day) in Oreochromis niloticus juveniles. Behavioural changes, feed consumption, mortality and biomass were evaluated. Besides, the levels of serum glucose, calcium, chloride, creatinine, alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase and blood cell morphology were determined at scheduled intervals. The 30 days of oral FFC-dosing caused 3.33% (1X) to 18.33% (10X) mortalities, reduced feed intake and biomass in a dosedependent manner. The sh fed the therapeutic dose recorded 1.25 folds increase in biomass; while the control group recorded 1.45 folds increase in 30 days. No signi cant erythrocyte morphological alterations were observed in the 1X group compared to the control. However, marked morphological alterations like tear-shaped, spindle-shaped and degenerative erythrocytes in higher dosing groups indicated FFC cytotoxicity. All the serum biomarkers of O. niloticus increased signi cantly on day 10 and day 30 FFC-dosing in a dose-dependent manner, except for calcium and chloride, which reduced signi cantly during the dosing period. Within 2 weeks of suspension of FFC-dosing, the serum biomarker levels became normal except for alkaline phosphatase and creatinine. The recovery of biomass, feed intake, serum biomarker levels and erythrocyte morphological changes suggested that the FFC induced changes are reversible. This study has, thus, proclaimed the safety of FFC at the therapeutic dose in O. niloticus.

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