Bioavailability and Tissue Distribution of Oxytetracycline Long-Acting Formulation in Sheep Under Tropical Conditions in Kwara State

Long-acting oxytetracycline formulations are widely used for treating bacterial respiratory and systemic infections in small ruminants across Nigeria, as the extended dosing interval of 72 hours is practically advantageous in extensive management systems. However, the pharmacokinetic behaviour of long-acting formulations is influenced by ambient temperature, muscle perfusion, and the physicochemical properties of the formulation vehicle, which may differ significantly from temperate conditions under which most reference data were generated. This study characterised the bioavailability and tissue distribution of a commercially available long-acting oxytetracycline formulation at 20 mg/kg administered subcutaneously in Yankasa sheep maintained under tropical field conditions in Ilorin, Kwara State, North Central Nigeria. Sixteen sheep were treated, with blood samples collected at 15 time points over 96 hours and tissue samples from lung, liver, kidney, and muscle collected at 24, 48, and 72 hours from sacrificed subgroups. Plasma and tissue oxytetracycline concentrations were determined by validated HPLC. Bioavailability was 78.4 percent, and maximum plasma concentration of 4.2 micrograms per millilitre was achieved at 4.6 hours post-injection. Tissue concentrations in lung exceeded plasma concentrations at all time points, supporting efficacy against respiratory pathogens. Residues in muscle exceeded the MRL of 200 micrograms per kilogram up to 96 hours post-injection, indicating that a withdrawal period longer than 96 hours may be required under tropical conditions compared with temperate guidelines. These findings have significant food safety implications for meat from treated sheep in Nigerian markets. Keywords: oxytetracycline, bioavailability, tissue residues, sheep, Kwara State.