Pharmacokinetics of tulathromycin in lactating goats

Tulathromycin is a novel long-acting semi-synthetic macrolide antibiotic of the triamilide group that is used to treat respiratory diseases in cattle and pigs. Tulathromycin may therefore be useful in treating respiratory diseases of goats. The objective of this study was to characterize the pharmacokinetics of tulathromycin in lactating goats. Five lactating goats received single IV and IM injections of tulathromycin (2.5 mg/kg) with a 6 week interval between injections. Plasma, milk and urine concentrations of tulathromycin were determined using a microbiological assay method and pharmacokinetic values were calculated. Following a single IV injection of tulathromycin formulation, tulathromycin had a high mean volume of distribution (16.1 L/kg), excellent penetration into milk, slow plasma clearance (126 mL kg−1 h−1) and a long elimination half time (88 h). Plasma protein binding was 23%. Tulathromycin was rapidly absorbed following a single IM injection; the mean maximum plasma concentration (0.73 μg/mL) was rapidly reached at 0.5 h and mean bioavailability was 95.8%. Tulathromycin concentrations exceeded 0.1 μg/mL in milk and 0.01 μg/mL in plasma for at least 7 days. We conclude that the pharmacokinetics of a single IM injection of tulathromycin (2.5 mg/kg) in lactating goats are similar to those in non-lactating goats and yearling cattle administered a similar dose of tulathromycin SC. Tulathromycin may therefore be effective in the treatment of respiratory disease in goats. Long withdrawal times (>19 days) are required for milk intended for human consumption when tulathromycin is administered accidentally to lactating dairy goats.