Serum and tissue concentrations of erythromycin in calves with induced pneumonic pasteurellosis

The effects of pneumonia on the pharmacokinetics of erythromycin administered IM and the tissue concentration changes with time were evaluated in 2-month-old calves. Pneumonia was induced by injection of Pasteurella haemolytica cultures through the thoracic wall into each lung. Six days prior to induction of pneumonia, erythromycin (15 mg/kg) was administered in a single IM dose. Erythromycin was administered again 48, 72, and 96 hours after injection of P haemolytica. On the third day of erythromycin administration (96 hours), the calves were serially euthanatized in groups of 4 calves each at 2, 5, 8, 12, 18, and 24 hours after the final dose was given. Tissue concentrations of erythromycin in kidney, liver, lung, muscle, CSF, and serum were determined. Neither the serum concentrations nor the overall pharmacokinetic values were significantly (P less than or equal to 0.05) changed by pneumonia. The concentrations of erythromycin were maximal at 5 hours for liver, muscle, and serum and at 8 hours for CSF, kidney, and lung. Serum and muscle concentrations were similar, whereas concentrations in CSF were lower than in serum and higher in kidney, liver, and lung. The lung/serum ratios were approximately 2.5 to 3 at 8 through 24 hours after IM administration. The peak concentration in lung was approximately 6 micrograms/g at 8 hours.