Pharmacokinetics of ivermectin in sheep following intravenous, intra‐abomasal or intraruminal administration*

Prichard, R.K., Steel, J.W., Lacey, E. & Hennessy, D.R. Pharmacokinetics of ivermectin in sheep following intravenous, intra‐abomasal, or intraruminal administration. J. vet. Pharmacol. Therap. 8, 88–94. The pharmacokinetics of ivermectin in plasma following intravenous, intra‐abomasal, and intraruminal administration to sheep was determined. When given intravenously, ivermectin was very slowly eliminated with a terminal half‐life of 178 h and a volume of distribution at steady state of 5.3 l/kg indicating sequestration in a temporary depot. Intra‐abomasal administration resulted in rapid absorption, a peak plasma concentration of 6O.6 ng/ml at 4.4 h, and 100% bioavailability. However, intraruminal administration produced a much lower peak concentration (17.6 ng/ml at 23.5 h) and bioavailability (25.1%). A subsequent in vitro study indicated that ivermectin may be rapidly metabolized in the rumen. Dr.J. W. Steel, CSIRO Division of Animal Health, McMaster Lahoratory, Private Bag No. 1, P.O., Glebe, NSW 2037, Australia.