Effect of oesophageal groove closure on the pharmacokinetic behaviour and efficacy of oxfendazole in sheep.

Direct intraabomasal oxfendazole (OFZ) administration resulted in the peak plasma OFZ concentration occurring sooner and the area under the plasma OFZ concentration curve being reduced when compared with intraruminal administration. Glucose given directly into the abomasum of fistulated sheep caused a substantially greater elevation of plasma glucose levels than did direct administration into the rumen. This difference was subsequently used to indicate rumen bypass, by oesophageal groove closure, after oral dosing with an OFZ formulation to which glucose had been added. When 17 penned sheep were drenched with OFZ containing added glucose, elevated plasma glucose levels indicated substantial rumen bypass in six of them and was negatively correlated with time to peak plasma OFZ concentration, area under the plasma OFZ curve and efficacy against thiabendazole resistant Haemonchus contortus. Among 60 grazing sheep there were indications of some degree of oesophageal groove closure in 42 per cent of them following drenching with OFZ and in these animals there was a significant reduction in OFZ efficiency as measured by faecal worm egg counts.