Anthelmintic evaluation of a thiabendazole-resistant strain of Ostertagia circumcincta recovered from sheep in England.

A total of 35 worm-free lambs were infected with a strain of Ostertagia circumcincta isolated earlier from sheep in Cheshire, England, and found to be resistant to thiabendazole (TBZ). When patency was established the sheep were divided into groups of six, and dosed orally with either TBZ (44 mg kg-1, 88 mg kg-1), fenbendazole (FBZ; 5 mg kg-1) or levamisole (7.5 mg kg-1) or not treated. Three of the remaining five animals were dosed with FBZ at 10 mg kg-1. Egg hatch tests, post-dosing faecal egg counts and post-mortem worm counts confirmed resistance to TBZ, and a degree of side-resistance to FBZ was also revealed. Only levamisole gave the clearance expected of modern anthelmintics.