Effectiveness of anthelmintic treatments against levamisole-resistant Ostertagia.

Adult worms from a population of Ostertagia consisting of 88% O. circumcincta and 12% O. trifurcata were found to survive a double drench (14 mg/kg) of levamisole. An anthelmintic assay on this population found that against levamisole the LD50 was 11.95 mg/kg and the LD99 was 67.75 mg/kg. Three benzimidazoles were also tested against this population and the LD50 and LD99 were respectively thiabendazole 11.8 and 144.9 mg/kg, oxfendazole 0.17 and 6.10 mg/kg and albendazole 0.08 and 2.55 mg/kg. The percentage of worms killed by the recommended dose rate was levamisole 24%; thiabendazole 91%; oxfendazole 99% and albendazole 99%.