Resistance to thiabendazole in a field population of Ostertagia circumcincta from sheep.

A population of Ostertagia circumcincta from sheep has been found which, in field and laboratory trials, showed a lack of response to the recommended dose (44 mg/kg) of thiabendazole, suggesting that a degree of resistance exists to this anthelmintic. Preliminary experiments indicated a less than expected response to levamisole. This was not confirmed in a laboratory trial and therefore resistance to this anthelmintic has not been shown conclusively.