[Two cases of Ostertagia spp. in sheep showing resistance to benzimidazole anthelmintics].

Two field isolates of Ostertagia spp. of sheep from the Swellendam and Malmesbury districts in the south-western Cape were shown to be resistant to benzimidazole anthelmintics. At therapeutic dosage rates albendazole was 32,5%, thiabendazole 0%, oxfendazole 14,9% and morantel 91,4% effective against the adult stage of Ostertagia spp. in naturally infested lambs at Swellendam. Efficacy against the adult stage of the same isolate of Ostertagia spp. in artificially infested sheep in the laboratory was tested at dosage rates of albendazole 3,8% mg/kg or 7,6 mg/kg, mebendazole 15 mg/kg, levamisole 7,5 mg/kg and morantel 14,5 mg/kg and varied from 0%, 0%, 1,9% 53,9% and 87,0% respectively. Albendazole, the only anthelmintic tested against the Malmesbury strain of Ostertagia spp. was 9,8% effective against the adult stage in naturally infested lambs. Trichostrongylus spp. and Nematodirus spp. simultaneously present in these animals were fully susceptible to all the anthelmintics tested.