A controlled trial was conducted to determine the anthelmintic efficacies of moxidectin and mebendazole. The compounds were administered orally at doses of 0.2 mg/kg bodyweight and 15.0 mg/kg bodyweight, respectively, to lambs infected experimentally with large numbers of gastrointestinal nematode larvae including a benzimidazole-resistant strain of Haemonchus contortus. Moxidectin was 100 per cent effective against adult H contortus, Ostertagia species, Trichostrongylus colubriformis, Cooperia curticei and fifth stage larvae of Oesophagostomum species and Chabertia ovina, but was only 76 per cent effective against adult Strongyloides papillosus. Mebendazole was 100 per cent effective against adult Ostertagia species and T colubriformis, but reduced the numbers of adult C curticei by only 39 per cent, S papillosus by 58 per cent, H contortus by 76 per cent, and fifth stage larvae of Oesophagostomum species by 79 per cent and C ovina by 72 per cent.
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