RESUMO
OBJECTIVE: To determine the efficacy of monepantel, a developmental compound from the amino-acetonitrile derivative class of anthelmintics, against field infections of gastrointestinal nematodes in sheep. PROCEDURES: Comparisons of efficacy (using standard faecal worm egg count reduction tests) and safety (on the basis of visual observations) were made in a large-scale field study in Australia, between groups of sheep treated with either an oral solution of monepantel or a registered anthelmintic. The sheep were naturally infected with the major gastrointestinal nematode genera present in Australia. RESULTS: The post-treatment efficacy results for monepantel were: at 7 days (+/-1 day) efficacy was >98%; at 14 days (+/-1 day) it was generally close to or >99%; and at 21 days (+/-1 day) efficacy was consistently >99%. A high proportion of the targeted nematode populations were confirmed as being resistant to one or more of the currently available anthelmintic classes. CONCLUSIONS: Monepantel when used under field conditions at a minimum dose rate of 2.5 mg/kg was highly effective against mixed-genus natural field infections of the major gastrointestinal nematode genera including Haemonchus, Teladorsagia (Ostertagia), Trichostrongylus, Nematodirus, Chabertia and Oesophagostomum. This result included efficacy against some populations resistant to the currently available broad-spectrum anthelmintics. Few Cooperia spp. were present to allow confirmation of efficacy against this genus. On no occasion after treatment did any commercial anthelmintic-treated groups have significantly lower faecal egg counts than the monepantel-treated groups. Monepantel was safe for the target animals and human operators when used in a field situation.