Efficacy of monepantel and anthelmintic combinations against multiple-resistant Haemonchus contortus in sheep, including characterisation of the nematode isolate.

Three experiments defined the resistance profile of a population of Haemonchus contortus, which was shown to express multiple resistances to the benzimidazole, levamisole, macrocyclic lactone and salicylanilide anthelmintic classes when given as a registered combination. Study 1 was a faecal egg count reduction (FECR) test and the efficacies for the anthelmintics were monepantel, 100%; abamectin+levamisole+oxfendazole, 40.0%; and abamectin+levamisole+oxfendazole+naphthalophos, 100%. No larvae were recovered from the post-treatment cultures for monepantel or the 4-way treatment, and for the 3-way treatment the culture was 100% Haemonchus spp. Efficacies in Study 2 were calculated from mean post-mortem nematode burdens of H. contortus and were levamisole+oxfendazole, 3.1%; abamectin+levamisole+oxfendazole, 5.0%; ivermectin, 0.4%; moxidectin, 28.4% and closantel, 70.2%. Study 3 was also a FECR test that resulted in efficacies of 100% for monepantel and 83.0% for a formulated 4-way combination of abamectin+levamisole+albendazole+closantel. Larvae recovered from the post-treatment culture for the combination-treated sheep were all Haemonchus spp. Multi-resistant parasites such as examined in these studies are a continuing challenge to be managed by farmers and their advisors. Control programs must be planned and well-managed, and should include on-farm testing for anthelmintic resistance, monitoring of nematode burdens (by FEC and larval culture) to determine appropriate treatment times and the management of pastures to reduce the overall parasite challenge. This should be in balance with the generation, use and maintenance of drug-susceptible nematode populations in refugia.