Anthelmintic resistance of Ostertagia ostertagi and Cooperia oncophora to macrocyclic lactones in cattle from the western United States

In June 2008, 122 yearling heifers with a history of anthelmintic resistance were obtained from pastures in northern California and transported to a dry lot facility in southwestern Idaho, USA. Fifty heifers with the highest fecal egg counts were selected for study enrollment. Candidates were equally randomized to treatment with either injectable ivermectin (Ivomec®, Merial, 0.2 mg kg−1 BW), injectable moxidectin (Cydectin®, Fort Dodge, 0.2 mg kg−1 BW), oral fenbendazole (Safe-Guard®, Intervet, 5.0 mg kg−1 BW), oral oxfendazole (Synanthic®, Fort Dodge, 4.5 mg kg−1 BW), or saline. At 14 days post-treatment, nematodes were recovered from the abomasum, small intestine, and large intestine. Parasitism was confirmed in the control group when 10/10 animals were infected with adult Ostertagia ostertagi and 9/10 animals with both developing and early L4 stages of O. ostertagi. Similarly, 9/10 animals were parasitized with adult Cooperia spp. Fenbendazole and oxfendazole efficacy verses controls were >90% against adult Cooperia spp., while moxidectin caused an 88% parasite reduction post-treatment (P < 0.05). Ivermectin treatment resulted in no reduction in adult Cooperia spp. Based on geometric mean percent reduction versus saline controls, all four treatments were ≥90% efficacious against adults of O. ostertagi, while moxidectin and fenbendazole were equally effective against developing and inhibited early L4 stages (P < 0.05). Ivermectin was not efficacious for developing or inhibited early L4 stages of O. ostertagi. Oxfendazole failed to decrease O. ostertagi developing L4 larvae by >90% but was efficacious for inhibited early L4 larvae. Based on the results of this study, a source of multi-species anthelmintic resistance in cattle has been identified in the western United States.