Anthelmintic resistance in Australian sheep nematode populations

The resistance status of gastro-intestinal nematodes to anthelmintics was evaluated on 881 sheep farms throughout Australia during 1991-92. Resistance was shown to be widespread. Overall, 85% of farms had sheep infected with nematodes resistant to benzimidazole, 65% to levamisole and 34% to combination (benzimidazole+levamisole) products. Resistance to ivermectin was not detected. On only 9% of farms did all anthelmintic groups reduce egg counts by greater than or equal to 95%. The culture of faeces from untreated sheep showed Telodorsagia circumcincta, Trichostrongylus spp, Chabertia ovina and Haemonchus contortus to be the principal species. The nature and prevalence of resistance was not significantly correlated with stocking rate. However, resistance to combination products was almost twice as prevalent on farms in areas with an average annual rainfall of greater than 500 mm.