Monitoring anthelmintic resistance in Queensland sheep flocks

Abstract

Development of anthelmintic resistance in Australian sheep flocks over the past 20 years has caused concern for sustainable sheep and wool production.1 Resistance was
first reported in 1968, and by the mid-1980s resistance of the important sheep nematode species (Haemonchus, Ostertagia and Trichostrongylus) to a range of anthelmintics had been described. In a survey conducted during 1991/92, the
prevalence of resistance to benzimidazole and levamisole anthelmintics was estimated to be 86 and 65%, respectively. Resistance to macrocyclic lactone anthelmintics was not detected, however resistance to ivermectin was subsequently reported. Factors associated with the emergence of resistance include excessive use of anthelmintics, underdosing and use of faulty equipment. Regional control programs, based on monitoring flock faecal egg counts (FECs), have been developed and promoted to control anthelmintic resistance. To better advise producers on the most effective anthelmintics to use, it is essential to have information on the regional anthelmintic resistance status of flocks. We describe a 2-year monitoring program for anthelmintic resistance in southern Queensland flocks that was initiated to provide this information.