Brown Besier, Department of Agriculture and Food WA, Albany
Resistance to the macrocyclic lactone (ML) group of anthelmintics is a serious problem for the Australian sheep industries. When the MLs were first introduced in the late 1980s, resistance was widespread to the older drenches, and the new drench type was a much-needed lifeline, which rapidly transformed worm control.
Unfortunately, the worms fought back very quickly. Resistance to Haemonchus contortus (barbers pole worm) and Teladorsagia (Ostertagia) circumcincta (small brown stomach worm) were both detected within less than 4 years of the release of ivermectin. The rest is history—ML resistance is now present on the vast majority of sheep farms in Australia (although the more potent MLs remain effective on many). We now know far more about the management of drench resistance, but this is of more help with new anthelmintic groups—we can’t turn the clock back once resistance develops.
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