Regional and temporal use of avermectins for ruminants in Australia.

The avermectin class of anthelmintics has been available for approximately 4 years (sheep) and 7 years (cattle) in Australia. They are highly efficacious against a very broad spectrum of nematode parasites and certain ectoparasites of livestock. To date there are no field reports of resistance to these compounds in either the cattle or sheep industry, despite the fact that nematode parasites of sheep show high levels of resistance to other broad spectrum anthelmintics. The avermectins have gained rapid acceptance, but their share of the anthelmintic market is less than expected, owing to several factors such as drought and recession of the rural economy. Nevertheless, it can be expected that this class of compounds will assume greater importance, particularly in the sheep industry, when greater awareness of the widespread failure of alternative broad spectrum anthelmintics, and the value of regional worm control programs become accepted by producers.