The development of anthelmintic resistance in ruminant livestock.

Despite the enormous advances over the last 50 years in the chemotherapeutic control of nematode parasites, the economic importance of these parasites to the grazing livestock industries remains as great as ever. Added to this, the emergence and rapid development of resistance to the new broad spectrum anthelmintics in the important nematode species now looms large as a major international threat. This particularly is of concern to the sheep industry, especially in the major sheep-raising countries of the Southern Hemisphere, but there are clear signs that the problem in Western Europe and North America is rapidly escalating. Resistance is also a serious concern in the intensive goat industry, but at present appears to occur only on isolated, individual cattle properties. To obtain a perspective of the development of anthelmintic resistance in relation to husbandry practices, this review focuses on the problem in the sheep industry in relation to its development, detection and management.