Drug resistance in nematodes: a paper tiger or a real problem?

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Abstract
The purpose of this review is to illustrate where drug resistance in parasitic nematodes has become a major problem. The mechanisms underlying anthelmintic resistance, the possible reasons for the development of anthelmintic resistance, and recommendations to minimize the further development of anthelmintic resistance in humans will be addressed. Resistance has developed to all drugs of the few anthelmintic classes currently available. Drug resistance has become a major threat to sheep production in Australia, New Zealand, Great Britain and South Africa. In addition, the treatment of nematode infections in horses has become insufficient because of anthelmintic resistance, whereas resistance in cattle has recently been found in New Zealand and southern America. In contrast, anthelmintic resistance to human helminths has only been reported anecdotally. The rate of development of anthelmintic resistance is determined by the resistance gene frequency and by selection pressure, as illustrated by the concept of 'worms in refugia' in ovine nematode infections. Although anthelmintic resistance has become a major constraint in livestock production of small ruminants, drug resistance is currently not a major issue in the treatment of human nematode infections. However, if recent community treatment programmes are pursued injudiciously, the inevitable evolutionary consequence of anthelmintic resistance in humans might emerge sooner rather than later.