Trends in Parasitology
Volume 20, Issue 10, October 2004, Pages 477-481
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Drug resistance in nematodes of veterinary importance: a status report

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Reports of drug resistance have been made in every livestock host and to every anthelmintic class. In some regions of world, the extremely high prevalence of multi-drug resistance (MDR) in nematodes of sheep and goats threatens the viability of small-ruminant industries. Resistance in nematodes of horses and cattle has not yet reached the levels seen in small ruminants, but evidence suggests that the problems of resistance, including MDR worms, are also increasing in these hosts. There is an urgent need to develop both novel non-chemical approaches for parasite control and molecular assays capable of detecting resistant worms.

Section snippets

Brief history of anthelmintic resistance

The initial reports of anthelmintic resistance were to the drug phenothiazine in the late 1950s and early 1960s, first in Haemonchus contortus (barber pole worm) of sheep [3] and then in cyathostomins (small strongyles) of horses 4, 5, 6. In 1961, thiabendazole was introduced as the first anthelmintic that combined efficacious broad-spectrum nematocide activity with low toxicity. The rapid acceptance and widespread use of thiabendazole and then other benzimidazole anthelmintics marked the

Anthelmintic resistance in cattle, horses and humans

Less attention has been given to the problem of anthelmintic resistance in cyathostomin nematodes of horses (now considered the principal parasitic pathogen of adult horses), although several studies have reported a prevalence of resistance to benzimidazole drugs greater than 75% [14]. Resistance to pyrantel (tetrahydropyrimidine class) appears to be much less common, but a recent study in southern USA found that over 40% of farms demonstrated resistance to this drug [34]. Interestingly, there

Implications of anthelmintic resistance

The serious problem of anthelmintic resistance is easily appreciated. But what can be done about it? Beginning with phenothiazine in the 1950s, followed by the benzimidazoles in the 1960s, the imidazothiazole–tetrahydropyrimidines in the 1970s and the avermectin–milbemycins in the 1980s, a new class of anthelmintics was introduced into the marketplace each decade. This arsenal of highly effective and relatively inexpensive drugs led to recommendations for parasite control that were based almost

Perspective

It is unlikely that development of new anthelmintics will rescue livestock producers from the inevitable losses in productivity and problems of animal welfare that result from a failure to control MDR worms adequately. Therefore, sincere efforts must be made to preserve the efficacy of the few drugs that remain effective. Now and in the future, anthelmintics must be thought of as highly valuable and limited resources to be preserved. The only realistic strategy for sustainable nematode parasite

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