Anthelmintic resistance in nematodes: extent, recent understanding and future directions for control and research.

Resistance has now been reported to all of the broad spectrum anthelmintic types currently available, namely to the benzimidazoles, levamisole/morantel and to ivermectin. The problem causes most concern for parasite control in sheep, but anthelmintic resistance has also been reported in nematodes of horses, goats, pigs and more recently cattle. Our understanding of the factors which select rapidly for resistance has increased and programmes of worm control which minimize selection for anthelmintic resistance are being developed and tested. One of the greatest problems encountered in attempting to reduce the selection for overt drug resistance is the need for more sensitive tests for developing resistance. In the long term, new approaches to chemotherapy and to overcoming anthelmintic resistance problems will arise from improving our understanding of the modes of action of, and mechanisms of resistance to, anthelmintics at the level of the receptor proteins and their genes.