Anthelmintic resistance.

Anthelmintic resistance is widespread in nematode parasites of sheep, goats and horses. Resistance is also developing in nematode parasites of cattle and has been detected in pig parasites. Benzimidazole, levamisole/morantel and ivermectin resistances occur in nematodes of sheep and goats and closantel resistance has been found in Haemonchus contortus. Anthelmintic resistance is likely to develop wherever anthelmintics are frequently used and be detected if it is investigated. Worm count or egg count reduction after treatment are useful for the detection of all types of anthelmintic resistances. More economical, faster and more sensitive in vitro assays for the detection of anthelmintic resistance have been developed. Some, such as the egg hatch assay are specific for a particular class of anthelmintic, whilst others such as larval development assays can be used with most anthelmintics. Improvements in our understanding of the biochemistry and molecular genetics of anthelmintic actions should lead to the development of more sensitive assays for the detection of anthelmintic resistance in individual nematodes. Levamisole/morantel resistance appears to be associated with alterations in cholinergic receptors in resistant nematodes. Ivermectin appears to act by binding to a glutamate receptor of a membrane chloride channel. This receptor has been expressed in vitro so that further studies of the interaction of ivermectin with this receptor and its possible alteration in ivermectin resistance will be feasible. Benzimidazole resistance in nematodes and fungi appears to be associated with an alteration in beta-tubulin genes which reduces or abolishes the high affinity binding of benzimidazoles for tubulin in these organisms. This knowledge can be exploited for DNA probes for benzimidazole resistance/susceptibility in individual organisms.