Anthelmintic Resistance in Ruminant Helminths: Where Are We Now and Where Are We Going?

Gastrointestinal helminths are an important cause of production losses in sheep and cattle. anthelmintic treatment provides a survival advantage for parasites carrying resistance alleles. Surviving parasites pass these alleles to their offspring so the allele frequency increases during subsequent parasite generations if selection is maintained. it is generally considered that anthelmintic resistance develops through the selection of ancient resistance alleles present in the parasite population. the use of anthelmintics has lead to the inevitable development of populations of helminths resistant to treat- ment. once resistance to an anthelmintic class has emerged within an individual cattle herd or sheep flock, the control of parasitic gastroenteritis using any of the chemicals in that anthelmintic class is compromised. Long-term reversion to susceptibility does not occur.

Consequently, the sustainability of helminth control based on the use of the currently available anthelmintics has become severely limited. although there are two new classes of anthelmintic available in some markets (amino acetonitrile derivatives and spiroindoles, the positive effect. of such valuable resources for the control of parasites might not last long if used following the same application strategies as the three broad spectrum anthelmintic classes (benzimidazoles [BZ], imidazothiazole [LV] and macrocy- clic lactone [ML]) which preceded them.

In sheep, t. circumcincta or H. contortus resistance to BZ, LV and/or ML anthelmintics is now commonplace in many countries worldwide. there is evidence of emerging ML resistance in roundworms of cattle in Europe. the dose-limiting species, Cooperia onco- phora, is often implicated. [...]