Anthelmintic resistance in sheep gastro-intestinal strongyles in Europe

The intensive use of anthelmintics for treating and control gastrointestinal strongyles affecting sheep has caused a global spread of parasite populations that are resistant to one or more parasiticide classes. This phenomenon, which represents a severe problem in the southern hemisphere, is recently becoming a threat also in Europe. In fact, the occurrence and possible spread of sheep strongyle populations resistant to benzimidazoles and tetrahydropyrimidines-imidazothiazoles and, with a lesser extent, to macrocyclic lactones has been recorded for the most important nematodes parasitizing sheep, i.e., Teladorsagia circumcincta, Haemonchus contortus and Trichostrongylus spp. in various countries. Of higher concern are also the recent descriptions of strongyle populations that are capable to survive to more than one anthelmintic class. These findings indicate that control programs based only on the use of anthelmintics are outdated and that novel and sustainable approaches, relying on a combination of integrated measures, are warranted, especially for preserving the still-effective molecules. The aim of the present article is to review the state-of-the-art of drug resistance in sheep strongyles in Europe, with a focus on current scenarios and measures necessary to limit the further expansion of drug resistance in sheep flocks.