Are Teladorsagia circumcincta (Nematoda) morphs equally able to survive under anthelmintic treatment in sheep on pastures?

The nematode Teladorsagia circumcincta is a polymorphic species, morphs T. c. trifurcata and T. c. circumcincta being the most frequent. Among other t raits, T. c. trifurcata has lower infectivity than T. c. circumcincta. Anth elmintic treatments that regularly remove populations should favour the mor e infective T. c. circumcincta. The present investigation was intended to t est the respective capabilities of the two morphs to withstand anthelmintic pressure. The nematode strain was susceptible to imidazothiazole and partl y resistant to benzimidazole at the onset of experiment. A 2-year grazing s eason study was undertaken under four levels of anthelmintic pressure [from untreated controls to frequently treated with a poorly efficient benzimida zole (fenbendazole) or an efficient imidazothiazole (levamisole)]. The stra in previously maintained in laboratory conditions regained its natural, ori ginal proportion of T. c. trifurcata during the first grazing season. The t wo morphs were equally resistant to fenbendazole; and the proportion of res istant nematodes did not vary between the two morphs. During the second gra zing season, the treatments indirectly depleted the proportions of T. c. tr ifurcata, probably due to the poor infectivity of this morph.