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 traits, T. c. trifurcata has lower infectivity than T. c. circumcincta. Anthelmintic treatments that regularly remove populations should favour the more infective T. c. circumcincta. The present investigation was intended to test the respective capabilities of the two morphs to withstand anthelmintic pressure. The nematode strain was susceptible to imidazothiazole and partly resistant to benzimidazole at the onset of experiment. A 2-year grazing season study was undertaken under four levels of anthelmintic pressure [from untreated controls to frequently treated with a poorly efficient benzimidazole (fenbendazole) or an efficient imidazothiazole (levamisole)]. The strain previously maintained in laboratory conditions regained its natural, original proportion of T. c. trifurcata during the first grazing season. The two morphs were equally resistant to fenbendazole; and the proportion of resistant nematodes did not vary between the two morphs. During the second grazing season, the treatments indirectly depleted the proportions of T. c. trifurcata, probably due to the poor infectivity of this morph.
Leignel
V
V
INRA, Station de Pathologie Aviaire et de Parasitologie, Nouzilly, France.
Cabaret
J
J
D000818:Animals; D000969:Antinematodal Agents / Q000627:therapeutic use*; D004351:Drug Resistance; D005273:Fenbendazole / Q000627:therapeutic use*; D007978:Levamisole / Q000627:therapeutic use*; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy* / Q000469:parasitology; D014251:Trichostrongyloidea / Q000187:drug effects / Q000254:growth & development*; D014252:Trichostrongyloidiasis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*
D000969:Antinematodal Agents; D007978:Levamisole; D005273:Fenbendazole