Characterisation of milbemycin-, avermectin-, imidazothiazole- and benzimidazole-resistant Teladorsagia circumcincta from a sheep flock.
Selection for milbemycin resistance in a population of Teladorsagia circumcincta was examined in a sheep flock in which a lack of persistence of an oral dose of 0.2 mg/kg moxidectin against T circumcincta had previously been identified. A faecal egg count reduction test also showed resistance to benzimidazole, levamisole and avermectin anthelmintic groups. Bioassays were used to compare the moxidectin-resistant T circumcincta with another previously characterised benzimidazole-, levamisole- and ivermectin-resistant (MTci5) strain that had been isolated from a sheep flock in the same region in south-east Scotland and with an anthelmintic-susceptible (MTci3) strain of T circumcincta. The mean ED(50) value (the concentration of drug required to prevent 50 per cent of eggs from hatching) obtained for thiabendazole in egg hatch assays was higher in the moxidectin-resistant T circumcincta than in the ivermectin-resistant MTci5 strain. The inclusion of the cytochrome p450 inhibitor piperonyl butoxide in larval feeding inhibition assays increased the level of ivermectin resistance in vitro in the ivermectin- and moxidectin-resistant populations, but not in the ivermectin-susceptible MTci3 strain of T circumcincta.
Sargison
N D
ND
Large Animal Practice, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG. neil.sargison@ed.ac.uk
Jackson
F
F
Wilson
D J
DJ
Bartley
D J
DJ
Penny
C D
CD
Gilleard
J S
JS
D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology / Q000627:therapeutic use*; D001562:Benzimidazoles / Q000494:pharmacology / Q000627:therapeutic use; D018432:Drug Resistance, Multiple; D005243:Feces / Q000469:parasitology; D007559:Ivermectin / Q000031:analogs & derivatives / Q000494:pharmacology / Q000627:therapeutic use; D018942:Macrolides / Q000494:pharmacology / Q000627:therapeutic use; D010270:Parasite Egg Count / Q000662:veterinary; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy*; D013830:Thiadiazoles / Q000494:pharmacology / Q000627:therapeutic use; D014251:Trichostrongyloidea / Q000187:drug effects* / Q000254:growth & development; D014252:Trichostrongyloidiasis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*
D000871:Anthelmintics; D001562:Benzimidazoles; D018942:Macrolides; D013830:Thiadiazoles; D007559:Ivermectin; C019264:avermectin; C031000:benzimidazole; C027837:moxidectin