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Characterisation of milbemycin‐, avermectin‐, imidazothiazole‐ and benzimidazole‐resistant Teladorsagia circumcincta from a sheep flock
Author(s) -
Sargison N. D.,
Jackson F.,
Wilson D. J.,
Bartley D. J.,
Penny C. D.,
Gilleard J. S.
Publication year - 2010
Publication title -
veterinary record
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.261
H-Index - 99
eISSN - 2042-7670
pISSN - 0042-4900
DOI - 10.1136/vr.b4853
Subject(s) - moxidectin , ivermectin , teladorsagia circumcincta , avermectin , anthelmintic , biology , levamisole , veterinary medicine , flock , abamectin , benzimidazole , zoology , larva , haemonchus contortus , chemistry , ecology , pesticide , medicine , organic chemistry , anatomy
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 .