Characterisation of milbemycin-, avermectin-, imidazothiazole- and benzimidazole-resistant Teladorsagia circumcincta from a sheep flock
- N. D. Sargison, BA, VetMB, PhD, DSHP, DipECSRHM, FRCVS1,
- F. Jackson, BSc, PhD2,
- D. J. Wilson, MA, BVMS, DSHP, DipECSRHM, MRCVS1,
- D. J. Bartley, BSc, PhD2,
- C. D. Penny, BVM&S, CertCHP, DBR, DipECBHM, MRCVS1 and
- J. S. Gilleard, BVSc, PhD, DipEVPC, MRCVS3
- 1 Large Animal Practice, Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG
- 2 Department of Parasitology, Moredun Research Institute, Pentlands Science Park, Bush Loan, Penicuik, Midlothian EH26 0PZ
- 3 Department of Comparative Biology and Experimental Medicine, Faculty of Veterinary Medicine, University of Calgary, 3330 Hospital Drive NW, Calgary, Alberta, Canada
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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 ED50 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.
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