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

Abstract

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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