The effective life of ivermectin on Western Australian sheep farms—A survival analysis R.J. Suter a, * , E.J. McKinnon b , N.R. Perkins c , R.B. Besier d a School of Veterinary and Biomedical Sciences, Division of Health Sciences, Murdoch University, Murdoch, WA 6150, Australia b Division of Science and Engineering, Murdoch University, Murdoch, WA 6150, Australia c EpiCentre, IVABS,Massey University, Palmerston North, New Zealand d Department of Agriculture, 444 Albany Highway, Albany, WA 6330, Australia Received 7 May 2004; received in revised form 28 July 2005; accepted 28 July 2005 Abstract A mail survey of 235 Western Australian sheep farmers who had performed faecal egg c reduction tests for anthelmintic resistance in 1999 or 2000 was conducted, with some telephone follow-up. A response of 56% was achieved. Resistance to ivermectin, a member of the ma lactone class of anthelmintics, had developed on 44% of the farms surveyed. We used time to occurrence of resistance to ascertain factors that contributed to extending the time ivermectin remained an effective drench on these farms (median time = 10.5 years). This time was sig longer when farmers implemented more worm control practices on their farms (P = 0.003). We developed a multivariable survival model that contained the following main effects: reduce drenching frequency, 0–2 flock treatments in 5 years (hazard ratio (HR) 0.52);availability of alternative effective anthelmintic classes on the farm (HR 0.30); always using safe pastures 0.23); and veterinarians as the primary source of worm control advice (HR 0.58). The relati these findings to the understanding of anthelmintic resistance is discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Ivermectin; Anthelmintic resistance; Sheep;Survivalanalysis;Farm management; Ostertagia circumcincta www.elsevier.com/locate/prevetmed Preventive Veterinary Medicine 72 (2005) 311–322 * Corresponding author. Tel.: +61 8 93602673; fax: +61 8 93602649. E-mail address: rsuter@murdoch.edu.au (R.J. Suter). 0167-5877/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2005.07.014