The effective life of ivermectin on Western Australian sheep farms--a survival analysis.
A mail survey of 235 Western Australian sheep farmers who had performed faecal egg count 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 macrocyclic 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 significantly 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: reduced winter 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 (HR 0.23); and veterinarians as the primary source of worm control advice (HR 0.58). The relationship of these findings to the understanding of anthelmintic resistance is discussed.
Suter
R J
RJ
School of Veterinary and Biomedical Sciences, Division of Health Sciences, Murdoch University, Murdoch, WA 6150, Australia. rsuter@murdoch.edu.au
McKinnon
E J
EJ
Perkins
N R
NR
Besier
R B
RB
D000383:Agriculture / Q000379:methods*; D000818:Animals; D000871:Anthelmintics / Q000494:pharmacology*; D004351:Drug Resistance; D006374:Helminthiasis, Animal / Q000453:epidemiology / Q000517:prevention & control*; D007559:Ivermectin / Q000494:pharmacology*; D012307:Risk Factors; D012621:Seasons; D012756:Sheep; D012757:Sheep Diseases / Q000453:epidemiology / Q000517:prevention & control*; D011795:Surveys and Questionnaires; D013997:Time Factors; D014904:Western Australia / Q000453:epidemiology
D000871:Anthelmintics; D007559:Ivermectin