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 anthelatintics, 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. (c) 2005 Elsevier B.V. All rights reserved.