Sheep-farm risk factors for ivermectin resistance in Ostertagia circumcincta in Western Australia

On-farm risk factors associated with ivermectin resistance on sheep farms in Western Australia were identified from data derived from a postal survey of 235 farmers who had conducted a fecal egg-count-reduction test in 1999, 2000, or both years. A response of 54% was achieved. We developed a logistic-regression model. Contributory main effects in the final model were selling 10% more sheep in 2000 than is the usual policy (OR = 4.00), farm purchased since 1975 (OR = 2.34), and number of winter flock anthelmintic treatments in the previous 5 years (OR = 1.04). A secondary logistic-regression model assessed risk factors for farms selling 10% more sheep than usual in 2000; these farmers appeared less committed to their sheep enterprises than other farmers. These results are discussed in relation to current hypotheses of anthelmintic resistance. This is the first time that the farmer's management of the flock has been implicated in the development of anthelmintic resistance.