Risk factor epidemiological studies of ivermectin resistant Ostertagia circumcinta on Western Australian sheep farms : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies in Epidemiology at Massey University, Palmerston North, New Zealand

Abstract

This study was designed to investigate the farm level epidemiology of ivermectin resistant Ostertagia circumcincta on Western Australian sheep farms. The study involved a postal survey and the results of that survey were used to develop statistical models to identify risk factors associated with ivermectin resistance. The survey was mailed to farmers in July 2001 who had conducted faecal egg count reduction tests on their properties in 1999 and 2000. The questionnaire contained questions about farm management practices, particularly those pertaining to worm control. Some mail and telephone follow-up was conducted. The response rate to the survey was 54%. The period prevalence for ivermectin resistance in Western Australia 1999 – 2000 as defined in this study was 38% (95% CI 29%, 46%) and for the period 1999 – 2001 was 44% (95% CI 39%, 58%) as some farms were diagnosed with ivermectin resistance in 2001. Two main effects models of anthelmintic resistance at the farm level were developed: a logistic regression model for risk factors for a farm having been diagnosed with ivermectin resistant Ostertagia circumcincta by 2000, and a Weibull parametric survival model studying the effective life defined as time to onset of resistance, for those farms using ivermectin. The logistic regression model contained three main effects variables: 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. The survival analysis model contained four main effects variables: winter drenching frequency, 0-2 vs. 3+ flock treatments in 5 years (RH 0.52); availability of alternative effective anthelmintic classes on the farm (RH 0.30); always using safe pastures (RH 0.23); and veterinarians as the primary source of worm control advice (RH 0.58). A major outcome of the study has been to identify that the farmer’s management of worm control in the sheep flock has an important influence on whether or not the farm develops anthelmintic (ivermectin) resistance.