A survey of farm management practices and their associations with anthelmintic resistance in sheep flocks in Ontario, Canada

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To describe parasite control and farm management practices commonly used by Ontario sheep farmers, and to determine whether any of these practices were associated with the level of anthelmintic resistance (AR) to ivermectin, fenbendazole or levamisole, we conducted fecal egg count reduction (FECR) tests in Ontario sheep flocks, and administered a questionnaire pertaining to farm practices that were considered putative risk factors for AR. In the previous 5 years, most of the producers had used ivermectin and fenbendazole drenches (95% and 68%, respectively), while only 11% had used levamisole drench. Producers treated their animals a mean of 2.6 times per year. Routine treatment was practiced by 82% of the producers; most ewes were treated either at lambing (55%) and/or at the beginning of winter housing (48%). The majority of the producers (82%) also used targeted or targeted selective treatment; however, it was often in addition to, rather than in lieu of, routine treatment. Twenty-five producers (66%) brought in new animals in the previous year. Many producers (45%) did not calibrate the drench gun before use. The mean FECR percentages following treatment with ivermectin, fenbendazole, and levamisole were 23.7%, 28.6% and 99.1%, respectively. Although univariable analyses identified several marginally significant risk factors (0.10>p>0.05), none were significant in the final model for ivermectin FECR percentage. In contrast, use of benzimidazoles in the previous 5 years was associated (p=0.01) with increased resistance (lower mean FECR percentage) to fenbendazole. Levamisole resistance could not be modeled due to the very low levels of resistance on the farms surveyed. This study: (1) provided a picture of management practices employed by Ontario sheep producers who were experiencing AR to one or more anthelmintic drugs on their farms; and (2) allowed us to identify areas for further AR risk factor research.