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Evaluating Strategies for Controlling Anthelmintic-Resistant Haemonchus contortus in Ontario Sheep Flocks

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Title: Evaluating Strategies for Controlling Anthelmintic-Resistant Haemonchus contortus in Ontario Sheep Flocks
Author: Westers, Trisha
Department: Department of Population Medicine
Program: Population Medicine
Advisor: Jones-Bitton, AndriaPeregrine, Andrew
Abstract: Widespread anthelmintic resistance has been documented primarily in Haemonchus sp. on sheep farms in Ontario, Canada. Access to an effective anthelmintic is therefore essential to both ensure animal welfare and reduce economic losses. There is, however, also a need to balance parasite control with management practices that help mitigate the selection of anthelmintic-resistant parasites. The primary objectives of this research were to: a) evaluate the efficacy of closantel, a novel anthelmintic to Canada, using fecal egg count reduction tests in periparturient ewes and grazing lambs on six sheep farms in Ontario; b) evaluate the ability of four targeted selective treatment (TST) indicators to predict Haemonchus-specific fecal egg counts (FEC) in recently lambed ewes; and c) evaluate whether TST was as effective in controlling Haemonchus sp. as whole-flock treatment (WT) using both a pilot longitudinal field study as well as a mathematical model developed for Ontario conditions. Closantel was found to be highly effective against anthelmintic-resistant Haemonchus sp. over two grazing seasons in both periparturient ewes and grazing lambs. Of the four TST indicators evaluated, FAMACHA© score was best able to predict Haemonchus-specific FEC. Further sensitivity and specificity analyses of FAMACHA© score were investigated using a receiver-operator characteristic curve; if FAMACHA© score was to be used as the sole indicator, ewes with FAMACHA© scores of 3-5 should be treated. Over the grazing season, Haemonchus larval pasture burdens and Haemonchus-specific FEC in both ewes and lambs were higher on TST farms compared with WT farms. However, there was no significant difference in the number of additional anthelmintic treatments given by producers nor was there a significant difference in number of parasite deaths between TST and WT farms. Model simulations determined that TST of ewes with FAMACHA© scores of 3-5 closely approximated Haemonchus sp. FECs of WT farms, thus indicating that TST can be used to help delay the development of anthelmintic resistance. Results from the closantel efficacy study have been used for the approval of closantel in Canada and the information from this thesis will be used to aid producers in using effective management practices to decrease the selection of anthelmintic-resistant parasites.
Date: 2016-10
Rights: Attribution-NonCommercial-NoDerivs 2.5 Canada
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Attribution-NonCommercial-NoDerivs 2.5 Canada Except where otherwise noted, this item's license is described as Attribution-NonCommercial-NoDerivs 2.5 Canada