Effect of targeted selective treatement with closantel at lambing on Haemonchus burdens on Ontario sheep farms with anthelmintic resistance

缠绕血圆线虫日益被认为是安大略省牧场绵羊生产的重要制约因素；它是严重贫血和低蛋白血症导致羔羊和成年绵羊发病和死亡的重要原因。该省最近的研究表明，这种寄生虫对绵羊最常用的两种驱虫剂具有很高的抗药性，即90%的受试农场对伊维菌素和芬苯达唑产生了抗药性。只有伊维菌素被批准用于绵羊，苯并咪唑(即芬苯达唑和阿苯达唑)只有在以额外的标签方式使用时才可用。在加拿大，没有其他驱虫剂可用于绵羊。因此，具有多重耐药性的农场没有合理的药物替代品来治疗寄生虫。因此，迫切需要改进控制策略。该项目将确定影响草地上螺旋杆菌感染性的参数，从而有助于加强对该寄生虫的控制计划。此外，通过概念验证研究，将评估氯桑泰(一种窄谱驱虫剂，具有抗螺旋体感染活性，以前未在加拿大使用)控制抗药性螺旋体感染的能力。由于母羊是春季草地缠绕线虫的主要污染源，这是由于产羔羊和哺乳期引起的围产期产卵上升，因此将评估两种处理绵羊产羔期的策略，即全群处理和有针对性地选择性处理母羊。结果评估将包括每种处理策略控制母羊及其羔羊感染和疾病的能力，以及控制牧场污染的能力--同时不增加对克山泰尔表现出抗药性的螺旋体的比例。由此产生的数据将有助于产生控制抗药性感染和减少羊场驱虫剂使用的新策略，从而有助于在安大略省的羊群中更谨慎地使用驱虫剂。这些数据还将有助于批准一种新的驱虫剂在加拿大用于绵羊。

Haemonchus contortus is increasingly being recognised as a significant constraint to sheep production on pasture in Ontario; it is a significant cause of morbidity and mortality in lambs and adult sheep from the effects of profound anaemia and hypoproteinemia. Recent work in the province has demonstrated a high prevalence of resistance in this parasite to the two most commonly used anthelmintics in sheep, i.e. resistance to ivermectin and fenbendazole has been demonstrated on >90% of tested farms. Only ivermectin is approved for use in sheep, and benzimidazoles (i.e. fenbendazole and albendazole) are only available when used in an extra label manner. No other anthelmintics are available for use in sheep in Canada. As a result, farms with multiple drug resistance have no reasonable pharmaceutical alternatives for parasite treatment. Improved control strategies are therefore urgently required. This project will define the parameters that influence the infectivity of H. contortus on pasture and thus help enhance control programs for the parasite. In addition, via a proof of concept study, the ability of closantel (a narrow-spectrum anthelmintic with activity against H. contortus, not previously used in Canada) to control drug-resistant H. contortus infections will be assessed. Since ewes are the primary source of pasture contamination with H. contortus in the spring due to the periparturient egg rise induced by lambing and lactation, two treatment strategies for ewes at lambing will be evaluated, i.e. whole flock versus targeted selective treatment of ewes. Outcomes assessment will include ability of each treatment strategy to control infection and disease in the ewes and in their lambs, and to control contamination of grazing pastures - whilst not increasing the proportion of H. contortus showing resistance to closantel. The resultant data will help to generate novel strategies for controlling drug-resistant infections and for reducing anthelmintic use on sheep farms, thus contributing to more prudent use of anthelmintics in Ontario flocks. The data will also assist in the approval of a new anthelmintic for use in sheep in Canada.