University of Edinburgh, Royal (Dick) School of Veterinary Studies, Farm Animal Practice, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG, United Kingdom.
Vet Parasitol. 2012 Oct 26;189(2-4):274-83. doi: 10.1016/j.vetpar.2012.04.020. Epub 2012 May 1.
A field study was conducted in a sheep flock in the south east of Scotland with a history of ivermectin resistance in Teladorsagia circumcincta. The objective of the study was to compare the effects of single anthelmintic treatments in ewes before turn-out onto pasture that was contaminated with a moderate level of overwintered, ivermectin resistant, T. circumcincta infective larvae. The ewes were treated according to label directions with either a long acting injectable formulation of moxidectin (1mg/kg; affording up to 14 weeks persistent action against macrocyclic lactone (ML)-susceptible T. circumcincta) or an oral formulation of moxidectin (0.2mg/kg; affording up to 5 weeks persistent action against ML-susceptible T. circumcincta). The lambs were enrolled in the normal management of the farm, and received a total of three oral ivermectin treatments during the 16 week study. The efficacy of both treatment strategies in controlling the periparturient rise in faecal nematode worm egg counts and subsequent pasture contamination was assessed from the faecal worm egg counts of the ewes and their lambs between lambing and weaning. Ewes that were treated with the oral formulation of moxidectin shed approximately 3.5 times more T. circumcincta eggs between lambing and weaning than ewes that were treated with the long acting formulation of moxidectin. This difference was reflected in the faecal worm egg counts of the lambs that were grazed alongside the different treatment groups of ewes. The results of the current study demonstrate persistent efficacy of the long acting formulation of moxidectin against an ivermectin resistant T. circumcincta population. The decreased pasture contamination after treatment could lead to improved lamb growth and a need for fewer anthelmintic treatments, thus potentially reducing one possible selection pressure for anthelmintic resistance. However, treatment with the long acting formulation of moxidectin would give rise to fewer susceptible nematodes being present in refugia, which could increase another possible selection pressure for anthelmintic resistance, depending on the subsequent grazing management of that pasture. The rationale for use of a persistent anthelmintic drug to control the periparturient rise in faecal ML-resistant T. circumcincta egg output of the ewes is discussed and potential differences in selection for macrocyclic lactone anthelmintic resistance using the different formulations of moxidectin are acknowledged.
在苏格兰东南部的一个羊群中进行了一项实地研究，该羊群中 Teladorsagia circumcincta 对伊维菌素具有耐药性。该研究的目的是比较在放牧前对感染中度越冬伊维菌素耐药 T. circumcincta 感染性幼虫的母羊进行单一驱虫处理的效果。根据标签说明，母羊分别接受长效注射型莫昔克丁（1mg/kg；对大环内酯类（ML）敏感的 T. circumcincta 具有长达 14 周的持续作用）或口服莫昔克丁（0.2mg/kg；对 ML 敏感的 T. circumcincta 具有长达 5 周的持续作用）处理。羔羊按照农场的正常管理程序进行接种，并在 16 周的研究期间接受了总共三次伊维菌素口服处理。从产羔和断奶期间母羊及其羔羊的粪便蠕虫卵计数评估了这两种治疗策略控制围产期粪便线虫卵计数升高和随后牧场污染的效果。与接受长效莫昔克丁制剂治疗的母羊相比，接受口服莫昔克丁制剂治疗的母羊在产羔和断奶期间排出的 T. circumcincta 卵大约多 3.5 倍。这一差异反映在与不同治疗组母羊一起放牧的羔羊的粪便蠕虫卵计数中。本研究的结果表明，长效莫昔克丁制剂对伊维菌素耐药的 T. circumcincta 种群具有持续的疗效。治疗后牧场污染的减少可能会导致羔羊生长更好，减少驱虫处理的次数，从而可能降低驱虫耐药的一个可能选择压力。然而，长效莫昔克丁制剂的治疗会导致在避难所中存在较少的敏感线虫，这可能会增加另一个可能的驱虫耐药选择压力，这取决于该牧场随后的放牧管理。讨论了使用持续驱虫药物来控制围产期粪便 ML 耐药 T. circumcincta 卵输出量增加的原理，并承认了使用不同莫昔克丁制剂对大环内酯类驱虫药耐药性的选择存在潜在差异。
