Department of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada.
Vet Parasitol. 2013 Mar 31;193(1-3):150-62. doi: 10.1016/j.vetpar.2012.11.014. Epub 2012 Nov 12.
Gastrointestinal nematodes (GIN) are a significant constraint to pasture-based sheep production worldwide. Anthelmintic resistance (AR) has been reported in most sheep-raising areas in the world, yet little is known about the AR status in Canada. This study was conducted to determine the frequency of AR in GIN in sheep flocks in Ontario, Canada. Forty-seven sheep flocks were enrolled in the study, and their level of parasitism was monitored monthly throughout a grazing season by analyzing owner-acquired fecal samples from 15 grazing lambs per flock. When the mean GIN fecal egg count (FEC) reached a threshold of 200 eggs per gram (epg), oral ivermectin was supplied to producers to check ivermectin efficacy; the reduction in mean FEC 14 days after ivermectin treatment was calculated. 'Drench failure' was defined as a reduction in mean FEC of <95%. In those flocks with apparent drench failure, researchers performed a Fecal Egg Count Reduction Test (FECRT), dividing sheep into 4 treatment groups (n=10-15): control (i.e. untreated), ivermectin, and, if sufficient numbers of animals - fenbendazole and levamisole. AR was defined as a reduction in mean FEC <95% and a lower 95% confidence interval <90%. Larval cultures were performed on pooled post-treatment FECRT samples. Larval Development Assays (LDAs) to detect the presence of resistance to thiabendazole and levamisole were performed prior to the ivermectin drench check on pooled owner-acquired fecal samples that reached the 200 epg threshold. Approximately 89% (42/47) of the farms reached the FEC threshold of 200 epg; 93% (39/42) of these farms performed an ivermectin drench check, and 88% (34/39) of these farms had drench failure. The FECRT was performed on 29 of the 34 farms. Resistance to ivermectin, fenbendazole and levamisole was demonstrated on 97% (28/29), 95% (19/20) and 6% (1/17) of the farms tested, respectively, with considerable variability in resistance levels among farms. Haemonchus sp. was the most commonly cultured parasite from post-treatment fecal samples. LDA results for 21 farms were available; of these, 14% (3/21) and 62% (13/21) had low and high levels of thiabendazole resistance, respectively, while none of the farms exhibited resistance to levamisole. Amongst these tested farms, resistance to both ivermectin and benzimidazoles was very common. These findings strongly suggest that AR, particularly in Haemonchus sp., is a serious problem in these sheep flocks. Thus, marked changes in GIN management need to be instituted immediately to mitigate a worsening situation.
胃肠道线虫（GIN）是全球牧场绵羊生产的重大制约因素。在世界上大多数绵羊养殖地区都报告了驱虫剂耐药性（AR），但对加拿大的 AR 状况知之甚少。本研究旨在确定加拿大安大略省绵羊群中 GIN 的 AR 频率。研究招募了 47 个绵羊群，通过分析每个羊群中 15 只放牧羔羊的所有者获得的粪便样本，在整个放牧季节每月监测其寄生虫感染程度。当平均 GIN 粪便卵计数（FEC）达到 200 个卵/克（epg）的阈值时，向生产者提供口服伊维菌素以检查伊维菌素的疗效；计算伊维菌素治疗后 14 天平均 FEC 的减少。“驱虫失败”定义为平均 FEC 减少<95%。在那些明显驱虫失败的羊群中，研究人员进行了粪便卵计数减少试验（FECRT），将绵羊分为 4 个治疗组（n=10-15）：对照组（即未治疗）、伊维菌素以及如果动物数量足够 - 芬苯达唑和左旋咪唑。AR 定义为平均 FEC 减少<95%，置信区间下限<90%。对治疗后 FECRT 样本进行了幼虫培养。在达到 200 epg 阈值的所有者获得的粪便样本中，进行了 thiabendazole 和左旋咪唑耐药性的幼虫发育测定（LDAs）。大约 89%（42/47）的农场达到了 200 epg 的 FEC 阈值；其中 93%（39/42）的农场进行了伊维菌素驱虫检查，其中 88%（34/39）的农场驱虫失败。在 34 个农场中的 29 个农场进行了 FECRT。在接受测试的农场中，分别有 97%（28/29）、95%（19/20）和 6%（1/17）的农场显示出对伊维菌素、芬苯达唑和左旋咪唑的耐药性，并且农场之间的耐药水平存在很大差异。从治疗后粪便样本中最常培养出的寄生虫是 Haemonchus sp.。可获得 21 个农场的 LDA 结果；其中，分别有 14%（3/21）和 62%（13/21）的农场具有低和高水平的噻苯达唑耐药性，而没有一个农场对左旋咪唑表现出耐药性。在所测试的这些农场中，对伊维菌素和苯并咪唑类药物的耐药性非常普遍。这些发现强烈表明，AR，特别是 Haemonchus sp.，是这些绵羊群中的一个严重问题。因此，需要立即对 GIN 管理进行重大改变，以减轻情况恶化。
