M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA.
M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA.
Int J Parasitol. 2022 Nov;52(12):787-798. doi: 10.1016/j.ijpara.2022.09.003. Epub 2022 Oct 13.
Macrocyclic lactones have been the most widely used drugs for equine parasite control during the past four decades. Unlike ivermectin, moxidectin exhibits efficacy against encysted cyathostomin larvae, and is reported to have persistent efficacy with substantially longer egg reappearance periods. However, shortened egg reappearance periods have been reported recently for both macrocyclic lactones, and these findings have raised several questions: (i) are egg reappearance period patterns different after ivermectin or moxidectin treatment? (ii) Are shortened egg reappearance periods associated with certain cyathostomin species or stages? (iii) How does moxidectin's larvicidal efficacy affect egg reappearance period? To address these questions, 36 horses at pasture, aged 2-5 years old, were randomly allocated to three treatment groups: 1, moxidectin; 2, ivermectin; and 3, untreated control. Strongylid fecal egg counts were measured on a weekly basis, and the egg reappearance period was 5 weeks for both compounds. Strongylid worm counts were determined for all horses: 18 were necropsied at 2 weeks post-treatment (PT), and the remaining 18 at 5 weeks PT. Worms were identified to species morphologically and by internal transcribed spacer-2 (ITS-2) rDNA metabarcoding. Moxidectin and ivermectin were 99.9% and 99.7% efficacious against adults at 2 weeks post treatment, whereas the respective efficacies against luminal L4s were 84.3% and 69.7%. At 5 weeks PT, adulticidal efficacy was 88.3% and 57.6% for moxidectin and ivermectin, respectively, while the efficacy against luminal L4s was 0% for both drugs. Moxidectin reduced early L3 counts by 18.1% and 8.0% at 2 or 5 weeks, while the efficacies against late L3s and mucosal L4s were 60.4% and 21.2% at the same intervals, respectively. The luminal L4s surviving ivermectin treatment were predominantly Cylicocyclus (Cyc.) insigne. The ITS-2 rDNA metabarcoding was in good agreement with morphologic species estimates but suggested differential activity between moxidectin and ivermectin for several species, most notably Cyc. insigne and Cylicocyclus nassatus. This study was a comprehensive investigation of current macrocyclic lactone efficacy patterns and provided important insight into potential mechanisms behind shortened egg reappearance periods.
大环内酯类药物在过去四十年中一直是马寄生虫控制最广泛使用的药物。与伊维菌素不同，莫昔克丁对囊尾幼虫具有疗效，并且据报道具有持久的疗效，其虫卵再次出现的时间明显延长。然而，最近大环内酯类药物的虫卵再次出现时间都有所缩短，这些发现提出了几个问题：（i）伊维菌素或莫昔克丁治疗后的虫卵再次出现时间模式是否不同？（ii）缩短的虫卵再次出现时间是否与特定的Cyathostomin 种或阶段有关？（iii）莫昔克丁的幼虫杀灭效果如何影响虫卵再次出现时间？为了解决这些问题，36 匹在牧场上的 2-5 岁马被随机分配到三组治疗组：1. 莫昔克丁；2. 伊维菌素；和 3. 未处理的对照组。每周测量 Strongylid 粪便虫卵计数，两种化合物的虫卵再次出现时间为 5 周。对所有马进行 Strongylid 蠕虫计数：18 匹马在治疗后 2 周（PT）进行剖检，其余 18 匹马在 5 周 PT 时进行剖检。通过形态学和内部转录间隔区-2（ITS-2）rDNA metabarcoding 鉴定蠕虫。莫昔克丁和伊维菌素在治疗后 2 周对成虫的有效率分别为 99.9%和 99.7%，而对腔 L4 的疗效分别为 84.3%和 69.7%。在 5 周 PT 时，莫昔克丁和伊维菌素对成虫的杀虫效果分别为 88.3%和 57.6%，而两种药物对腔 L4 的疗效均为 0%。莫昔克丁在 2 或 5 周时分别减少早期 L3 计数 18.1%和 8.0%，而对晚期 L3 和粘膜 L4 的疗效分别为 60.4%和 21.2%。伊维菌素治疗后存活的腔 L4 主要为 Cylicocyclus（Cyc.） insignis。ITS-2 rDNA metabarcoding 与形态学物种估计结果吻合良好，但表明莫昔克丁和伊维菌素对几种物种的活性存在差异，最显著的是 Cyc. insignis 和 Cylicocyclus nassatus。本研究全面调查了当前大环内酯类药物的疗效模式，并为缩短虫卵再次出现时间的潜在机制提供了重要的见解。
