Shortened egg reappearance periods of equine cyathostomins following ivermectin or moxidectin treatment: morphological and molecular investigation of efficacy and species composition

Martin K. Nielsen, Ashley E. Steuer, Haley P. Anderson, Stefan Gavriliuc, Alyssa B. Carpenter, Elizabeth M. Redman, John S. Gilleard, Craig R. Reinemeyer, Jocelyn Poissant

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)787-798
Number of pages12
JournalInternational Journal for Parasitology
Issue number12
StatePublished - Nov 2022

Bibliographical note

Funding Information:
The authors are grateful to Drs. Kenton Morgan and Nathan Voris from Zoetis (USA) for providing financial support for the study. Mackenzie Smith, Taylor Blasey, Hannah Cox, Abby Vetter, Russell Davis, John Hines, Ben Hines, Avery Martin, Grace Perrin, Jessica Scare, and Holli Gravatte are warmly acknowledged for their hard work in collecting and processing the samples in the study. The farm crew - Chad Tucker, Mason Mulholland, Chip Stamper, Felicia Stamper, and Courtney Floyd - are appreciated for their excellent care and maintenance of the horses and help with the logistics of the project. The team at the Veterinary Diagnostic Laboratory, University of Kentucky (USA) - Sara Welsh, Amanda Burkhart, Susan Minnis, and Eva Langlois - are thanked for their help and support with the necropsy procedures of the project.

Publisher Copyright:
© 2022 Australian Society for Parasitology


  • Anthelmintic resistance
  • Horse
  • Macrocyclic lactones
  • Strongyle

ASJC Scopus subject areas

  • Parasitology
  • Infectious Diseases


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