Shortened egg reappearance periods of equine cyathostomins following ivermectin or moxidectin treatment: morphological and molecular investigation of efficacy and species composition.
Abstract: 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.
Copyright © 2022 Australian Society for Parasitology. Published by Elsevier Ltd. All rights reserved.
Publication Date: 2022-10-13 PubMed ID: 36244428DOI: 10.1016/j.ijpara.2022.09.003Google Scholar: Lookup
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- Journal Article
- Randomized Controlled Trial
- Veterinary
- Research Support
- Non-U.S. Gov't
Summary
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The study investigates the efficiency of two equine parasite drugs, ivermectin and moxidectin, in controlling cyathostomins infection and the potential cause for the inabilities of these drugs to prevent rapid reappearance of parasite eggs.
Objective of the Study
- The study aims to answer several questions raised by the recent finding of shortened egg reappearance periods observed after the treatment of horses with macrocyclic lactones (ivermectin and moxidectin). The key questions are:
- Are the patterns of egg reappearance periods after ivermectin or moxidectin treatment different?
- Are certain cyathostomin species or stages associated with shortened egg reappearance periods?
- How does moxidectin’s efficacy against larval forms impact the egg reappearance period?
Methodology
- A total of 36 horses pasturing and aged between 2-5 years old were randomly assigned to one of the three treatment groups: moxidectin, ivermectin, or untreated as control.
- Fecal egg counts of strongylid worms were determined weekly, and egg reappearance periods were measured. The determination of worm counts was also performed for all horses, with half necropsied at 2 weeks after treatment and the other half at 5 weeks after treatment.
- Species identification of worms was done using morphological methods and by using a genomic method called ITS-2 rDNA metabarcoding.
Results
- Both moxidectin and ivermectin showed high efficacy against adults at 2 weeks post-treatment, with moxidectin proving marginally more effective.
- While the larvicidal efficacy for luminal L4s at 2 weeks post-treatment was noticeably lower than adulticidal efficacy, moxidectin was more effective than ivermectin.
- By 5 weeks post-treatment, moxidectin maintained relatively higher efficacy against adults, while both drugs showed 0% efficacy against luminal L4s.
- Moxidectin was found to be more effective against early L3 counts and late L3s and mucosal L4s at both 2 and 5 weeks post-treatment.
- It was noted that luminal L4s surviving ivermectin treatment were predominantly of the species Cyc. insigne.
- The genomic method used for species identification was broadly in agreement with morphological estimates and suggested differential activity of moxidectin and ivermectin for several species.
Conclusion
- The findings of the investigation provide crucial insight into the efficacy patterns of currently used macrocyclic lactone drugs, and potential reasons for the shortened egg reappearance periods.
- The research implies that further understanding of these mechanisms can assist in enhancing the efficacy of the treatments for equine parasites and offer better control over the rate of egg reappearance.
Cite This Article
APA
Nielsen MK, Steuer AE, Anderson HP, Gavriliuc S, Carpenter AB, Redman EM, Gilleard JS, Reinemeyer CR, Poissant J.
(2022).
Shortened egg reappearance periods of equine cyathostomins following ivermectin or moxidectin treatment: morphological and molecular investigation of efficacy and species composition.
Int J Parasitol, 52(12), 787-798.
https://doi.org/10.1016/j.ijpara.2022.09.003 Publication
Researcher Affiliations
- M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA. Electronic address: martin.nielsen@uky.edu.
- 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.
- Department of Ecosystem and Public Health, University of Calgary, Calgary, Alberta, Canada.
- M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA.
- Department of Comparative Biology and Experimental Medicine, University of Calgary, Calgary, Alberta, Canada.
- Department of Comparative Biology and Experimental Medicine, University of Calgary, Calgary, Alberta, Canada.
- East Tennessee Clinical Research, Rockwood, TN, USA.
- Department of Ecosystem and Public Health, University of Calgary, Calgary, Alberta, Canada.
MeSH Terms
- Animals
- Anthelmintics / therapeutic use
- Anthelmintics / pharmacology
- DNA, Ribosomal
- Drug Resistance
- Feces / parasitology
- Horse Diseases / drug therapy
- Horse Diseases / parasitology
- Horses
- Ivermectin / therapeutic use
- Macrolides / therapeutic use
- Parasite Egg Count / veterinary
- Strongyle Infections, Equine / drug therapy
- Strongyle Infections, Equine / parasitology
- Strongyloidea / genetics
Citations
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