FREE SHIPPING over $40
OVER 10000 FIVE-STAR REVIEWS
5% OFF ALL SUBSCRIPTIONS
100% HAPPINESS GUARANTEE
Analyze Diet
0
International journal for parasitology. Drugs and drug resistance2020; 14; 99-104; doi: 10.1016/j.ijpddr.2020.09.004

Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm.

Abstract: Anthelmintic resistance in equine cyathostomins is both widespread and highly prevalent in the benzimidazole and tetrahydropyrimidine classes; however, reports of resistance to macrocyclic lactone (ML) drugs are sparse and sporadic. This study reports a case of clear ML resistance in a group of Thoroughbred yearlings imported from Ireland to the US in 2019. Fecal egg count reduction (FECR) following ivermectin administered in February 2020 demonstrated 100% reduction in the US bred yearlings, but 93.5%, 70.5%, and 74.5% reduction in three groups of the imported yearlings. The two former groups were then retreated with ivermectin, yielding FECRs of 33.8% and 23.5%, respectively. Horses from these two groups were then assigned randomly to two possible treatments; moxidectin or a triple combination of moxidectin, oxibendazole, and pyrantel pamoate. The groups treated with moxidectin had FECRs of 90.2%, 57.3%, and 50.0%, while the triple combination had a 100% FECR in all treated groups. Subsequently, the efficacy of ivermectin was reassessed in June 2020 yielding FECRs of 99.8%, 87.7%, and 62.0% in the three imported groups. The FECRs of the US bred yearlings all remained in the 99-100% range. This is the first study to clearly demonstrate ML resistance in cyathostomins and to confirm the suspicion through reassessment. These data demonstrate that ML-resistant cyathostomins were imported from Ireland and serve to illustrate that the global movement of horses has the potential to quickly spread ML-resistant parasite isolates around the world. The equine industry is strongly encouraged to routinely monitor anthelmintic efficacy, so occurrence of ML resistant cyathostomins can be detected and appropriate interventions implemented as early as possible.
Copyright © 2020 The Author(s). Published by Elsevier Ltd.. All rights reserved.
Get Full Text
Publication Date: 2020-09-30 PubMed ID: 33022574PubMed Central: PMC7548974DOI: 10.1016/j.ijpddr.2020.09.004Google Scholar: Lookup
The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
LinkedInCopy
  • Case Reports
  • Journal Article

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

The research paper discusses a case of imported macrocyclic lactone-resistant cyathostomins in a thoroughbred farm in the US. It highlights how resistance to anthelmintic drugs can be rapidly dispersed due to the global movement of horses.

Study Background

  • The study starts by acknowledging the global issue of anthelmintic resistance in cyathostomins. Cyathostomins are a type of parasitic worm that infect equines such as horses.
  • The most commonly observed resistance is against benzimidazole and tetrahydropyrimidine types of anthelmintics. However, macrocyclic lactones (ML), another type of anthelmintic, have shown resilience with sparse and sporadic reports of resistance.

Study Findings

  • The researchers discuss their investigation into a case of apparent ML resistance in Thoroughbred yearlings that were imported from Ireland to the US in 2019.
  • They conducted fecal egg count reduction (FECR) tests after administering ivermectin (an ML drug) and found that while the US-bred yearlings showed a 100% FECR, the imported yearlings showed lower FECRs of 93.5%, 70.5%, and 74.5% in three different groups.
  • The two groups showing the least response were treated with ivermectin again and showed 33.8% and 23.5% FECRs.
  • After an organized assignment of the poorly responding horses, they were treated with either moxidectin alone or a triple combination of moxidectin, oxibendazole, and pyrantel pamoate.
  • The FECRs improved in the groups treated with moxidectin (90.2%, 57.3%, and 50.0%) and the groups treated with the triple combination showed a complete (100%) FECR.
  • However, upon reassessment with ivermectin, the FECRs in the imported groups were again inadequate, validating the ML resistance.

Conclusions and Recommendations

  • This study is the first to highlight ML resistance in cyathostomins, underscoring the need for continuous monitoring and reassessment.
  • The results drive home the point that ML-resistant cyathostomins were imported from Ireland and these could be a source of rapid global dissemination of ML-resistant parasite isolates.
  • The authors call for a regular monitoring of anthelmintic efficacy to detect occurrences of ML-resistant cyathostomins and to implement appropriate interventions as early as possible to control the spread.

Cite This Article

APA
Nielsen MK, Banahan M, Kaplan RM. (2020). Importation of macrocyclic lactone resistant cyathostomins on a US thoroughbred farm. Int J Parasitol Drugs Drug Resist, 14, 99-104. https://doi.org/10.1016/j.ijpddr.2020.09.004

Publication

International journal for parasitology. Drugs and drug resistance
ISSN: 2211-3207
NlmUniqueID: 101576715
Country: Netherlands
Language: English
Volume: 14
Pages: 99-104

Researcher Affiliations

Nielsen, M K
  • M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, KY, USA. Electronic address: martin.nielsen@uky.edu.
Banahan, M
  • Godolphin, Jonabell Farm, 3333 Bowman Mill Road, Lexington, KY, USA.
Kaplan, R M
  • Department of Infectious Diseases, University of Georgia, GA, USA.

MeSH Terms

  • Animals
  • Anthelmintics / pharmacology
  • Drug Resistance
  • Farms
  • Feces
  • Horse Diseases / drug therapy
  • Horse Diseases / parasitology
  • Horses
  • Humans
  • Ireland
  • Ivermectin / therapeutic use
  • Lactones
  • Parasite Egg Count

Conflict of Interest Statement

The authors declare no conflicts of interest.

References

This article includes 37 references

Citations

This article has been cited 21 times.
  1. Klass LG, Krücken J, Mbedi S, Sparmann S, Schenk T, Andreotti S, von Samson-Himmelstjerna G. Characterizing mixed strongyle infections in foals and broodmares using cytochrome c oxidase subunit I deep amplicon sequencing. Parasit Vectors 2026 Jan 3;19(1):65.
    doi: 10.1186/s13071-025-07192-1pubmed: 41484633google scholar: lookup
  2. Nielsen MK, Pyatt A, Perrett J, Tydén E, van Doorn D, Pihl TH, Schmidt JS, von Samson-Himmelstjerna G, Beasley A, Abbas G, Jabbar A. Global equine parasite control guidelines: Consensus or confusion?. Int J Parasitol Drugs Drug Resist 2025 Aug;28:100600.
    doi: 10.1016/j.ijpddr.2025.100600pubmed: 40472642google scholar: lookup
  3. do Carmo TA, Fonseca JDS, Braga FR, Paz-Silva A, de Soutello RVG, de Araújo JV. Exploring the Use of Helminthophagous Fungi in the Control of Helminthoses in Horses: A Review. Animals (Basel) 2025 Mar 18;15(6).
    doi: 10.3390/ani15060864pubmed: 40150393google scholar: lookup
  4. Beasley A, Abbas G, Hughes K, El-Hage C, Jacobson C, Bauquier J, Wilkes E, Carrigan P, Cudmore L, Hurley J, Beveridge I, Nielsen M, Jabbar A. Australian guidelines for equine internal parasite management. Aust Vet J 2025 Apr;103(4):151-158.
    doi: 10.1111/avj.13424pubmed: 39837536google scholar: lookup
  5. Bull KE, Hodgkinson J, Allen K, Poissant J, Peachey LE. Quantitative DNA metabarcoding reveals species composition of a macrocyclic lactone and pyrantel resistant cyathostomin population in the UK. Int J Parasitol Drugs Drug Resist 2025 Apr;27:100576.
    doi: 10.1016/j.ijpddr.2024.100576pubmed: 39778419google scholar: lookup
  6. Hedberg Alm Y, Tydén E, Martin F, Lernå J, Halvarsson P. Farm size and biosecurity measures associated with Strongylus vulgaris infection in horses. Equine Vet J 2025 May;57(3):703-711.
    doi: 10.1111/evj.14212pubmed: 39171858google scholar: lookup
  7. Elghryani N, Lawlor A, McOwan T, de Waal T. Unravelling the Effectiveness of Anthelmintic Treatments on Equine Strongyles on Irish Farms. Animals (Basel) 2024 Jul 2;14(13).
    doi: 10.3390/ani14131958pubmed: 38998069google scholar: lookup
  8. Northcote HM, Wititkornkul B, Cutress DJ, Allen ND, Brophy PM, Wonfor RE, Morphew RM. A dominance of Mu class glutathione transferases within the equine tapeworm Anoplocephala perfoliata. Parasitology 2024 Mar;151(3):282-294.
    doi: 10.1017/S0031182024000015pubmed: 38200699google scholar: lookup
  9. Ng'etich AI, Amoah ID, Bux F, Kumari S. Anthelmintic resistance in soil-transmitted helminths: One-Health considerations. Parasitol Res 2023 Dec 20;123(1):62.
    doi: 10.1007/s00436-023-08088-8pubmed: 38114766google scholar: lookup
  10. Abbas G, Stevenson MA, Bauquier J, Beasley A, Jacobson C, El-Hage C, Wilkes EJA, Carrigan P, Cudmore L, Hurley J, Beveridge I, Nielsen MK, Hughes KJ, Jabbar A. Assessment of worm control practices recommended by equine veterinarians in Australia. Front Vet Sci 2023;10:1305360.
    doi: 10.3389/fvets.2023.1305360pubmed: 38026649google scholar: lookup
  11. Buono F, Veneziano V, Veronesi F, Molento MB. Horse and donkey parasitology: differences and analogies for a correct diagnostic and management of major helminth infections. Parasitology 2023 Oct;150(12):1119-1138.
    doi: 10.1017/S0031182023000525pubmed: 37221816google scholar: lookup
  12. Boelow H, Krücken J, von Samson-Himmelstjerna G. Epidemiological study on factors influencing the occurrence of helminth eggs in horses in Germany based on sent-in diagnostic samples. Parasitol Res 2023 Mar;122(3):749-767.
    doi: 10.1007/s00436-022-07765-4pubmed: 36627515google scholar: lookup
  13. Macdonald SL, Abbas G, Ghafar A, Gauci CG, Bauquier J, El-Hage C, Tennent-Brown B, Wilkes EJA, Beasley A, Jacobson C, Cudmore L, Carrigan P, Hurley J, Beveridge I, Hughes KJ, Nielsen MK, Jabbar A. Egg reappearance periods of anthelmintics against equine cyathostomins: The state of play revisited. Int J Parasitol Drugs Drug Resist 2023 Apr;21:28-39.
    doi: 10.1016/j.ijpddr.2022.12.002pubmed: 36543048google scholar: lookup
  14. Osterman-Lind E, Hedberg Alm Y, Hassler H, Wilderoth H, Thorolfson H, Tydén E. Evaluation of Strategies to Reduce Equine Strongyle Infective Larvae on Pasture and Study of Larval Migration and Overwintering in a Nordic Climate. Animals (Basel) 2022 Nov 10;12(22).
    doi: 10.3390/ani12223093pubmed: 36428321google scholar: lookup
  15. Nielsen MK. Anthelmintic resistance in equine nematodes: Current status and emerging trends. Int J Parasitol Drugs Drug Resist 2022 Dec;20:76-88.
    doi: 10.1016/j.ijpddr.2022.10.005pubmed: 36342004google scholar: lookup
  16. Malsa J, Courtot É, Boisseau M, Dumont B, Gombault P, Kuzmina TA, Basiaga M, Lluch J, Annonay G, Dhorne-Pollet S, Mach N, Sutra JF, Wimel L, Dubois C, Guégnard F, Serreau D, Lespine A, Sallé G, Fleurance G. Effect of sainfoin (Onobrychis viciifolia) on cyathostomin eggs excretion, larval development, larval community structure and efficacy of ivermectin treatment in horses. Parasitology 2022 Sep;149(11):1439-1449.
    doi: 10.1017/S0031182022000853pubmed: 35929352google scholar: lookup
  17. Nápravníková J, Várady M, Vadlejch J. Total Failure of Fenbendazole to Control Strongylid Infections in Czech Horse Operations. Front Vet Sci 2022;9:833204.
    doi: 10.3389/fvets.2022.833204pubmed: 35265696google scholar: lookup
  18. Steuer AE, Anderson HP, Shepherd T, Clark M, Scare JA, Gravatte HS, Nielsen MK. Parasite dynamics in untreated horses through one calendar year. Parasit Vectors 2022 Feb 8;15(1):50.
    doi: 10.1186/s13071-022-05168-zpubmed: 35135605google scholar: lookup
  19. Abbas G, Ghafar A, Hurley J, Bauquier J, Beasley A, Wilkes EJA, Jacobson C, El-Hage C, Cudmore L, Carrigan P, Tennent-Brown B, Gauci CG, Nielsen MK, Hughes KJ, Beveridge I, Jabbar A. Cyathostomin resistance to moxidectin and combinations of anthelmintics in Australian horses. Parasit Vectors 2021 Dec 4;14(1):597.
    doi: 10.1186/s13071-021-05103-8pubmed: 34863271google scholar: lookup
  20. Johnson ACB, Biddle AS. The Use of Molecular Profiling to Track Equine Reinfection Rates of Cyathostomin Species Following Anthelmintic Administration. Animals (Basel) 2021 May 9;11(5).
    doi: 10.3390/ani11051345pubmed: 34065099google scholar: lookup
  21. Roelfstra L, Quartier M, Pfister K. Preliminary Data from Six Years of Selective Anthelmintic Treatment on Five Horse Farms in France and Switzerland. Animals (Basel) 2020 Dec 15;10(12).
    doi: 10.3390/ani10122395pubmed: 33333748google scholar: lookup
Subscribe to our newsletter
Join 99,727 horse owners like you who receive our email updates on horse health and nutrition.