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Home / Equine Research Bank / Int J Parasitol Drugs Drug Resist / Anthelmintic resistance in equine nematodes: Current status ...
International journal for parasitology. Drugs and drug resistance2022; 20; 76-88; doi: 10.1016/j.ijpddr.2022.10.005

Anthelmintic resistance in equine nematodes: Current status and emerging trends.

Abstract: Anthelmintic resistance is reported in equine nematodes with increasing frequency in recent years, and no new anthelmintic classes have been introduced during the past 40 years. This manuscript reviews published literature describing anthelmintic resistance in cyathostomins, Parascaris spp., and Oxyuris equi with special emphasis on larvicidal efficacy against encysted cyathostomin larvae and strongylid egg reappearance periods (ERP). Resistance to benzimidazoles and pyrimidines is highly prevalent in cyathostomin populations around the world, and macrocyclic lactone resistance has been documented in cyathostomins in recent years as well. Two recent studies have documented resistance to the larvicidal regimen of fenbendazole, whereas the larvicidal efficacy of moxidectin is variable, but with no evidence of a reduction from historic levels. In the 1990s, ERP estimates were 8-10 and 12-16 weeks for ivermectin and moxidectin, respectively, while several studies published after year 2000 found ERPs to be 5 weeks for both compounds. This is a clear change in anthelmintic performance, but it remains unclear if this is due to development of anthelmintic resistance or selection for other biological traits leading to a quicker resumption of strongylid egg shedding following anthelmintic treatment. Macrocyclic lactone resistance is common in Parascaris spp. around the world, but recent reports suggests that resistance to the two other classes should be monitored as well. Finally, O. equi has been reported resistant to ivermectin and moxidectin in countries representing four continents. In conclusion, multi-drug resistance is becoming the norm in managed cyathostomin populations around the world, and a similar pattern may be emerging in Parascaris spp. More work is required to understand the mechanisms behind the shortened ERPs, and researchers and veterinarians around the world are encouraged to routinely monitor anthelmintic efficacy against equine nematodes.
Copyright © 2022 The Author. Published by Elsevier Ltd.. All rights reserved.
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Publication Date: 2022-10-26 PubMed ID: 36342004PubMed Central: PMC9630620DOI: 10.1016/j.ijpddr.2022.10.005Google Scholar: Lookup
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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.

This study reviews literature on anthelmintic resistance (drug resistance in worms) found in equine (horse) nematodes. It emphasizes the increasing occurrence of resistance and notes the lack of new drug classes within the past 40 years. The paper focusses particularly on resistance in cyathostomins, Parascaris spp., and Oxyuris equi nematodes, indicating occurrences of global multi-drug resistance, with a need for more understanding and regular monitoring of anthelmintic efficacy.

Overview of Anthelmintic Resistance in Equine Nematodes

  • The paper takes a deep dive into the growing frequency of anthelmintic resistance among equine nematodes. In the process, it calls out the unfortunate truth that no new anthelmintic drugs have been introduced over the past four decades.
  • It then focuses on anthelmintic resistance observed in three types of equine nematodes: cyathostomins, Parascaris spp., and Oxyuris equi. In each case, the authors document the state of resistance and note emerging trends.
  • The emphasis is put on the efficacy of larvicides against encysted cyathostomin larvae and the time it takes for strongylid eggs to reappear after treatment (ERP).

Resistance in Different Nematode Species

  • The article declares that resistance to two kinds of anthelmintic drugs (benzimidazoles and pyrimidines) is now rampant in cyathostomin populations globally. Furthermore, resistance to a third class of drugs, macrocyclic lactones, has increasingly been found in cyathostomins.
  • The article references two studies showcasing resistance to fenbendazole, a larvicidal regimen, while it also points out that the efficacy of another larvicide, moxidectin, varies essentially with no clear evidence of reduced impact compared to its historic levels.
  • Macrocyclic lactone resistance has been commonly found in Parascaris spp. There are recent reports that the resistance to the other two drug families should also be monitored, suggesting an imminent risk of multi-drug resistance in this species. O. equi, on the other hand, has been reported resistant to two anthelmintics, ivermectin, and moxidectin, in different countries of four continents.

Changes in Anthelmintic Performance Over Time

  • The study marks a notable change in the ERP estimates of anthelmintics. In the 1990s, the ERP values estimated were 8-10 and 12-16 weeks for ivermectin and moxidectin respectively. However, several studies conducted after the year 2000 found ERP values to be as low as 5 weeks for both compounds. This presents evidence of a significant shift in anthelmintic performance over the decades.
  • While there has been a clear change in anthelmintic efficiency, the study underlines that it is yet unclear whether this is due to development of drug resistance or if it’s a result of selection for other biological traits that lead to quicker resumption of egg shedding after treatment.

Conclusion and Future Implications

  • The research concludes that multi-drug resistance is becoming the norm in managed cyathostomin populations globally. It also indicates that a similar trend may soon be seen in Parascaris spp.
  • The paper urges the necessity to better comprehend the reasons behind the observed shortened ERPs. It accentuates the need for constant monitoring of anthelmintic efficacy against equine nematodes. Researchers and veterinarians globally are encouraged to partake in these efforts.

Cite This Article

APA
Nielsen MK. (2022). Anthelmintic resistance in equine nematodes: Current status and emerging trends. Int J Parasitol Drugs Drug Resist, 20, 76-88. https://doi.org/10.1016/j.ijpddr.2022.10.005

Publication

International journal for parasitology. Drugs and drug resistance
ISSN: 2211-3207
NlmUniqueID: 101576715
Country: Netherlands
Language: English
Volume: 20
Pages: 76-88

Researcher Affiliations

Nielsen, M K
  • M.H. Gluck Equine Research Center, Department of Veterinary Science, University of Kentucky, Lexington, USA. Electronic address: martin.nielsen@uky.edu.

MeSH Terms

  • Horses
  • Animals
  • Horse Diseases / drug therapy
  • Anthelmintics / pharmacology
  • Anthelmintics / therapeutic use
  • Fenbendazole
  • Macrolides / therapeutic use
  • Ivermectin / therapeutic use
  • Ascaridoidea
  • Drug Resistance
  • Parasite Egg Count
  • Feces

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