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Parasitology research2008; 103(1); 209-215; doi: 10.1007/s00436-008-0959-7

Field studies indicating reduced activity of ivermectin on small strongyles in horses on a farm in Central Kentucky.

Abstract: Field studies (n=6) were completed on evaluation of activity of ivermectin (200 microg/kg) paste formulation against small strongyles in horses (foals, yearlings, and older animals) on a farm (Farm MC) in Central Kentucky in late 2006 and during 2007. A girth tape was used to estimate body weights which were then used to calculate the proper dose rate of ivermectin. The foals, yearlings, and some of the older horses were born and raised on the farm. However, most of the older horses which were not raised on the farm had been there for several years. The horse herd was given ivermectin exclusively, usually four times a year, since 1990. An exception was that during the foal's period of life fenbendazole, pyrantel pamoate, and oxibendazole were given occasionally besides ivermectin. Efficacy of drug activity was determined by pretreatment and posttreatment counts of strongyle eggs per gram of feces (EPGs). Culture of strongyle eggs in feces from some of the horses showed that only small strongyle larvae were present. The research included two studies (A and B) in foals (n=24) and four studies (C, D, E, and F) in yearlings (n=13) alone or with older horses (n=10). For each of the studies (B through F), there was a treated and a nontreated group. These groups were switched for each treatment, i.e., the treated group in one study was the nontreated group in the next study and vice versa. Eggs per gram of feces counts were determined at 1- or 2-week posttreatment intervals for 4 weeks for study A and 6 weeks for studies B through F. Also, for studies B, E, and F, counts of EPGs were done either two or three times during the third week posttreatment. The studies showed a similar posttreatment pattern of strongyle EPG counts beginning to return at about 4 weeks and increasing at 5 and 6 weeks posttreatment. Two horses in study E and one in study F had low EPG values toward the end of the third week posttreatment. The results of this ivermectin investigation showed that the strongyle EPG counts started returning about twice as quickly post-ivermectin-treatment of horses than when the drug was first marketed in the early 1980s.
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Publication Date: 2008-04-05 PubMed ID: 18389281DOI: 10.1007/s00436-008-0959-7Google 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.

The study evaluates the efficacy of ivermectin in treating small strongyles (a type of parasitic worm) in horses on a farm in Central Kentucky. The research observed that the effects of the drug diminished more quickly post-treatment compared to when ivermectin was first introduced in the 1980s.

Methodology

  • The study involved six field trials on a farm (designated Farm MC) in Central Kentucky in late 2006 and 2007.
  • The horses treated included foals, yearlings, and older horses, some of which were born and raised on the farm, while others had been there for several years.
  • The treatment consisted of a 200 microgram per kilogram dose of ivermectin paste, usually administered four times a year.
  • Exceptionally, during the foals’ youthful period, fenbendazole, pyrantel pamoate, and oxibendazole were used alongside ivermectin.
  • The effectiveness of the ivermectin was assessed through pre- and post-treatment counts of strongyle eggs per gram of feces (EPGs). Cultures of strongyle eggs confirmed the presence of only small strongyle larvae.

Findings of the Study

  • The study was segmented into two parts (Studies A and B) involving foals (24 in number) and four parts Studies C, D, E, and F) with yearlings, either alone (13 in number) or alongside older horses (10).
  • Each study involved a treated and non-treated group, and these were switched for subsequent treatments to minimize bias.
  • Egg counts (EPGs) occurred at one or two-week intervals – four weeks for Study A and six weeks for the others. In Studies B, E, and F, EPG counts were performed two or three times in the third week post-treatment.
  • Results indicated an increase in EPG counts beginning around four weeks post-treatment and increasing at five and six weeks. A few horses in Studies E and F showed low EPG values towards the end of the third week.

Conclusions

  • The research concluded that the effectiveness of ivermectin treatment on small strongyles showed signs of diminishing earlier than when first introduced in the early 1980s – the EPG counts began to rebound about twice as quickly compared to earlier records.
  • Though the reasons for these results were not stated, it may suggest some level of resistance development among the parasites or a reduction in drug potency over time.

Cite This Article

APA
Lyons ET, Tolliver SC, Ionita M, Lewellen A, Collins SS. (2008). Field studies indicating reduced activity of ivermectin on small strongyles in horses on a farm in Central Kentucky. Parasitol Res, 103(1), 209-215. https://doi.org/10.1007/s00436-008-0959-7

Publication

Parasitology research
ISSN: 0932-0113
NlmUniqueID: 8703571
Country: Germany
Language: English
Volume: 103
Issue: 1
Pages: 209-215

Researcher Affiliations

Lyons, E T
  • Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY 4056-0099, USA. elyons1@uky.edu
Tolliver, S C
    Ionita, M
      Lewellen, A
        Collins, S S

          MeSH Terms

          • Animals
          • Anthelmintics / pharmacology
          • Horse Diseases / drug therapy
          • Horse Diseases / parasitology
          • Horses
          • Ivermectin / pharmacology
          • Kentucky / epidemiology
          • Phylogeny
          • Strongyle Infections, Equine / drug therapy
          • Strongyle Infections, Equine / epidemiology
          • Strongyle Infections, Equine / parasitology
          • Strongylus / drug effects

          References

          This article includes 10 references
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          Citations

          This article has been cited 24 times.
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