Undergraduate Honors Thesis Projects

Date of Award

Spring 2026

Document Type

Honors Paper

Degree Name

Equine Pre-Veterinary/Pre-Graduate Studies-BS

Department

Equine Science

Advisor

Dr. Steffanie Burk

First Committee Member

Dr. Steffanie Burk

Second Committee Member

Dr. Sheri Birmingham

Third Committee Member

Dr. Cynthia Laurie-Rose

Keywords

small strongyle, equine, anthelmintic, resistance

Subject Categories

Animal Sciences | Higher Education | Other Animal Sciences

Abstract

Cyathostomins, commonly referred to as small strongyles, are among the most prevalent gastrointestinal parasites affecting horses worldwide. Control of these parasites has historically relied on the use of anthelmintic drugs; however, widespread resistance has developed in many cyathostomin populations, particularly against benzimidazoles and tetrahydropyridines. Monitoring anthelmintic efficiency through fecal egg count reduction tests (FECRT) and evaluating egg reappearance periods (ERP) are essential tools for identifying resistance and guiding sustainable parasite control strategies. The objective of this study was to evaluate the efficacy and ERP of two commonly used equine anthelmintics, fenbendazole and moxidectin, in horses in Central Ohio. A total of 57 horses meeting inclusion criteria were enrolled and randomly assigned to one of three groups: control (n=20), fenbendazole treatment (n=19), or moxidectin treatment (n=18). Horses were required to have a high egg count of ≥200 eggs per gram (EPG) of feces and no history of deworming within the previous eight weeks. All three groups had fecal samples collected on day 0 prior to treatment, day 1, and week 1. Subsequently, both treatment groups had fecal samples collected for 11 more weeks following treatment, totaling 12 weeks. Fecal egg counts (FEC) were performed using the Modified Stoll Method, and FECRT values calculated according to the American Association of Equine Practitioners (AAEP) guidelines. Fenbendazole treatment resulted in minimal and inconsistent reductions in FEC, with mean FECRT values ranging from –27.46% to 42.69%. The upper confidence intervals (CI) never exceeded the 80% efficacy threshold for benzimidazoles, indicating treatment failure. Consequently, the ERP for fenbendazole in this study was determined to be 0 weeks, suggesting substantial resistance within the parasite populations. In contrast, moxidectin produced rapid and sustained suppression of egg shedding, with FECR values exceeding 99% through week 9 and remaining above the 90% efficacy threshold for macrocyclic lactones throughout the 12-week observational period. The ERP for moxidectin was therefore greater than 12 weeks. These findings indicate severe resistance to fenbendazole in the studied horse sample, while moxidectin remained highly effective. Continued monitoring of anthelmintic efficacy using FECRT and ERP analysis is essential for identifying emerging resistance and supporting evidence-based parasite control programs in equine management.

Licensing Permission

Copyright, all rights reserved. Fair Use

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Acknowledgement 2

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