Three-year evaluation of anthelmintic treatment efficacy against equine strongyles in France using FECRT.

Overview

• This study assessed the effectiveness of four anthelmintic drugs against equine strongyles in France over three years using fecal egg count reduction tests (FECRTs).
• It revealed widespread resistance to all tested anthelmintics, highlighting the urgent need for improved parasite management strategies.

Background and Purpose

• Strongyles are parasitic worms commonly infecting horses, controlled primarily via anthelmintic (AH) drugs.
• Resistance to these drugs is a growing global concern, threatening effective parasite control.
• Despite this, current data on drug efficacy and resistance in France were limited prior to this study.
• The researchers aimed to evaluate the long-term efficacy of four commonly used AH drugs in France: fenbendazole, pyrantel, ivermectin, and moxidectin.
• They used the fecal egg count reduction test (FECRT), a standard method to assess drug efficacy by comparing parasite egg shedding before and after treatment.

Methods

• The study took place over three years across 70 equine facilities in France.
• They performed 104 complete FECRTs on groups comprising 749 horses.
• Facilities included breeding farms, riding schools, experimental research platforms, and even a zoological park.
• Resistance was defined based on insufficient reduction in fecal egg counts after treatment with each AH drug.

Key Findings

• Fenbendazole resistance:
  • Detected in 8 out of 9 tested groups (89%).
  • Found in 7 out of 8 facilities (88%) where fenbendazole was tested.
  • High upper confidence intervals suggest widespread prevalence.
• Pyrantel resistance:
  • Identified in 15 out of 22 groups (68%).
  • Present in 12 out of 17 facilities (71%) using pyrantel.
• Ivermectin resistance:
  • Found in 14 out of 51 groups (27%).
  • Detected in 11 out of 41 facilities (27%).
  • Although the prevalence is lower compared to other drugs, the upper confidence intervals indicate potential underestimation.
• Moxidectin resistance:
  • Identified in 5 out of 22 groups (23%).
  • Found in 4 out of 18 facilities (22%).

Geographical and Facility-specific Patterns

• Resistance was mainly observed in Normandy, a key equine breeding region.
• Breeding farms had the greatest incidence of resistant populations (25 out of 70 facilities).
• Riding schools that offer boarding (3 facilities), research experimental platforms (2 facilities), and even a zoological park showed some resistance, indicating spread across diverse settings.

Conclusions and Implications

• The study provides the first comprehensive field evidence in France of resistance to all four major AH drug classes used against equine strongyles.
• This widespread resistance poses a significant challenge to parasite control in horses.
• It highlights the critical need for changes in regulatory policies to promote:
  • Diagnostic-based treatments such as targeted selective treatment guided by fecal egg counts, rather than routine blanket deworming.
  • Strategies that reduce selective pressure on parasite populations, which drive the development of resistance.
• Ultimately, adopting evidence-based treatment protocols could help prolong the efficacy of existing drugs and sustain equine health.