Treatment efficacy of pyrantel, fenbendazole and macrocyclic lactones in equine strongyles in Germany using FECRT and the model eggCounts.

Overview

• This study evaluated how well three common deworming drugs—pyrantel, fenbendazole, and macrocyclic lactones—work against small strongyle parasites in horses in Germany.
• Using two different methods to measure drug effectiveness, the research found that while macrocyclic lactones remain very effective, fenbendazole shows significant resistance, and pyrantel has reduced but still generally good efficacy.

Background and Objective

• Small strongyles (cyathostomins) are common intestinal parasites of horses that can develop resistance to deworming medications.
• Resistance to anthelmintics (deworming drugs) is a growing issue globally, including in Germany, but recent data on the current status of drug efficacy against these parasites is limited.
• The study specifically aimed to assess the present-day effectiveness of three anthelmintics — pyrantel, fenbendazole, and macrocyclic lactones — in horses under field conditions in Germany.

Methods

• A total of 1670 horses were included, resulting in 1913 deworming events analyzed.
• Treatments were distributed as follows:
  • 1682 with pyrantel
  • 56 with fenbendazole
  • 162 with macrocyclic lactones
  • 13 with other substances like herbs
• Effectiveness was measured by the Faecal Egg Count Reduction (FECR), which evaluates the drop in parasite egg counts in feces before and after treatment.
• Two calculation methods were employed:
  • The traditional Faecal Egg Count Reduction Test (FECRT)
  • The statistical eggCounts model, which can provide confidence intervals and account for variability
• Interpretation of efficacy followed established guidelines by the American Association of Equine Practitioners (AAEP) and the World Association for the Advancement of Veterinary Parasitology (WAAVP).

Results

• Percent of treatments with effective FECR (>95% reduction in egg counts):
  • Pyrantel: 68.7% of treatments were fully effective
  • Fenbendazole: only 21.4% effective
  • Macrocyclic lactones: 98.1% effective, indicating high efficacy
• Resistance classification based on FECRT and eggCounts:
  • Pyrantel showed “low resistance”:
    • FECRT upper/lower confidence limits: 91.1% / 89.5%
    • eggCounts: 88.7% / 88.1%
  • Fenbendazole was classified as “resistant”:
    • FECRT: 65.0% / 49.1%
    • eggCounts: 50.4% / 42.9%
  • Macrocyclic lactones were classified as “susceptible” with FECRT (100.1% / 99.0%) and “low resistant” with eggCounts (99.8% / 99.5%), indicating they remain highly effective.

Conclusions and Implications

• Macrocyclic lactones still provide very effective control of small strongyles in horses in Germany.
• Fenbendazole shows widespread resistance and is largely ineffective for treating these parasites.
• Pyrantel has reduced effectiveness in about 31.3% of cases but remains mostly effective, supporting its continued use.
• Routine post-treatment monitoring (e.g., fecal egg counts 10-14 days after treatment) is recommended to detect reduced drug efficacy early and manage resistance.
• These findings support targeted anthelmintic use and regular monitoring to maintain effective parasite control and slow resistance development.