Management of drug-resistant cyathostominosis on a breeding farm in central North Carolina.
Authors: Little D, Flowers J R, Hammerberg B H, Gardner S Y
Journal: Equine veterinary journal
Summary
# Management of Drug-Resistant Cyathostominosis: An Alternative Strategy for Breeding Operations A North Carolina breeding farm documented resistance to both fenbendazole and pyrantel in cyathostomins after nine years of rapid-rotation anthelmintic protocols, necessitating a fundamental shift in parasite management strategy. Rather than continuing conventional dosing schedules, the farm implemented targeted ivermectin treatment based on faecal egg count thresholds—administering to mares shedding >200 eggs/g and young stock >100 eggs/g—whilst monitoring changes in resistance patterns and drug efficacy over 30 months. The targeted approach achieved acceptable control in broodmares and substantially reduced patent infections across successive foal crops, with treatment frequency dropping by 78 per cent in mares and 533 per cent in foals, despite evidence that egg reappearance time shortened during the second year of the study. Whilst this selective treatment model proved economically viable and clinically effective in this population, the authors emphasise that ivermectin efficacy requires close monitoring, as the declining egg reappearance time suggests the potential for resistance development with continued use. For equine professionals managing breeding herds with documented multidrug resistance, this work demonstrates that strategic, threshold-based treatment can reduce anthelmintic burden whilst maintaining herd health—though practitioners should implement regular faecal monitoring and remain alert to emerging resistance patterns.
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Practical Takeaways
- •Rapid rotation anthelmintic protocols do not prevent resistance development; monitor for resistance using faecal worm egg count reduction tests
- •Targeted ivermectin treatment based on faecal egg count thresholds (>200 epg in adults, >100 epg in foals <2 years) can reduce treatment frequency while managing drug-resistant strongyles
- •Ivermectin efficacy must be monitored closely as egg reappearance times may shorten over time, indicating emerging resistance to this drug class
Key Findings
- •Cyathostomins resistant to both fenbendazole and pyrantel were documented on a breeding farm using rapid rotation anthelmintic protocols for 9 years
- •Targeted ivermectin treatment based on faecal egg count thresholds achieved acceptable control in mares and improved control in foal crops over 30 months
- •Egg reappearance time after ivermectin treatment was <8 weeks more frequently in year 2 than year 1, suggesting emerging resistance
- •Anthelmintic treatment frequency was reduced by 77.6% in mares and 533% in foals compared to previous rotation protocols