Anthelmintic efficacy on UK Thoroughbred stud farms.

Area of Science:

• Veterinary Parasitology
• Equine Health
• Anthelmintic Resistance

Background:

• Anthelmintic drugs have been used for over 40 years to control horse nematodes.
• Indiscriminate use may lead to reduced drug efficacy and resistance.

Purpose of the Study:

• To investigate the efficacy of four broad-spectrum anthelmintic drugs against horse nematodes.
• To assess potential emerging anthelmintic resistance in Thoroughbred stud farms.

Main Methods:

• Faecal egg count reduction tests (FECRT) were conducted on 16 Thoroughbred farms.
• 429 horses were treated with ivermectin, moxidectin, pyrantel, or fenbendazole.
• Egg reappearance periods (ERP) were monitored for up to 12 weeks post-treatment.

Main Results:

• Moxidectin was highly efficacious against strongyles (99.8-100% reduction).
• Reduced efficacy of pyrantel was observed in yearlings (0-73% reduction), but not in mares (98-99.4% reduction).
• Fenbendazole showed consistently low efficacy (0.4-41% reduction).
• Ivermectin efficacy varied (85.7-100% reduction), with reduced efficacy in one yearling group.
• Efficacy against Parascaris equorum varied, with fenbendazole showing acceptable results (97.5-99.9%) but ivermectin reduced efficacy (25.5-91.2%).
• ERP for moxidectin, ivermectin, and pyrantel were shorter than previously recorded.

Conclusions:

• Moxidectin and ivermectin demonstrated acceptable efficacy at 14 days post-treatment.
• Shortened ERP suggest reduced effectiveness and impending multi-drug resistance in strongyles.
• Strategic anthelmintic drug use is crucial to mitigate resistance development.