Shortened egg reappearance after ivermectin or moxidectin use in horses in the UK.
Authors: Daniels S P, Proudman C J
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary Daniels and Proudman (2016) investigated whether macrocyclic lactones retain their expected efficacy in UK equine populations by tracking egg reappearance periods (ERP) following ivermectin or moxidectin treatment in horses with persistently positive faecal egg counts—a marker of potential anthelmintic resistance. Across a high-risk pleasure horse population, they documented substantially shortened ERPs compared to historical data: ivermectin-treated premises showed reappearance within 5–6 weeks (versus the expected 8–10 weeks), whilst moxidectin-treated premises ranged from 10–12 weeks (versus the expected >13 weeks), with the earliest reappearance recorded at just 5 weeks post-treatment. These findings suggest genuine shifts in parasite population dynamics rather than sporadic treatment failures, indicating that reliance on standard dosing intervals may leave horses underprotected for longer than practitioners currently assume. The practical implication is substantial: farriers, vets and yard managers should consider more frequent faecal egg monitoring and potentially shortened treatment intervals in high-risk populations, alongside reviewing grazing management and pasture contamination as complementary control strategies rather than assuming macrocyclic lactones provide the traditional protection window.
Read the full abstract on PubMed
Practical Takeaways
- •Ivermectin and moxidectin egg reappearance periods are shorter than historically expected, requiring more frequent faecal monitoring and potentially more frequent dosing intervals to maintain strongyle control
- •Individual horse variation in ERP is significant; monitor faecal egg counts regularly rather than relying on standard retreatment intervals to detect early egg reappearance in your horses
- •Consider anthelmintic resistance as a likely factor if you observe positive faecal egg counts sooner than 8-10 weeks post-treatment; discuss alternative parasite control strategies with your veterinarian
Key Findings
- •Earliest egg reappearance period (ERP) after ivermectin or moxidectin was 5 weeks, substantially shorter than previously published values of 8-10 weeks (ivermectin) and >13 weeks (moxidectin)
- •Moxidectin showed ERP ≥12 weeks in 5 of 16 premises studied, but only ≥10 weeks when single-horse premises were grouped (n=61)
- •Ivermectin demonstrated ERP ≥5 weeks across premises, with ≥6 weeks in single-horse premises (n=31)
- •Field data from UK pleasure horses at high strongyle infection risk demonstrate shortened ERPs compared to drug marketing historical data, suggesting potential anthelmintic resistance or reduced efficacy