Efficacy of moxidectin against cyathostomins after long-term use in a large herd of draught horses with a high stocking density.
Authors: Schumacher J, Livesey L, DeGraves F, Blagburn B, Ziska S, Caldwell M, Brock K
Journal: The Veterinary record
Summary
# Editorial Summary Researchers investigated whether cyathostomin resistance to moxidectin had emerged in a large draught horse herd managed intensively on limited pasture, given the anthelmintic had been used routinely for five consecutive years. Using a longitudinal faecal egg count protocol, they assessed parasite burden immediately pre-treatment and at 7, 30, 60 and 90 days post-administration of moxidectin gel. Whilst egg counts showed significant suppression at days 7 and 30, counts returned to pre-treatment levels by days 60 and 90, yet this pattern was consistent with normal pharmacokinetics rather than genuine resistance. The findings suggest that although moxidectin's efficacy window is relatively short in this population, no anthelmintic resistance had developed despite five years of continuous use—though the rapid rebound in egg counts underscores the importance of strategic dosing intervals and integrated parasite management in high-stocking-density operations to prevent future resistance selection.
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Practical Takeaways
- •Moxidectin remains effective against cyathostomins even after 5 years of regular use in high-density grazing situations, with no resistance detected
- •The early reappearance of eggs (by 60-90 days) suggests relatively short duration of protection and may indicate need for frequent monitoring of parasite burden in intensively managed herds
- •High stocking density alone does not appear to accelerate resistance development to moxidectin, though continued vigilance through faecal egg count monitoring remains prudent
Key Findings
- •Faecal egg counts were significantly reduced at 7 and 30 days post-moxidectin treatment
- •Faecal egg counts returned to pretreatment levels by 60-90 days post-treatment
- •No evidence of moxidectin resistance developed after 5 years of routine use in the herd