In our October 2008 issue, we told you about studies that had found small strongyle fecal egg counts can become positive again in as short a period as 30 days after ivermectin treatment. This likely represents the emergence of strains with ultra-short life cycles that will flourish on the more traditional eight-week deworming timing.
Now a study from Auburn University has found the same is true for moxidectin (Quest). A herd of draft horses that had been routinely dewormed with moxidectin for five years was checked for eggs at 0, 7, 30, 60 and 90 days after deworming. Egg counts were reduced at 7 and 30 days, but not 60 and 90.
Bottom Line:Small strongyles with very short life cycles may now be a widespread problem.
If you’re deworming at intervals longer than 30 days, regardless of the drug, you should be checking a fecal at the 6- or 8-week mark to make sure you actually have small strongyles controlled. This is particularly important with very young or very old horses, and those being kept under crowded conditions.