In the fall of 2006 at weaning time, I noticed a problem - a problem that I had no explanation for: my weanlings had large roundworms! My foals had been dewormed from the time they were four weeks old. I started with Safeguard for the first deworming and then switched to an ivermectin product, and I was deworming every four weeks. Yet here I was looking at a weanling that had just pooped out a large roundworm. Even though they weren’t yet due to be dewormed again, I rounded up all the weanlings and hit them all with another dose of ivermectin. Then I watched and waited, and checked poo for worms for the next several days. I knew at least one foal had large roundworms yet only one or two manure piles showed evidence of dead worms. I had a niggling feeling in the back of my head that told me to deworm them again with another product, so a week after the ivermectin deworming, I hit them with Strongid. HOLY CRAP! Or should I say wormy crap… Every poo pile I checked for the next several days showed dead large roundworms.