Intestinal parasites can be a major problem in horses and hopefully all of you with horses are working with your vet to set up an effective deworming program. However, there is a major problem at this point in that many parasites are becoming resistant to the dewormers that are available. In the past, parasites called large strongyles were the major problems in horses but these seem to be effectively controlled. The small strongyles are a different story as many of these parasites have become resistant to a major class of dewormers called benzimidazoles. An example of this class of dewormer is fenbendazole that is sold under the trade name Panacur or Safeguard. A recent study indicated that the use of only Safeguard on a herd of horses for just 18 months led to resistance of the small strongyles to the drug. And the parasites in young horses were more resistant than those in older horses.
So rotation of dewormers may be most effective. This is nothing new as most people usually rotate between deworming products. However, the question is when to rotate and will this be effective in parasites that are resistant? To determine if rotation was effective, the same horses that were resistant to safeguard were dewormed every 90 days with a rotation of ivermectin, Strongid, a high dose of Safeguard, and Quest. Results indicated that by rotating the dewormers, all of the wormers were effective, even safeguard that was previously resistant in this group of horses. Instead of just buying dewormers at the feed store, consult with your vet for their recommendations on controlling parasites in your horses.