The American Association of Equine Practitioners is aware of the problem of the worms developing resistance to the de- wormers we currently have access to. One thing you can do to prevent further development of resistance to the de wormers we are currently using is to make sure to give your horse a dose that is adequate for his weight. If you dose your horse for less than his weight, you will make all the worms sick, and kill NONE of them. They will develop resistance to the de- wormer. Many states are successfully incorporating a system of only de-worming the horses that are shedders of parasite eggs. It seems that only a small portion of the horses seem to be carrying most of the parasites. We call these horses “shedders”. If we only de-worm the “shedders”, theoretically we will decrease the chance of resistance developing. The way this procedure is implemented is by testing the horses manure for the presence of parasite eggs before the deworming and again about 2/3 of the way through the manufacturer’s recommendation for the duration of action of the de- wormer. The horses should have no or a low number of parasite eggs in the feces at this point. Those who have high egg counts are the ones we want to concentrate on keeping on a more frequent de- worming schedule. Those who do not have very many eggs can be de- wormed less frequently. If your horse does show a high egg count after a certain type of de-wormer, you can try using a different ingredient (don’t just buy a different name brand of de- wormer, make sure you are using a different ingredient). It may be that the parasites at your farm have developed resistance to the particular de-wormer ingredient that you are using. Rotation of dewormers will help prevent resistance from developing.

I have not had great success using the fecal test to determine which horses to de- worm and which ones to not de- worm here in Texas. Perhaps this method will work if the horses are not exposed to any pasture. Most of my clients have horses that graze on some type of pasture, and the ones who do not have parasites in their feces can still develop L4 larvae at their anterior mesenteric artery when they are not de- wormed for an extended period of time. The L4 larvae can be present in large numbers with a fecal egg count (FEC) of zero. We have also found that if we check the feces in the clinic we find many more parasite eggs than the lab does if we send the fecals out. It seems that the eggs are very fragile and seem to disintegrate before they reach the lab, but if the feces are checked while it is still very fresh, more eggs are visualized.

In this part of central Texas, if your horses are on any pasture at all, it is my recommendation to de-worm them every 2-3 months with alternating de- wormers.

Parasite load appears to be more linked with the horse’s strength of immunity than anything else. The best way to protect your horse from worms is to keep his immune system very strong. See other articles in the website for how to build a healthy immune system in your horse.

We have an example deworming schedule below:

  • Jan/Feb: Double Dose, (usually 2 tubes for a 1,000lb horse) of Pyrantel Pamoate (Strongid)
  • Mar/April: Oxibendazole or Fenbendazole (Anthelcide EQ or Panacur)
  • May/June: Ivermectin
  • July/Aug: Quest Plus (moxidectin and praziquantel)
  • Sept/Oct: Oxibendazole or Fenbendazole (Anthelcide EQ or Panacur)
  • Nov/Dec: Praziquantel Combination (Equimax or Quest Plus)

One of my clients found that the Valley Vet “Premium One-Year Wormer Pak” had all of the necessary dewormers in it if you buy one extra tube of Quest Plus, at a very reasonable cost.