Treating equine parasites is more difficult these days due to many parasites being resistant to various dewormers. The most common parasite in adult horses at this point is the small strongyle. The most common parasite in foals is the roundworm. A study was recently performed by the staff at Kansas State to determine the amount of resistance of these parasites to the dewormer fenbendazole that is sold under the trade names of Panacur and Safeguard. And yes, that is the same fenbendazole that some are using for cancer treatment in humans. Fecal samples were taken to check the number of parasite eggs before deworming. After deworming, a second fecal egg count showed an average strongyle egg reduction of 71.8%. However, this average varied widely between farms, with some locations experiencing only 40% reduction while others had a total elimination of strongyle eggs following deworming.
Fenbendazole was more effective in treating foals with roundworms as the reduction of eggs after deworming was over 98 percent. So fenbendazole is effective at treating most roundworms in foals but its effectiveness at treating small strongyles in adult horses, even at a double dose for five days in a row, is questionable. For this reason, it is important to have your veterinarian check your horse’s feces for parasite eggs before and after deworming. You could be using a product that is ineffective, which costs you money and can affect the health of your horses. Certainly giving a one-time dose of fenbendazole to an adult horse for small strongyles is likely to be ineffective as these horses were given a double dose for five days with only a moderate response.