Horse deworming strategies have been around for centuries. Some of these, like herbal remedies, were less effective, while others became the benchmark for horse keeping practices today. As we’ve gained more knowledge on how parasites and antiparasitic drugs work, though, it has become clear that just because something is the standard doesn’t mean it’s the best way to continue.
Past approaches to deworming have contributed to parasite resistance in worms, which means the usual deworming treatments might not work much longer. Quarter Horse News asked Dr. Jennifer Voellinger, an equine veterinarian based in Roanoke, Texas, how to best deworm horses today without contributing to resistance that could negatively affect the equine population of tomorrow.
Stop Constant Rotation
Historically, horse owners believed that the best protocol for deworming horses was to rotate between different deworming products every two months. The idea behind this was that by using a different type of dewormer, like ivermectin or fenbendazole, each deworming, you could target a broad spectrum of parasites.
This is no longer a great strategy, Voellinger said. One, it drives parasite resistance, which is the ability of parasites to survive treatment with drugs that were generally effective against them in the past. Once parasites develop this capability, it never goes away. Two, it’s unnecessary to deworm that often.
“There seems to be no correlation between deworming frequency and horse health; therefore, there doesn’t appear to be any negative effects to decreasing deworming frequency,” Voellinger said. “The goal is to keep parasites under control, not eradicate them in the horse, as that is impossible, anyway.”
Submit a Fecal Sample
As more became known about parasite drug resistance, veterinarians started recommending fecal egg counts instead. A fecal egg count is a quantitative assessment of how many and what kind of parasite eggs a horse is shedding at the time the test is taken. The test is performed on a manure sample collected from your horse.
If the horse is harboring worms, a fecal egg count will verify what type of worms they are, so you can then know which dewormer to use. In adult horses, the most common parasites are small strongyles, though they are also often plagued by roundworms, large strongyles, pinworms and tapeworms. Each of these requires the right dewormer to be effective.
“A fecal egg count will also help you find high shedding horses — the horses that tend to have the higher egg count,” Voellinger suggested. “These are the 20% of horses that are shedding 80% of the eggs onto the pasture. That way, we can target treat those horses more.”
Follow-Up on the Results
While fecal egg counts can be enlightening for determining which parasite eggs your horse might be shedding, they don’t tell you everything. For instance, you won’t know if your horse has worms (all horses have worms); how many worms it has; or if worms are affecting the horse’s health.
“Horses are supposed to have worms all the time,” Voellinger said. “Adult worms really don’t cause any problems in the horse. It’s the larvae that cause tissue damage, but they don’t create eggs like the adult worms. Therefore, there is no correlation between egg counts and worm counts in the horse.”
A fecal egg count will also vary depending on when you take the sample. A follow-up test should be performed two weeks after the first egg count is done to see if there are any changes.
Treat Young Horses Differently
According to the American Association of Equine Practitioners, fecal egg counts are not advised to be used as the only method for determining deworming schedules in horses under 3. Instead, the organization recommends foals receive a minimum of four anthelmintic treatments in their first year. The first dosage, at two months, should be a drug that targets ascarids, or roundworms, as foals have a high incidence of these parasites. Further treatments can be administered at five months, nine months and 12 months.
During weaning, fecal egg counts can be used to determine whether the horse has more ascarid or strongyle-type worms, and then the correct dewormer can be given accordingly. At nine months, treatment should target tapeworms and strongyles, then strongyles again when the horse reaches a year in age. From that point on, horses under three years of age should receive about three yearly treatments; once they turn three, using fecal egg counts can become standard.
Follow Your Vet’s Recommendations
In light of the most recent research, “it is no longer advised to go to the local feed store and buy a pack of dewormers for the year,” Voellinger said. In fact, if your horse isn’t shedding high volumes of parasite eggs, it might be able to go for a period of time before needing treatment. While that seems backwards according to the old school rotational deworming methods, it’s an important step in preventing parasitic drug resistance.
Deworming protocols will vary on depending on your horse, your property and other environmental factors, Voellinger added. Working with your veterinarian to develop the right procedure is always the best plan of action.
The American Association of Equine Practitioners (AAEP) also has released guidelines with additional information about controlling internal parasites.