Imagine that every fifth Monday, when you arrive at work, your boss hands everyone an antibiotic to ingest. “Just in case one of you is ill, I’d like everyone to take this,” she says. Would you be happy to oblige, or would you be a bit hesitant to take pills every few weeks, just in case someone in your office might be sick? I’m guessing that most people would be reluctant to take drugs when they felt just fine. We’ve all heard about the dangers of antibiotic-resistant bacteria, and most of us are justifiably concerned about using antibiotics unnecessarily. We wouldn’t randomly administer antibiotics without suspecting that a person is sick, so why would we randomly administer dewormers to our horses, just in case one might be affected by intestinal parasites? Unfortunately, this is exactly what we’ve been doing for years in the horse industry. To make matters worse, for decades we’ve been randomly administering a different dewormer to our horses every few weeks, simply hoping that one of them will be effective. “Rotation deworming” seemed like a good idea when it was introduced in 1966, but forty years later, we’ve realized the error of our ways.
Deworming of horses is primarily geared toward the elimination of strongyles, the most common and important intestinal parasite of the species. There are two types of strongyles, large strongyles and small strongyles. Large strongyles are much more susceptible to dewomers, and have become less of a problem than small strongyles. Small strongyles (cyathostomes) are ingested from pasture, travel into the intestine, penetrate the wall of the intestine and then develop in the intestinal wall for a period of time. They finally emerge from the intestinal wall and mature into adult worms in the intestine. There, they shed eggs into the manure, which hatch on pasture and develop into new infective parasites. Under some circumstances, the parasites may stay within the wall of the intestine for a prolonged period of time, becoming what is called “encysted small strongyles.” Other types of intestinal parasites, such as tapeworms, bots, pinworms and ascarids, are important, but we’ll discuss that in a future article.
Just like bacteria have gradually become resistant to antibiotics, equine small strongyles have become resistant to commonly used dewormers. Over-use of these products has allowed only the “strongest and smartest” of the parasites to survive, leaving “super-bug” type worms to infect our horses. A similar phenomenon has been observed in goats, and the problem has become quite severe. Dewormer resistance is so common in goat parasites that we now recommend deworming goats only when it is absolutely necessary. Each goat is observed for signs of anemia (pale eyes and gums), and dewormed only when significant anemia (from blood-sucking parasites) has set in. Can you imagine waiting until your jumper, barrel horse or best buddy becomes pale and lethargic before deworming him?
So if rotation deworming is a bad idea, what’s the alternative? Veterinarians now recommend identifying horses with active parasite infections, and deworming only those that need treatment. A simple fecal egg-count can be performed at a veterinarian’s office, and will determine each horse’s individual parasite load. The fecal egg-count (a McMaster’s count) identifies eggs that are shed into the manure by adult worms living in the intestine. Interestingly, horses seem to be genetically susceptible or resistant to intestinal parasites. On any given farm, about 20% of the horses will shed 80% of the parasite eggs (indicating that they harbor 80% of the adult worms). Some horses will be almost completely resistant to parasites (in my experience, some Morgan horses appear to be particularly hardy), and some will consistently have very high fecal egg counts, indicating that they’re very susceptible to infection. Most horses will fall somewhere in the middle. Veterinarians classify horses into “high,” “medium,” or “low-shedder” categories based on serial fecal egg counts. Once a horse is placed into an appropriate category, recommendations for follow-up fecals and management can be made.
When performing fecal egg counts, the number of parasite eggs are counted and multiplied to give a number of “eggs per gram of feces.” Cut-off values (usually between 200 and 500 eggs per gram) are assigned based on farm and management practices, and all horses with a fecal egg count over the cut-off value are dewormed. Horses under the cut-off value are not dewormed, since their parasite burden is low enough to cause no harm to their body. The horses that are not dewormed also serve as a source of “good, weak parasites” for the pasture. This ensures that the majority of parasites on a farm are not “super bugs” and will remain susceptible to dewormers. We call this population “refugia.” Refugia is the exact opposite of selective breeding for the biggest, strongest horses. We’re selecting for the weakest, puniest parasites.
So which dewormers do we use when a horse has a high fecal egg count? Unfortunately, our options are dwindling fast. The longer a dewormer has been around, the more “wise” to it strongyles have become. It appears that up to 98% of farms in the US have parasite populations that are resistant to dewormers of the benzimidazole class (fenbendazole, oxybendazole, “Panacur,” “Safe Guard”) and a large portion of farms have worms that are resistant to pyrantel (“Strongid”). In addition, evidence is building that resistance to Ivermectin (Zimectrin) is becoming a reality, and resistance to Moxidectin (Quest) is on the horizon. No new dewomers are anticipated to hit the market in the next 10 years, so judicious use of currently available products is imperative.
In addition to identifying “high-shedding” horses, your veterinarian can also help you determine which dewormers are effective against the parasites on your particular farm. If there’s concern about a dewormer’s effectiveness on your farm, a fecal-egg-count-reduction test should be performed on a few horses. This test involves measuring the fecal egg count following deworming, to ensure that the worms have been killed by the product you purchased, and resistance isn’t occurring.
So what do we do about all this “doom and gloom?” As horse owners, it’s important that we step up and make an effort to do the right thing for our horses. Rotation deworming is a thing of the past; it’s time to start having fecal egg counts performed and only using products that are effective. Working closely with your veterinarian is the best way to accomplish this goal and ensure that our horses are fit and healthy for generations to come. Lucky low-shedding horses may only need deworming once yearly with an appropriate product. Heavy shedders may need deworming every 3-4 months. Your veterinarian can also help you implement farm management practices that reduce pasture contamination and parasite spread.
For those that are concerned about the cost of implementing a reactive, modern parasite control program on their farm, I encourage you to consider not only the cost, but the future of the horse. Most farms will break nearly even or spend a few dollars more to develop an effective parasite control regimen. Fecal egg counts from Foundation Equine are $25 each. The average horse will need around $60- $90 worth of dewormers per year if rotation is used, or $50 to $75 worth of fecal egg counts (plus one or more $10 dewormer) on a reactive program (depending on whether the horse is a high or low shedder). It’s pretty much the same cost to do the RIGHT thing!