Farm-Wise Equine Parasite Control Strategies

When you’ve got a barn full of horses with different parasite burdens, fecal egg count testing can help you deworm each one properly

Change can be hard, especially for horse owners. We follow the seemingly indisputable rules of horse care and management that have been handed down to us from generations past. And we can be taken aback when someone dares question our decades-old practices.

One tradition that must be bucked, however, is deworming horses at regular intervals. Blanket parasite control strategies have led to widespread drug resistance in many parasite populations. And if farm and boarding barn owners don’t transition to a more targeted approach, then the resistance of all parasite populations to all drugs on the market could very soon become a reality.

Each horse has his own individual needs when it comes to parasite control. But when you’re responsible for deworming a number of horses on one property, how do you meet each one’s needs? The answer lies in collecting a fecal sample prior to deworming and calculating the number of parasite eggs within it.

“There’s really no way to build a parasite control program without using fecal egg counts (FEC),” says Martin Nielsen, DVM, PhD, Dipl. ACVM, associate professor of parasitology at the University of Kentucky’s Maxwell H. Gluck Equine Research Center, in Lexington. Nielsen is one of the foremost experts in the field of equine parasitology and chair of the American Association of Equine Practitioners (AAEP) Parasite Control Guidelines (aaep.org/parasite-control-guidelines) committee.

Horse owners that don’t use egg counts can have a false sense of security; they might assume the anthelmintic (deworming) products they are using are working when, in reality, they have no way of knowing. “I feel like I spend all my time advocating for this, but these habits are slow to change,” he says.

Fecal Egg Count Cheat Sheet

John Haffner, DVM, associate professor at Middle Tennessee State University’s School of Agribusiness and ­Agriscience, in Murfreesboro, oversees the deworming program for the university’s 50-horse herd. “When I started practice years ago, we dewormed every horse as often as we could,” he says. “Some horses got wormed monthly. It may have been necessary back then, but with the advent of (the anthelmintic) ivermectin and the decline of large strongyles, it doesn’t make sense to deworm horses like that anymore. All horses aren’t suffering from heavy parasite burdens.”

“It’s not just that you might be spending money on a drug that isn’t working, but with a (parasite) population that is resistant to that class of drug, you can actually increase the intensity of resistance in that population by using a drug that doesn’t work anymore,” says Craig Reinemeyer, DVM, PhD, president of East Tennessee Clinical Research Inc., in Rockwood, and another leading equine parasitology expert.

So, with these veterinarians’ help, let’s find out how to craft an appropriate deworming program for every horse in your barn.

It’s All About Resistance

Horse owners currently use three drug classes to fight parasites: benzimidazoles (e.g., fenbendazole and oxibendazole), pyrimidines (pyrantel salts such as pyrantel pamoate and pyrantel tartrate), and macrocyclic lactones (e.g., ivermectin and moxidectin alone or combined with praziquantel).

These drugs’ efficacy, which Reinemeyer says was at least 95% when they were first approved, has changed over the past few decades.

“There’s no big broad-spectrum, ­umbrella-type product anymore that we can just give and know it gets everything in the horse,” says Nielsen. “There’s also not any product that we can just discard and kick out and never use anymore. Each of the products currently available has some resistance issues in some equine parasites, but each of them still has a use for some parasites infecting horses.”

For instance, Nielsen says, small strongyles (cyathostomins, the parasite of most concern in adult horses) show widespread resistance against all benzimidazoles and pyrantel salts and the beginnings of resistance against the macrocyclic lactones. In fact, he says he’d be surprised to find a farm that doesn’t have resistance issues against those first two drug classes.

In many places, moxidectin and ivermectin only suppress strongyle egg production (by paralyzing and killing internal parasites) for three to five weeks before counts return to pretreatment levels, he says. This is in contrast to the typical egg reappearance periods we used to see of three months for moxidectin and six to eight weeks for ivermectin. 

Reinemeyer adds that veterinarians are finding ivermectin resistance in most ascarid (roundworm, which is prevalent in foals) and some pinworm populations in the United States. And we might also start seeing more widespread ascarid resistance to the pyrantel salts, he says.

These resistance issues concern Nielsen because they’re only likely to increase in all drug classes. “Even the cheapest dewormer becomes expensive if it doesn’t work at all,” he says. And wasted money is one thing; the health risk that you’re running with your horses is another.

While Nielsen and others have been working to develop new anthelmintic products (read about these at TheHorse.com/36473), they’re not out of the research phase yet.

Developing a Strategy

The goal of any parasite control program, says Nielsen, is to reduce the level of egg shedding in your herd to, in turn, reduce pasture contamination and parasite infection. Before developing a deworming strategy for your farm, consider your area’s active grazing season and the age of each horse.

Then measure each horse’s fecal egg count to determine his shedding status. Low shedders pass no more than 200 eggs per gram of feces; moderate ­shedders pass 200-500 eggs per gram; and high shedders pass 500 or more eggs per gram, says Reinemeyer. This is crucial to know, he explains, because only 20% of your horse population passes about 80% of all parasite eggs on your property. Once you’ve tested a mature horse several times to determine his shedding status, his classification is unlikely to change.

Elizabeth Houtsma and her husband, Greg Houtsma, DVM, own Hillside H Ranch, a small Warmblood breeding farm in Warrensburg, Missouri, as well as Midwest Performance Equine and Warrensburg Animal Hospital. They switched from a traditional rotational dewormer schedule and began using FEC testing in 2005 due to parasite resistance concerns.

“Because of our fecal egg count testing, we’ve been able to determine which horses are high shedders and which horses are not,” says Elizabeth Houtsma, adding that she and her husband test any horse new to their property before deworming, after deworming, and periodically through the first year to determine its shedding level. They only test longtime residents once a year or every other year, unless they suspect a ­problem. They test horses under 2 and seniors more frequently.

For adult horses in your herd, perform FECs during your region’s active grazing period—typically once in the spring and again in the fall after worms have ­accumulated all year—and deworm as needed.

Juvenile horses require an alternate approach, says Nielsen, because foals, yearlings, and young horses have different parasites and higher burdens and are more susceptible to disease. “It’s like two different species of animals,” he says, adding that a young horse’s age is particularly important; as his immune system develops, it’s better equipped to eradicate parasites.

Around four to six months of age, horses’ immune systems develop to a point they’re able to kick out the ascarids; this is when strongyles start increasing in number, says Nielsen. Fecal egg counts at weaning time can help reveal if a young horse has reached this point, and what type of parasites he has, so you can administer the appropriate dewormer.

Horses that are being transported, competing, or in training tend to have higher egg counts, but not necessarily due to higher parasite burdens, says Nielsen. More likely, these horses have weaker immune responses due to stress, which allows the parasites they already have to produce more eggs. Therefore, run FECs on any transient horse or horse in training at least twice a year. If one becomes a high shedder, you can change his treatment regimen.

“It’s just nice to make sure you get those horses treated so they’re not shedding and contaminating the environment everywhere they go and after they come home,” says Nielsen.

Older horses and those with pituitary pars intermedia dysfunction (PPID or equine Cushing’s disease), equine metabolic syndrome, or that are immunocompromised also have special parasite control needs. Researchers have shown that horses over 20 are significantly more likely to have high egg counts than middle-aged horses. Elizabeth Houstma says she’s seen this in her own population of mares: The geriatric ones are typically high ­shedders.

Nielsen says these horses might need an additional treatment a month to six weeks after the initial one because eggs might start reappearing sooner.

Deworming Tips

As we’ve discussed, a horse’s shedding status dictates how frequently he should be dewormed. However, our sources say adult horses, regardless of fecal egg count, should be dewormed once or twice a year, usually in the spring and fall, to help eradicate small strongyles, tapeworms, and sometimes pinworms—the three worm species of most concern for adult horses.

If you experience winter where you live, you won’t need to deworm during that season because the cold prevents parasites from developing into an infective stage, says Nielsen.

Because FECs only identify small strongyle and ascarid burdens, owners should administer a dewormer against tapeworms (e.g., praziquantel or pyrantel pamoate) at least once a year to control them. If you notice bot eggs on horses’ legs or around their manes, you can administer an effective drug class (ivermectin and moxidectin) to help prevent the larvae from making their way into the horse’s mouth and gastrointestinal tract. 

Testing Your Anthelmintics

To evaluate the efficacy of a drug class on your property, also be sure to perform fecal egg count reduction tests (there’s an acronym for that, too: FECRT). Have your veterinarian perform an FEC on each horse before treatment, deworm that horse as necessary, test a subsequent fecal sample from the same animal roughly 10 to 14 days later, and compare the two.

“You need to do it for every group of horses or class of anthelmintic that you use on the farm,” says Reinemeyer. “You should get at least a 98% reduction of egg counts post-treatment with ivermectin and moxidectin and at least 90% with the other dewormers. If you’re not getting that now, then it means that the worms have changed. If you demonstrate that a particular class is worthless on your farm, you should never use it again,” for routine deworming. That drug, however, might still be useful against other parasites.

For those drug classes that are effective, recheck them about every two years, he adds.

Not all horses need an FECRT. You can determine a product’s efficacy by testing about six horses in a herd that are known to have moderate to high egg counts, says Nielsen. If results come back showing any positives, the parasites on your property are resistant to that product.

Take-Home Message

Thanks to current widespread drug resistance in many parasite populations, it’s important for horse owners to understand that each horse has individual deworming needs. Those needs depend on the horse’s parasite shedding status and the farm’s drug resistance status, both of which can be determined using fecal egg counts.

To develop a parasite control program for your farm, review the AAEP’s guidelines and talk to your veterinarian. Our sources agree that all facility managers and farm owners should adopt these protocols to avoid ending up with a world in which equine parasites are ­uncontrollable.