Deworming Decoder

Understand the latest parasite control recommendations to better protect your horse.

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Herd of horses

Picture this: You’ve moved your horse to a new boarding stable. In your initial conversation with the barn manager, she tells you that all horses will be given a dose of antibiotics on the first of each month.

That sounds strange to you, but the manager says it’s just a precaution in case one horse might be sick.

If this scenario sounds perfectly ridiculous to you—and it should!—stop and consider that deworming horses every two months and rotating dewormer products is really just as irresponsible and is an unnecessary use of drugs.

Fighting Parasites

But wait. Shouldn’t you be concerned about internal parasites in your horse and do everything possible to get rid of them?

Yes. And no.

Yes, you need to have a responsible deworming program. No, you shouldn’t try to eradicate every worm in your horse. (Not that it would even be possible.)

The truth about equine parasite control is that there is no black and white, one-size-fits-all program. If you want to learn how to protect your horse now and in the future, read on to learn about the latest deworming recommendations and how to put them to use.

Getting Realistic

Here’s a basic fact: All grazing animals—including your horse—have internal parasites. The way this typically happens is that horses pick up infective larvae while grazing; those larvae migrate in the horse’s body, mature and lay eggs. These are excreted in the horse’s manure, and thus the cycle continues.

Before the advent of the first modern-era deworming drugs, many cases of colic with fatal outcomes were attributed to gastrointestinal parasites. In the 1960s, researchers discovered deworming drugs that were effective in fighting internal parasites. Many horse owners began routine use of dewormers, following the early recommendations of deworming every 60 days and rotating products.

Today, the large strongyles (“bloodworms”) that were once a common and dangerous equine intestinal parasite are now considered rare.

That’s good news, isn’t it? The fact that widespread use of dewormers over the past five decades has all but eliminated a deadly parasite is positive news. However, the downside is that other commonly found parasites have developed resistance to deworming drugs. This can occur when a dewormer is used repeatedly and frequently.

Once the parasite population contains a significant number of resistant worms, they can no longer treated with that drug class. The big concern is that we currently have only three basic chemical classes of deworming products to control internal equine parasites:

  1. benzimidazoles (fenbendazole and oxibendazole)
  2. tetrahydropyrimidines (pyrantel salts)
  3. avermectin/milbemycins (ivermectin and moxidectin), also referred to as macrocyclic lactones

“We thought for a while that rotating products would be useful, but the data is loud and clear: it doesn’t do anything to prevent or reduce resistance,” says Martin Nielsen, DVM, Ph.D., DipEVPC, associate professor at the University of Kentucky.

Beware of Resistance

Dewormer product labels don’t mention which parasites have shown resistance to particular drugs, so you need to do your homework. Below are some common equine internal parasites that have shown resistance to the drug classes noted.

  • Small strongyles:
    Widespread resistance to benzimidazoles and pyrantel salts has been reported
  • Pinworms:
    Resistance to ivermectin has been reported
  • Large roundworms (ascarids):
    Resistance to ivermectin and pyrantel salts has been reported

Common Mistakes

If resistance is a concern, then how can you protect your horse from parasites? You start by avoiding common mistakes.

“The most common mistake made by horse owners is to not use fecal egg counts, and to deworm far too often,” says Nielsen, whose focus of study is equine parasitology, including understanding the level of parasite resistance to deworming drugs.

“Some horse owners try to deworm their herds down to ‘ground zero,’” observes Craig Reinemeyer, DVM, Ph.D., a veterinary parasitologist and president of East Tennessee Clinical Research in Knoxville. “In terms of equine health, such intensity is unnecessary, and is likely to have unintended consequences.” (Reinemeyer and Nielsen are the authors of Handbook of Equine Parasite Control, published by Wiley-Blackwell.)

The unintended consequences of deworming too often and not using fecal testing are twofold:

  • You may contribute to resistance
  • Your horse may not be as protected as you think

“You need to know what you’re treating and if the treatment worked,” says Nielsen. “It’s as simple as that. Most adult horses do not need more than a couple of treatments a year. The risk of sticking to the old regimes is that we will just keep getting more resistance. And there are no new dewormers on the horizon, so we will eventually run out of treatment options.”

It would help to think of dewormer products with the same caution you have for other drugs, such as antibiotics, that are occasionally administered to your horse when needed. Because anyone can go to the equine supply store and buy dewormer, we tend to forget these are FDA-regulated drugs that should be used wisely and only as necessary.

Two horses grazing together
Worm eggs pass in manure and horses pick up the infective worm larvae while grazing, creating a constant cycle of reinfection. Photo: mholka/shutterstock

How to Use Fecal Testing

In order to use dewormer drugs most effectively, you need to talk with your veterinarian and create a selective deworming program targeted to your individual horse, based on his age, exposure to parasites, climate and season.

Any responsible modern deworming program must incorporate fecal testing, which is a catch-all term for fecal egg counts (FEC) and fecal egg count reduction tests (FECRT).

Reinemeyer explains that fecal egg count testing is the only practical test currently available to demonstrate resistance to deworming drugs, and to identify which horses carry a heavy or light parasite egg count, which in turn determines how often—or not—they need treatment. (Note: Such testing applies to mature horses. Foals and young horses are more at risk for parasite infection and therefore need more frequent deworming than older horses.)

“Fecal egg counting is the way forward,” says Nielsen. “It can be performed at any time, but it makes the most sense to do it during or around the grazing season.”

Performing an egg count when you deworm and then again 10 to 14 days later will clearly reveal how effectively the deworming drug worked. If results show parasite resistance, your veterinarian can recommend a different dewormer, depending on the parasites involved.

Unfortunately, many horse owners still aren’t utilizing fecal testing in their deworming program, but for the sake of the horses, parasitologists hope this changes.

Reinemeyer explains that if you’re not using fecal testing, you have no way of knowing whether deworming treatments are actually working and what kind of parasites the horse harbors.

“This means horse owners are likely to choose an ineffective product, and then they have accomplished nothing,” he adds.

Identifying the Shedders

Fecal testing can also reveal whether your horse tends to carry a high or low egg burden. This refers to internal parasite eggs in the horse’s body that are shed in manure and into his environment.

Most horses fall into the category of “low shedders” (less than 200 eggs per gram of feces when tested). These horses typically only need deworming treatment once or twice a year.

“High shedders” (greater than 500 eggs per gram) are horses that are more susceptible to parasites and may need to be dewormed more often per year than low shedders. And about one-third of herd members might be “moderate shedders,” with egg counts between 200 to 500 eggs per gram (EPG).

Your veterinarian can review the fecal test results with you and use those results to determine what time of year deworming will be most effective for your horse, how many times treatment is needed, and which drug class of dewormer to use, depending on the parasites you’re trying to control.

How to Collect a Fecal Sample

Turn a zip-top plastic bag inside out. Place your hand inside and pick up several “apples” of very fresh manure. Turn the bag right side out, squeeze out excess air and close securely. For most accurate results, keep the sample refrigerated or on ice until your vet picks it up or you drop it off at the lab for testing.

Time for a Change

“Resistance to change is probably the single biggest obstacle to effective adult education,” says Reinemeyer.

Getting horse owners to change traditional deworming practices and follow the latest recommendations is a challenge. But it’s important that we heed the most current research. Otherwise, we will find ourselves with even more limited options to controlling internal equine parasites.

For the health of your horse, make it a point to talk with your veterinarian about updating your deworming program and, if you’re not doing it yet, to incorporate fecal testing. As a responsible horse owner, you want to follow an effective deworming protocol that provides adequate control of parasites without over-treatment that can further the development of resistance to dewormer drugs.

CYNTHIA McFARLAND is an Ocala, Florida-based freelance writer, horse owner and avid trail rider. The author of nine books, her latest is The Horseman’s Guide to Tack and Equipment.


This article originally appeared in the February 2018 issue of Horse Illustrated magazine. Click here to subscribe!

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