How Often to Deworm Horses?

by | Dec 5, 2018

Most horse owners know that internal parasites can cause all sorts of problems for your equine friend. But opinions differ wildly on how often horses should be treated for these pests – ranging from every four weeks to just twice a year. Advances in veterinary science have made it possible to tailor-make your deworming programme specifically for your horse. Here’s how to do it

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Why Are Worms Bad for Horses?

Internal parasites, commonly known as “worms”, are nasty little invertebrates that make their homes inside your horse’s digestive system. Like all parasites, these critters can be extremely problematic.

The equine digestive system is as delicate as it is essential for your horse’s health and well-being. Worms can cause many different problems. Firstly, these worms obviously have to live on something, and that generally turns out to be one of two things: either your horse’s blood or his partially digested food. Neither is good for your horse’s overall health, and he will lose weight and struggle to perform. With a large enough worm burden, your horse could even become anemic or suffer from malnutrition despite being fed correctly.

Worms are also a major cause of colic. Not only can they cause intestinal pain by biting into the horse’s intestinal walls, but if present in large enough numbers, they can cause a physical blockage somewhere in your horse’s intestine. This is a form of impaction colic that can be deadly if it develops into a perforation or a torsion.

So, it’s essential to make sure your horse’s belly is worm-free. But how do you know if he has worms or not?

Signs of a Worm Burden

Almost all horses have some worms in their systems. Very small amounts of worms won’t be generally harmful to your horse; it’s only when their numbers get out of hand that they can cause these health problems.

Visible signs of a heavy worm burden include:

• Loss of weight

• Lethargy

• Poor performance

• Dull coat with long hair

• Colic

• Pale mucus membranes (anemia)

• A large, low-slung belly, known as a “worm belly”. (This can be confused with a pregnant belly or with a “hay belly” in non-working horses who consume large amounts of roughage, and is not always an indication of a worm burden).

Most horses with significant worm burdens, however, are asymptomatic – you won’t see any of these clinical signs. By the time the horse is visibly “wormy”, his worm burden is astronomical. Your horse could be crawling with worms and you wouldn’t be able to see a thing. The idea is to prevent him from ever having the symptoms of a worm burden, and this is best done using fecal egg counts.

Fecal egg counts are effectively worm tests. These are done on small samples of fresh manure. Your vet or even animal medication company will inspect the manure under a microscope and count the number of worm eggs present per gram (EPG). As a general rule of thumb, horses with an egg count of more than 400 EPG should be dewormed and then have another egg count done to ensure that the anthelmintic (dewormer) worked.

As a general rule, fecal egg counts should be done at least four times a year, depending on your worm burden and management system. Horses on pastures or in group paddocks are at far greater risk of ingesting worms than horses who are stabled and live in individual paddocks. Grazing horses are at the highest risk of worms, especially young horses, as they have no immunity against the worms yet.

Choosing the Right Dewormer

A major problem in the equine world today is that many worms are becoming resistant to deworming drugs, just as some bacteria mutate to become resistant to antibiotics. Deworming your horse blindly “just in case” – or even using the traditional rotational deworming schedule – is a recipe for disaster. You might as well be taking a course of antibiotics to prevent yourself from getting sick: it might work once, but all you’re really doing is creating super-worms who aren’t affected by anthelmintic drugs.

For this reason, the anthelmintic you choose has to be specifically targeted for the type of worms that you have. Your vet will be able to tell you exactly what type of worms your horse has based on his fecal egg count.

When buying an anthelmintic, it’s important to check its active ingredient. Here’s a list of some popular active ingredients, their commercial forms, and the worms they’re effective against.

• Ivermectin: Effective against redworms, roundworms, pinworms, threadworms, and bots. A popular and extremely safe dewormer. Contained in: Duramectin, Zimecterin, Bimectin

• Pyrantel Pamoate: Effective against redworms, roundworms, pinworms, threadworms, and tapeworms; a good option if there is resistance to ivermectin. Contained in: Exodus, Pyrantel Paste

• Oxibendazole: Effective against redworms, roundworms, pinworms, stomach hairworms and large-mouthed stomach worms. Can be used multiple times per day for very heavy worm burdens. Contained in: Anthelcide EQ

• Moxidectin: One of the few ingredients effective against encysted small strongyles. Easy to overdose and must be used with care. Contained in: Quest


Intestinal parasites are a persistent problem that can even be life-threatening to your horse. That’s why it’s far the best to work closely with a veterinarian when working out a deworming schedule for your horses. It’s also essential to take steps towards preventing anthelmintic resistance in order to keep your horse worm-free for years to come.

About The Author

<a href="" target="_self">Firn Hyde</a>

Firn Hyde

I'm a young horsewoman living in a tiny home on a horse farm in South Africa with three dogs, two pigs, a longsuffering man, and God's grace. I run a stableyard and compete in dressage with two kind geldings who keep me happy and a psychotic mare who keeps me humble. For the past two years, I've been writing for a living, and I enjoy every opportunity to combine my two passions.

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