Horse Dewormers: Is Your Protocol Up to Date, or Outdated?

Horse owners tend to follow tradition. This is especially obvious when it comes to deworming — but that's not necessarily a good thing.

Rotating horse dewormer products and treating horses every two months are outdated programs that originated in the 1960s. Yet plenty of horse owners still follow these old routines.

"Many horse owners were taught to do rotational deworming. People are notoriously slow to adopt new information even when that information is 100 percent correct," observes David Ramey, DVM, whose practice, Ramey Equine, has been based in the Los Angeles, California, area since 1987. Ramey's equine patients include breeds from warmbloods to mini donkeys, ranging from world champion show horses to backyard companions.

Ramey encourages horse owners to follow strategic deworming guidelines, which focus on using fecal testing and deworming according to those results so that each horse is only treated when necessary.

Which horse dewormer is right for your horse?

You should work with your veterinarian to manage your horse's deworming schedule, just as you manage his vaccination and dental care.

"A deworming schedule should be individualized to the needs of the horse," says Ramey. "Deworming is easy, so people think, ‘I'll just take care of this myself.' But this doesn't mean you shouldn't get expert advice, if only for the reassurance you're doing the right thing."

Two horses on the same property, receiving the same care, can have very different loads of parasites. For example, horses can develop some immunity to parasites. A horse's age also impacts how often you should deworm. This is why you need a strategic deworming program that is individualized for each horse, rather than putting all horses on the same plan.

Over 35 years in veterinary practice, Ramey has adhered to the following philosophy: Don't give medicine unless the horse needs medicine. This includes horse dewormer drugs. "They are largely safe and effective, but there are side effects and there is also the concern of resistance with antibiotics and dewormers," he notes.

Ivermectin availability

What should you do if ivermectin is not in stock?

In this era of COVID-19 and supply chain issues, horse owners are concerned about possible shortages of ivermectin deworming products.

Fear has been a potent driver during the pandemic, but don't let it overwhelm your deworming plan.

"It's not that ivermectin is effective and all the other dewormers aren't. It all depends on the population of parasites you're dealing with," says Ramey.

For example, a horse dewormer with fenbendazole as the active ingredient targets some of the same parasites as an ivermectin dewormer. This could be an option instead of ivermectin, depending on which parasites you are treating. Your veterinarian can strategize with you as far as ivermectin alternatives if your preferred horse dewormer is suddenly in short supply — or not currently available.

Dewormer active ingredients

Although there are numerous brands of horse dewormer products on the market, they fall into just three basic chemical classes. Once you know the parasite(s) that you need to target, you need to read the product label to make sure the active ingredient is effective against those parasites.

• Benzimidazoles (fenbendazole/oxibendazole) target large strongyles, small strongyles (not encysted), ascarids and pinworms
• Pyrimidines (pyrantel) target large strongyles, small strongyles, ascarids, tapeworms (if given as a double dose) and pinworms
• Macrocyclic lactones (ivermectin/moxidectin) target large strongyles, small strongyles (including encysted — moxidectin only), ascarids, pinworms and bots

Praziquantel is an additional minor-class drug that targets tapeworms. In the U.S. you will only find praziquantel in horse dewormer products featuring macrocyclic lactones¹.

Farnam offers deworming products in each of the three chemical classes

Oral paste or pelleted horse dewormers are sometimes referred to as "purge" dewormers because they're used as a single dose, or purge, to kill parasites within the horse. Daily dewormers are designed to be fed every day to prevent parasite infection.

Here's a quick rundown on each product:

IverCare® (ivermectin paste) 1.87% (anthelmintic and boticide):

• Broad-spectrum dewormer (see label for all parasites and stages)  
• Removes worms and bots with one single dose
• Paste dewormer administered with easy-to-use Sure-Grip™ syringe
• One syringe treats up to 1,500 pounds

FenCare® Safe-Guard (fenbendazole) 1.96% Type B Medicated Feed Equine Dewormer

• Controls large strongyles, small strongyles, pinworms and ascarids
• Palatable, apple-flavored pellet formula mixes in with horse's feed, making it easy to treat horses that are hard to deworm with paste
• One 5-ounce packet treats up to 1,250 pounds
• Also approved for pregnant mares and stallions

PyrantelCare® Daily Dewormer 2.11% (pyrantel tartrate) Equine Anthelmintic

• Designed to be fed daily
• Medicated pellets for daily feeding provide continuous protection against the most common species and stages of equine parasites, including large and small strongyles, ascarids and pinworms
• Top dress the palatable pellets on the horse's grain each day
• For horses of all ages, including foals, mares at any stage of pregnancy or lactation and breeding stallions

Consult with your veterinarian to create a strategic deworming schedule for your individual horse, including which products to use and when to use them. For example, if you have young horses, they will likely require treatment with a different dewormer than your mature horses at times.

Resistance and why it matters

Many horse owners have faithfully stuck to a "one-size-fits-all" deworming plan for years. However, when you automatically deworm every two months, you will inevitably be treating horses that don't require it.

It's not possible for a horse dewormer to kill every single internal parasite at every stage. However, with continued deworming and repeated exposure, the parasites that aren't killed by the treatment can develop a tolerance to the deworming drugs. The parasites then become even more resistant to treatment. Once the parasite population contains a significant number of resistant worms, drugs are no longer effective against them.

Unfortunately, we are now experiencing widespread resistance to horse dewormer drugs in the case of some common parasites, including small strongyles and ascarids. With the advent of ivermectin in the 1980s, problems with large strongyles, once a scourge, have become more rare.

Because we only have the three classes of deworming drugs available, resistance is a serious issue, as it limits the available options to control parasite populations.

Currently, large strongyles do not show resistance to any of the three chemical classes of dewormers. However, resistance is a cause for concern among some common parasites^1,2:

Small strongyles

• Widespread resistance to fenbendazole, oxibendazole
• Moderate resistance to pyrantel
• Emerging/early resistance to macrocyclic lactones

Large roundworms (ascarids)

• Widespread resistance to macrocyclic lactones
• Emerging/early resistance to fenbendazole, oxibendazole, pyrantel

Parasites of concern

Of the many internal parasites that can affect horses, the most common and concerning are:

• Small strongyles (cyathostomins)
• Roundworms (ascarids)
• Tapeworms
• Large strongyles (bloodworms or redworms)
• Pinworms
• Bots
• Threadworms

According to the American Association of Equine Practitioners (AAEP), "probably the most important, in terms of health risk, are the first three: small strongyles, roundworms, and tapeworms."

Fecal testing as a tool

Deworming is an essential part of equine health care, but the goal is parasite control, not the complete elimination of every single parasite, which isn't possible — or necessary.

Before you deworm, the first step should be determining which parasites are the concern.

Fecal testing is currently the most accurate way to detect the presence of adult strongyles and roundworms, two of the most common parasite species.

"Medicine always works best with a real diagnosis," says Ramey. "Fortunately, with parasites, you have a pretty definitive diagnostic tool available in fecal testing. It is in the 90 percent range of accuracy in determining if parasites are present and if so, which ones."

Fecal egg count testing is a simple, but effective way to determine if the horse's manure contains parasite eggs and if the parasite infection is light, moderate or heavy. When such fecal tests are done before and after deworming, it can also show if the deworming product used is effective — or not.

Your veterinarian can help you schedule fecal testing. A few fresh manure "apples" are collected in a plastic bag and the sample is examined in a veterinary lab to identify the number of parasite eggs per gram (epg).

Results of less than 200 epg are considered a light parasite load, while 500 to 1000 epg is a high load. A high epg test indicates the horse needs deworming. High epg results may also suggest that a different deworming drug should be used and/or that the interval between deworming treatments should be shortened for this particular horse. You may also consider adding a daily dewormer to the horse's feed program, so get your veterinarian's recommendation.

Fecal tests aren't reliable when checking for the presence of tapeworms, bots or pinworms. Ask your veterinarian if these parasites are of concern in your area, and how to test for them.

General recommendations for treating a horse for tapeworms are based on his potential exposure, which includes region and exposure to pasture. Horses in a stall or dry lot have far less exposure than horses on pasture, but there is still some risk.

Consider your horse's environment

Most of the internal equine parasites of concern are picked up on grass. In other words, time on pasture is when horses get exposed to these parasites.

"If horses are in a dry environment without grass, parasite transmission is very difficult," says Ramey, noting that horses in arid environments may be barely exposed to parasites at all.

For horses that don't spend any time turned out on grass, pasture-borne parasites, such as strongyles and tapeworms, are less prominent. Your veterinarian can determine the best time for a deworming treatment if your horse has no grazing season.

Management practices to reduce parasites

You can't rely on horse dewormers alone to protect your horse from parasites. The answer to parasite resistance is not to deworm more frequently, but to develop management strategies so parasite eggs don't get concentrated in the horse's environment.

"You also need to look at how you're managing pastures and manure, which gets into non-chemical means of parasite control," says Ramey.

Non-chemical management practices include:

• Feed hay so it's off the ground
• Remove manure from pastures where horses are turned out
• Limit the number of horses on a pasture to reduce contamination with parasite eggs and larvae
• Avoid spreading manure and used bedding on pastures currently used for grazing

Ramey encourages horse owners to use fecal testing to monitor parasite load, only deworm as needed, and to use management practices that reduce the chance of parasite infection, including daily manure removal.

References:

1 https://extension.psu.edu/decoding-dewormers-types-resistance-concerns-and-use-for-horses

2 www.aaep.org