Fighting the Dewormer Resistance


Horse Illustrated MagazineWhen Sarah’s horse almost died from a severe case of small strongyles, she was stunned. He was on a regular deworming program and she rotated between drug classes, just as she’d been advised. Despite her best intentions, she’d unwittingly put him at risk.

Even after deworming, some worms hold a genetic trait that makes them resistance to anthelmintics“There’s a big difference between treating a horse with a deworming product and actually having effective parasite control,” says Ray M. Kaplan, DVM, Ph.D., DEVPC, professor of parasitology at the University of Georgia’s College of Veterinary Medicine. “I’ve seen quite a few cases of horses that died or almost died from small strongyles, and they were being frequently dewormed. Horses tolerate small strongyles rather well, so you don’t know you have a problem until you have a real disaster.”

At the heart of the issue is parasite resistance to anthelmintic (deworming) drugs. Major changes are necessary in the treatment of equine parasites in order to avoid serious trouble down the road. To learn more, we went to the experts. What we discovered should change the way horse owners look at deworming forever.

Anthelmintic Resistance
As hard as it is to believe, deworming wasn’t part of routine horse health care until the late 1950s and early 1960s. Prior to that, deworming products weren’t highly effective and had side effects that could be more dangerous than the worms they were supposed to destroy.

Unfortunately, eliminating parasites in the horse isn’t an exact science, thanks to drug resistance. Resistance to deworming drugs was seen as early as the 1960s. The danger of resistance is that horse owners have a false sense of security that their horses are protected, when in reality, horses can be at risk if the drugs aren’t doing their job. So what causes certain parasites to become resistant to anthelmintic drugs?

“Some individual worms have an innate capacity to survive treatment with various anthelmintics,” explains Craig Reinemeyer, DVM, Ph.D., president of East Tennessee Clinical Research in Knoxville. “This trait is passed on genetically, and it would remain rare unless those individuals enjoyed some type of advantage over the rest of the population, known as a ‘selection pressure.’ Deworming is the pressure that selects for anthelmintic resistance.

“After a dewormer is administered, all of the susceptible worms die off. [The non-resistant worms] won’t begin to reproduce and lay eggs for anywhere from four to 12 weeks after dosing,” says Reinemeyer. “In the interim, the resistant individuals that were not killed are [immediately] able to reproduce in the absence of competition. Thus, the frequency of their genes increases slowly but surely in the population. When the same drugs are used exclusively, or when treatments are administered with excessive frequency, the resistant worms are the only ones in the whole population that can still reproduce, so these two practices accelerate the rate of resistance development.” Once the resistant worms make up a significant part of the population, drugs aren’t effective in treating them.

Get Informed
Parasite resistance has been a factor for years, but the average horse owner may not be aware of it. What’s even more disconcerting is the fact that many practicing equine vets, even recent graduates, aren’t up-to-date in current advances on the subject. But Kaplan doesn’t blame them.

“There aren’t that many veterinary parasitologists who specialize in large animals anymore, and when you don’t have a specialist teaching you the latest ideas, you just end up hearing the same dogma that gets passed down,” he notes. “Over the past few decades, there has been a great reduction in funding for parasitology research, so fewer veterinarians are receiving specialty training in equine parasitology. Deworming drugs are so cheap and easy to administer that many veterinarians are not very involved any more. The end result is that many veterinarians are not aware of the severity of the problem of parasite resistance, so the ‘same old, same old’ just gets repeated over and over again.”

Both Kaplan and Reinemeyer emphasize that the current approach to parasite control is outdated and no longer meets the health needs of today’s horses. The good news is that there’s a simple way to determine which drugs are effective for your horse so you know he is protected.

FEC tests can determine the level of deworming medicine necessaryEffective Testing
Fecal egg counts (FEC) and fecal egg count reduction testing (FECRT) are simple ways to determine if your horse needs to be dewormed and which deworming products are effective. An FEC will establish if a horse is a high or low egg-shedder (contaminator), while an FECRT will allow you to determine if the deworming drugs you’re using are effective.

Although not foolproof, such testing is presently the best way to identify parasite resistance, particularly in strongyles, which cause the majority of parasite-related health problems. If horse owners start asking their veterinarians to perform FECs and FECRTs, they will become common services.

You’ll need to collect a fresh manure sample from each horse you want tested. Pick up one or two “balls” of manure in a sealable plastic bag and label it with the horse’s name. Unless the test is going to be performed immediately, you’ll need to keep the sample on ice in a cooler or in a refrigerator.

Deliver the sample to your veterinarian, who will run an FEC that counts the number of parasite eggs per gram (EPG) in the manure. This test typically costs about $10 to $25.

The best way to know if a deworming product is working effectively is to have your vet conduct an FECRT by performing an FEC just before deworming your horse and again 10 to 14 days later. A horse must have at least 100 EPG at first count to properly interpret the results. The veterinarian or technician performing the test compares the EPG in the manure after deworming to the EPG prior to deworming.

Ideally, you want to see a 90 percent or greater reduction in egg count, as this tells you the drug is still working. If there is less than a 90 percent reduction, the effectiveness of the drug is questionable. When you see less than an 80 percent reduction, this means that the drug is not effective and is a sign of parasite resistance to that specific drug product. Each time you use a drug that you haven’t previously tested with an FECRT, you should run the test. Keep in mind that some drugs are sold under many different brand names, so check the labels for the actual chemicals used.

Individual Results May Vary
One main problem with a traditional deworming program is that all horses are treated the same. However, horses are individuals, and their susceptibility to parasites varies greatly.

Research has shown that about 25 to 50 percent of horses will have zero or very low egg counts, even if they haven’t been dewormed. These horses are referred to as low egg-shedders. Only about 20 to 30 percent of horses will tend to have high egg counts after the benefits of recent deworming have expired. These horses, termed high egg-shedders, or contaminators, are simply more susceptible because their immune system responds differently to parasites than those of horses with naturally low egg counts.

Once a horse is 3 to 4 years old, his immunity to parasites is basically established. After you’ve tested a horse of this age (or older) several times, you will know how to classify him—whether he is a low, moderate or high egg-shedder.

“When you test is important. If you treat with an effective drug, this will cause the egg count to be low, so you’d have to wait until the effectiveness of that drug is over to see if the egg count rises again,” says Kaplan.

“We don’t know why some horses carry more parasites than others, but we do know it’s a lifelong characteristic that’s transmitted genetically,” notes Reinemeyer. “Sooner or later, we will identify the genetic basis for this. I think within a decade or two, we’ll be able to take a blood sample from a foal and learn if he’s going to be susceptible to parasites or not during his lifetime.”

Change in Protocols
You’ve likely heard that you should deworm your horse every eight weeks or so and that you should rotate between classes of drugs throughout the year. Say goodbye to those theories. “The protocols of deworming every two months were developed 40 to 50 years ago and were based on different parasite problems,” says Kaplan. “The major parasites we have now are small strongyles, and that program wasn’t developed to address them.”

The original reason for this line of thinking was that in the 1960s, there weren’t the different classes of deworming drugs or broad-spectrum products available today. People rotated between products just to cover all the bases.

Although one thought behind rotating deworming drugs was to prevent parasites from becoming resistant, studies have shown this isn’t the case. Additionally, rotation of products can hide clinical effects caused by parasites when you are using an effective product in conjunction with one that is ineffective, since one is killing parasites but the other isn’t killing them to the same degree.

Many horse owners simply opt to just keep deworming rather than using egg count tests, but this practice contributes to the problem of parasite resistance and puts horses at risk by using drugs that aren’t effective in those animals.

“By treating horses that don’t need treatment, you make the resistance worse,” says Kaplan. “If you’re not monitoring with egg count testing, you have no idea when your program starts to fail. Every time I’ve seen serious outbreaks of parasitic diseases on a farm, it’s not because they weren’t deworming—it’s because the drugs they were using weren’t effective.

“The best way to design a parasite control program is to use egg count tests to determine which horses have high egg counts and which have low counts,” continues Kaplan. “Develop a baseline program to cover those with low egg counts and use drugs that are effective. Then treat the high egg count horses additionally.”

Management Practices
Parasite problems are typically management problems, which means informed horse owners can win the battle against worms. In addition to using FECs and FECRTs, proper management is essential. Parasites aren’t out to get your horse. They’re simply present in the environment and horses unknowingly ingest them, allowing the parasite cycle to continue.

“The environment has a far greater impact on parasite transmission than anything else,” says Reinemeyer. “We can’t control the weather, but there are aspects that you can exploit, and (parasite) transmission patterns are pretty predictable within certain climates.”

If your horse is turned out on pasture, it’s a given that he is exposed to infective strongyle larvae. Horses in stalls and dry lots have much less exposure simply because their environments aren’t conducive to parasite survival.

After female strongyles lay eggs in the horse’s gut, the eggs pass out of the horse in the manure. (Ascarids and tapeworms also pass from the horse in the egg stage.) When conditions are favorable (45 to 85 degrees Fahrenheit with some moisture), those eggs hatch. The emerging larvae develop within the fecal pat and eventually crawl into the grass thatch near the ground, where they are ingested by grazing horses, and the cycle starts all over again.

If you’ve ever watched horses grazing, you already know that they avoid eating near manure piles. The issue gets complicated when pastures are overstocked and grazed too low. This increases the chance of horses picking up infective larvae, as they are eating closer to the grass thatch at ground level.

While you may have been advised to drag or harrow pastures to break up manure piles, this is not necessarily a good idea. Horses will naturally leave “rough” areas in their field where they defecate. These roughs contain 15 times more larvae than uncontaminated areas, known as “lawns,” where horses prefer to graze. When well-meaning horse owners drag the pasture to break up piles, they are actually helping parasites by spreading the larvae that were restricted to a small area near the roughs. Heavy rainfall (1 inch or more) can also spread the larvae across the grazing areas.

In a perfect world, if we removed the piles of manure every day from the horse’s environment, this would greatly reduce the transmission of parasites. Of course, this isn’t always practical.

Reinemeyer notes that hot, dry weather is the only effective time to drag. If you want to drag your horse’s pasture or paddock, do so during the summer on a hot, sunny morning. Ideally, horses should be kept off the pasture for at least two weeks in Southern states and up to four weeks in Northern states after dragging.

“The single greatest misconception about transmission is that cold weather kills the larvae,” explains Reinemeyer. “After the eggs hatch and larvae reach the infective stage, they can survive all winter. Parasites die after their energy is used up, and because they use less energy in cold weather, they live longer when it’s cold.”

Though infective larvae already present in the pasture when cold weather hits can survive reasonably well, any newly deposited worm eggs passed in manure are rapidly killed by the freezing temperatures, so no new larvae are added until the weather warms up again.

Strategic Deworming
Parasites are to blame for a wide variety of health issues, including colic, weight loss, diarrhea, lethargy, poor performance, bleeding ulcers, anemia and more. These issues can be serious and, in some cases, fatal. Fortunately, many horses have a fairly strong natural immunity to parasites, so while horse owners need to be proactive, they needn’t be terrified.

As mentioned earlier, strongyles cause most parasite-related health problems in adult horses. They can be found in two basic groups: large strongyles (also known as bloodworms) and small strongyles (also referred to as cyathostomes or cyathostomins). Tapeworms also cause health issues and are more common than previously suspected. In young horses, roundworms (ascarids) can be a serious problem.

Both Kaplan and Reinemeyer urge horse owners to think of parasite control as an annual cycle. Treatment should start at the time of year when transmission is likely, and this depends on where you live. In the South, the parasite control cycle begins in late summer/early fall. In northern states, the cycle begins in late winter/early spring.

One of the biggest challenges in the effort to control equine parasites is that we are limited to using three basic chemical classes of dewormers: 1) benzimidazoles (fenbendazole, oxfendazole, oxibendazole); 2) tetrahydropyrimidines (pyrantel salts); and 3) avermectin/ milbemycins (ivermectin and moxidectin). Avermectin/ milbemycins are also referred to as macrocyclic lactones. At this time, resistance to benzimidazoles is the most widespread. Although strongyles have shown the least resistance to ivermectin and moxidectin products, it’s unrealistic to think these drugs will always remain as effective as they are now.

“All the dewormers, with exception of moxidectin, only kill the adult worms. But the adults aren’t that harmful; it’s the larval stage that causes more problems,” notes Kaplan.

“Drug companies are trying to come up with new products, but it’s difficult,” says Reinemeyer. “We desperately need more products that will kill worms. There’s nothing very promising in the pipeline for horses right now, but they are working on it.”

It’s important for both veterinarians and horse owners to educate themselves with the latest knowledge on parasites so deworming practices can be adjusted to tackle the rising resistance problem. Fecal testing is simple and relatively inexpensive, and we should take full advantage of this valuable tool.

Further Reading
What is a Fecal Egg Count Reduction Test?
New Thinking on Deworming

Cynthia McFarland is an Ocala, Fla.-based freelance writer and horse owner whose latest book is The Foaling Primer.

This article originally appeared in the April 2010 issue of Horse Illustrated. Click here to subscribe.


  1. Excellent! As a new horse owner, I have just been following the deworming schedule — every 2 months — at the farm where I board. I am going to have the FECT done and get my horse on his own specific schedule. Thanks.

  2. Yes, it is definitely a good idea to get the fecal count done ASAP. I thought I was following a good worming plan but my two horses had way too many worms but didn’t “look wormy” (emaciated and rough coat)as I expected they would look if they had worms. I was concerned about other problems because they didn’t have much energy.


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