Rethinking Parasite Control in Horses A new approach to equine parasite control.

Rethinking Parasite Control in Horses

by Doug Thal, DVM DABVP

Equine parasites

Equine parasites are becoming resistant to deworming compounds. Additionally, outdated worming practices are making the problem worse.
In the last few years, there has been increasing evidence of worm resistance to common deworming compounds. If worms develop resistance to all de-wormers, we will no longer be able to protect our horses from worm-associated diseases.

It is natural for healthy horses to have some parasites. Craig Reinemeyer DVM Ph.D., a renowned expert on equine parasitology states the problem well.

“Equine parasites have co-evolved with the horse over 60 million years of evolution. They are unique to the horse, and they can only survive if the horse survives. It doesn’t make sense that to burn down their only home. We need to manage parasites, not eradicate them. Our efforts at eradication are what have led us quickly to resistance.”

As horse owners and caretakers, it is very important that you take an intelligent approach to parasite control. Simply worming your horses with a rotation of deworming pastes is no longer considered the best approach. Of course, you need to keep parasite numbers managed in your horses. But that need has to be balanced with better methods if we are going to slow the onset of resistance. There are more than 150 species of internal parasites that affect horses.

The most important groups include:

  • Ascarids – roundworms
  • Large strongyles – bloodworms
  • Small Strongyles 
  • Pinworms 
  • Bots 
  • Tapeworms. 

Internal parasites have a life-cycle that involves stages within the horse and stages in the environment. Parasites released into the environment take time to develop to infectious stages. The climate must be right for this development to take place. Management to reduce parasites, therefore, must take into account the parasites likely to be encountered, their life cycles and your geographic region’s climate. Most species have stages (eggs or larvae) that are passed from horse to pasture in manure. Any or all of these parasites may be present in the horse at one time and may be at different stages in their life cycles. Some worm species can lay hundreds of thousands of eggs per day, so parasite loads can grow quickly both on pasture and in horses. The different de-wormers have varying effectiveness against the types and stages of parasites. Intelligent control of parasites must take all these factors and many others into account.


Horses with parasites can show a wide variety of signs. The specific signs depend upon the specific parasites affecting them, and the numbers of those particular parasite species. It is common knowledge that heavy parasite loads can cause problems for horses, including gastrointestinal irritation and damage, reduced nutrient uptake and generally poor health. Horses affected by large parasite loads look like we would expect — pot-bellied, rough coated and thin. They may experience colic or have diarrhea.

Specific parasites cause specific conditions, depending upon the organs they affect. Here are some examples, but there are many, many more:

  • Pinworms cause skin irritation and itching to the area around the anus and tail head.
  • Thelazia eyeworms cause eye inflammation.
  • Thelazia eyeworms cause eye inflammation.
  • Large strongyles can cause life-threatening damage. The adults of this species localize in the large arteries supplying blood to the intestine. They can result in loss of blood supply to segments of intestine, a potentially fatal problem which results in signs of colic.
  • Bots are not thought to cause many problems at all, although the egg-laying adult flies are highly irritating to horses.

– The most common species of Tapeworm lives in a specific segment of small intestine, and in large numbers can cause irritation and blockage of the intestine. – Lungworms are common in donkeys and cause coughing, usually in horses stabled with donkeys. – Large roundworms in young horses (Ascarids), can cause ill thrift and, at times, complete obstruction of the intestine that usually occurs after deworming. – Habronema worms transmitted by flies cause non-healing wounds called Summer Sores. Certain parasite stages can damage vital tissues like lungs or liver causing illness related to that organ. This usually occurs as larval stages migrate through the horse’s system to complete their life cycles. Small numbers of intestinal parasites usually don’t cause obviously visible illness in horses. The parasites operate “behind the scenes”. Horses may look perfectly healthy or show only very subtle signs. And you wouldn’t expect to see anything in the manure – parasite eggs and the worms themselves are mostly tiny or microscopic and so are rarely visible in manure.

Over the past half-century, deworming compounds have been developed that have drastically reduced parasite problems in horses. “Thrombo-embolic colic” was a common and often fatal condition involving blockage of the blood supply to parts of the intestine. The condition is caused by Strongylus vulgaris, the large Strongyle. This worm has almost been eliminated by the worming campaign of the last 40 years and so this condition is now very rare. There are far fewer severe parasite-related problems than there were 40 years ago. There have been several main “chemical classes” of dewormers that have been developed over that time. These have drastically reduced the number of parasites in horses. But the cost of this effective campaign has been the development of resistance to these compounds, especially in small Strongyle parasites. This has resulted in the growing ineffectiveness of these drugs.

Recently, researchers have shown that this resistance is a greater problem than we thought and that it is progressing rapidly. We are beginning to see resistance to even our newest and most potent chemical class. Before the advent of paste dewormers, veterinarians were very involved in equine parasite control. Veterinarians typically tube-wormed horses, meaning that they passed a stomach tube and dosed a large quantity of the chosen chemical directly into the stomach.

In the last 25 years, paste formulations of the common chemical classes have become increasingly available and very cheap. Horse owners have rotated dewormers casually, not understanding the problem, and under the false assumption that “more is better”. Veterinarians, including myself, were complicit in this approach. Ten years ago, I consistently recommended a fast rotation program to my clients without understanding the potential implications. We now need to completely re-think how we deworm our horses. Misconception and lack of awareness by the industry led to the development of a very large problem.

Fecal egg counts involve microscopic examination of manure for quantifying the number of parasite eggs of different types. We have learned certain things about equine parasites from doing fecal egg counts. We know we can make the worms disappear from the manure by using deworming compounds. We assumed that if we rotated compounds, that parasites that were not killed by one class, would be killed by the next. This idea worked well when the 3 main chemical classes each killed the majority of parasites. However, this is becoming less and less true.

Key Points

Two out of three main classes of chemical no longer kill parasites adequately. Parasites have become resistant to them. Their continued indiscriminate use will quickly result in complete resistance.

Rotation using ineffective compounds ensures complete resistance to them while creating a false sense of security. The potency of the effective dewormers “covers up” the inadequacy of the others in the rotation.

We are already seeing pockets of resistance to ivermectin and moxidectin and it is inevitable that this will increase. Use of these compounds without fecal testing will ensure a short effective life for them.

There are no new chemical compounds in the works right now. Research and development are costly and take time. Our emphasis should be on extending the effectiveness of the drugs we have.

Understanding Parasite Resistance

Parasites develop resistance to the chemicals used to kill them. Resistance is based on individual worm genetics and selection for these genetic traits, i.e. natural selection. Most parasites are killed by a properly administered dewormer. Out of thousands of worms, there may be a few surviving (resistant) parasites that have genetic differences that allow them to tolerate the chemical.

These few survivors can then occasionally interbreed with other similarly resistant parasites, leading to a higher number of resistant parasites in the next generation. These offspring survive and propagate, resulting in the next generation being comprised of even more resistant parasites. The process is invisible to the horse owner.

The greater the percentage of the worm population that is exposed to a compound, the greater the pressure for the parasite populations to develop resistance against it, and the faster the percentage of parasites becomes the new resistant type.
Indiscriminate de-wormer use has and will accelerate the onset of resistance. It is inevitable that this process will take place with enough time and exposure to these compounds. Our goal should be to make that period as long as possible for each of our de-wormers. How do we achieve this? By minimizing the exposure to these compounds through targeted deworming.

The best way to prevent the development of resistance to these compounds is not to use them at all. That would completely eliminate any selective advantage to resistance. Obviously, this is not feasible because our horses would again succumb to the effects of parasites. However, leaving a segment of the parasite population with minimal exposure to these chemicals will slow the onset of resistance. Those susceptible parasites are allowed to go on living and competing with resistant parasites. This is known as preserving “refugia” within the population.

We can move toward this concept by only deworming horses that have higher fecal egg counts. For the others, we drastically reduce the number of deworming treatments. This requires an understanding of which horses have higher worm burdens and shed more into the environment.
This knowledge requires fecal testing. Fecal Egg Counts (Fecal Exams) are the cornerstone of targeted deworming. Here is how they work:
Worm eggs are shed in the manure. Most worm egg types can be identified and counted using a microscope. Accurate testing requires knowledge of the technique and practice. There are limitations to FEC, as some species are difficult to diagnose this way. Sometimes, eggs may only be shed intermittently, meaning that they can be missed. Regardless, FEC is the best indicator we have of worm burden in horses.
By using fecal egg count results, horses are broken into 3 groups, those with high parasite burden and shedding into the environment, those with moderate, and those with minimal. Only those in group 1 are dewormed frequently, about 3 times per year. The others are dewormed less. The goals of this new approach are optimal horse health for all horses in the herd, reduced dependency on chemicals and reduced contribution to the resistance problem, and improved fecal diagnostics to monitor the effectiveness of the program.
The key to this new approach to deworming includes working with your veterinarian. There is no perfect de-wormer and no standard program. Fecal testing guides the program. Horses at different ages and stages have varying needs for parasite control. Twenty percent of horses in a group shed 80% of the total parasites. Young, growing horses have some special needs. Young foals are especially susceptible to ascarid (roundworm) infestation and may benefit from deworming with an appropriate compound at 30-60 day intervals until they build some natural resistance.
Climatic conditions and season of year influence parasite levels in the horse and on pasture and are critical factors to address. The goal is not to kill all parasites, but to keep parasite loads to a level compatible with health, and leave a reservoir (refugia) of parasites in as many horses as practical. Based on this new paradigm, in our veterinary practice, we recommend a fecal exam on every horse twice annually, in our area in May and October. Testing is the only way to determine the effectiveness of a parasite control program and to detect the development of resistant parasites.
Collecting a fecal sample is easy. Simply pick up 1 fresh fecal ball in a zip lock bag, squeeze the air out of the bag, label it with your horse’s name, and drop it by your veterinarian’s office. You can store a fresh sample up to 12 hours if you keep it refrigerated. At our clinic, we do this testing in-house. We provide our clients with the results within 24 hours.
It is important that the sample is taken at least 3 months after deworming, and 4 months after deworming with a Quest compound. Otherwise, the effect from the prior deworming confuses the results.
If horses are on a continuous dewormer like Strongid C, they can be tested at any time.
Your vet assesses your overall program. This includes the susceptibility of your horses, management, your geographic region, and environment. They help you tailor your program to these factors and to the results of Fecal Egg Counts.
They perform a fecal egg count on your samples and determine which horses are low (< 200 EPG), moderate (200-500 EPG) or heavy (>500 EPG) shedders. They identify the specific parasite species, allowing a determination to be made of the most effective deworming compound. You will then deworm the horses with the appropriate compound. 
Once annually at least, they will perform a FECRT (Fecal Egg count reduction test). To do this, you submit a second sample two weeks after deworming. The veterinarian performs another fecal egg count. If the drugs are working the way they should, there should be almost no parasite eggs in the second sample.
A customized approach for worming is then tailored to the situation. Horses are dewormed with the appropriate compound based on their category. Low shedders are dewormed once or twice annually, spring and fall. Moderate shedders are dewormed three times annually. Heavy shedders are dewormed up to 4 times annually. The cost of fecal testing should be offset by significant savings in the purchase of deworming compounds.