Parasite Control: Strategic Deworming
In the last decade, one important topic among both horse owners and veterinarians is parasite resistance; that is, the development of parasites which resist being killed by the deworming medication given to a horse.

KEY POINTS about parasite resistance:
1. Two out of three main classes of dewormers no longer kill parasites adequately, if at all. Parasites have become resistant to them. Their continued indiscriminate use will only quickly result in complete resistance.
2. Rotational deworming using ineffective compounds ensures complete resistance to them while creating a false sense of security.
3. We are already seeing pockets of resistance to ivermectin and it is inevitable that this will increase. Moxidectin (Quest) is a related compound. It is only a matter of time before we see resistance to it too. Use of these compounds without fecal testing will ensure a short effective life for them.
4. There are no new chemical compounds in the works right now. Research and development is costly and takes time. Our emphasis should be on extending the effectiveness of what we have.

Internal parasites can cause serious disease in horses without leading to external signs obvious to horse owners. Parasite eggs and worms are tiny or microscopic and rarely visible in manure. Significant parasite loads cause all sorts of problems for horses.They can reduce immunity, cause gastrointestinal irritation and damage, reduce nutrient uptake and cause generalized un-thriftiness. While these “hidden” problems are more common, parasite damage can also lead directly to colic and death. In the last few years, evidence for worm resistance to common dewormers has emerged. This is a serious problem and requires a re-examination of our current methods of parasite control in horses.

There are more than 150 species of internal parasites that afflict horses. Some frequently discussed groups include:

  • Ascarids (Roundworms)
  • Large strongyles (Bloodworms)
  • Small Strongyles
  • Pinworms
  • Bots
  • Tapeworms
  • Lungworms

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 and season are key factors affecting the survival and development of these stages. Management must take into account the life cycles andthe 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, but they 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.

Different parasites harm the horse in different ways, depending on their location within the horse and life cycle. Some parasites cause severe, life threatening damage. An example is the large strongyle, the adults of which localize in the large arteries supplying blood to the intestine. These worms cause damage to the arteries and result in loss of blood supply to segments of intestine, a potentially fatal problem which results in colic signs. Certain parasites damage vital tissues like lungs or liver. This usually occurs as larval stages migrate through the horse’s system to complete their life cycles.

Parasites can cause obstructions and ulcerations within the horse’s digestive tract. The most common example of this is obstructions caused by large roundworms in young horses (Ascarids). Bot flies cause irritation to skin as they lay eggs. Pinworms cause skin irritation and itching to the area around the anus and tail head. The most common species of Tapeworm lives in a specific segment of small intestine and can cause irritation and blockage at that site. Lungworms are common in donkeys and cause clinical signs of coughing, usually in horses stabled with donkeys. These are just some examples of the many syndromes that parasites can be involved in. More common than the obvious diseases discussed above is low-grade disease in horses that might otherwise appear to be perfectly healthy. Signs of infestation may include:

  • Dull, rough hair coat
  • Lethargy or decreased stamina
  • Weight loss
  • Coughing and/or nasal discharge
  • Tail rubbing and hair loss
  • Resistance to the bit due to mouth lesions
  • Colic
  • Summer sores
  • Depression
  • Loss of appetite
  • General unthriftiness or loss of condition
  • Diarrhea

Over the past half-century, deworming compounds have been developed which have drastically reduced parasite problems in horses. This has been a great improvement for horse health, and there are far fewer severe parasite-related problems than in the past. There have been several main “chemical classes” of dewormers that have been developed over that time. However, parasites have gradually developed resistance to these compounds, resulting in their growing ineffectiveness. 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. Historically, before deworming medications were available, many horses died from parasite infestation. When effective deworming products were initially developed, veterinarians tube wormed horses, meaning they passed a stomach tube and dosed a large quantity of the chosen chemical directly into the stomach. In the last 25 years, paste dewormer formulation were developed, which made it possible for horse owners to treat their own horses with deworming medication. This was good for a variety of reasons. First, these medications were safer for the horse, meaning less risk of accidental overdose. Second, no nasogastric tube was required, meaning the horses generally tolerated the procedure better. Finally, it was much cheaper. Historically, veterinarians recommended that owners rotate dewormers every 4-6 weeks. In light of parasite resistance, these recommendations need to be reexamined.

Daily dewormers, were developed as feed additives in order to provide a low-level, constant dose of deworming medication. No offense intended to those that developed or use these products, this is about as good an idea as treating a bacterial infection with a daily, ½ dose of antibiotic. You’re not killing a whole lot of bugs, and the majority of those that survive have been exposed to just enough drug to make them resistant to a higher dose. The infection remains, and you have created super-bugs; the same applies to daily dewormers.

We have learned certain things about equine parasites from doing fecal egg counts. We learned that we can make the worms disappear from the manure by using these 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, in the last 10 years, this has proven to no longer necessarily be true. We now know that in our area the majority of farms have parasites that are resistant to at least one entire class (remember there are 3 total classes) of dewormers, and 50% of farms in our area have parasites that are resistant to two classes of dewormers. This means that on 50% of farms, parasites are resistant to 2/3 classes of dewormers, and there is only 1 class that is consistently effective. Scientists also predict that if the historical rotational deworming strategy is not changed, that 100% of parasites will be resistant to ALL 3/3 classes of dewormers within the decade! That is a scary thought, especially considering that there are no new dewormers in the pipelines for production-YIKES!

Most parasites are killed by a properly administered and effective 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 i.e. natural selection. 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, and in the next generation there are more resistant parasites. Traditional rotational dewormer use has lead to resistance, and will accelerate the onset of further resistance. It is inevitable that this process will take place with enough time and exposure to these deworming compounds. Our goal should be to make that period as long as possible for each of our dewormers. How do we achieve this? By minimizing the exposure to these compounds through targeted deworming.

The best way to prevent development of resistance to these deworming 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 resistance. Those susceptible parasites are allowed to go on living and competing with those that have developed resistance. This is known as preserving “refugia” within the population. Put another way, one of the best ways to diminish resistance is to DILUTE it. It’s all about competition. If the nonresistant (“naïve”) parasites are able to exist and compete with the resistant parasites, they will lessen the numbers of resistant parasites. So, we have to deworm some horses less frequently, to allow some parasites to live and grow without exposure to deworming products. The trick is figuring out which horses need more deworming, and which horses only need it less. We can move toward this concept by only targeting those horses that have higher fecal egg counts. This requires knowing which horses have higher worm burdens and shed more into the environment. This knowledge requires fecal testing. By using fecal egg count results, horses are broken into 3 groups, those with high parasite burden and shedding high numbers of parasites into the environment, those with moderate counts, and those with minimal counts. Only those in the high shedding group are dewormed frequently, the others are dewormed far less often. 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 is working together with the veterinarians at McGee Equine Clinic. It is no longer acceptable to randomly treat horses or keep horses on a typical deworming rotation calendar. There is no perfect dewormer and no standard program. Fecal testing guides the program. Horses at different ages and stages have varying needs for parasite control. 20% of horses in a group shed 80% of the total parasites. Young, growing horses have some special needs. In addition, an individual horse’s parasite burden may change during the course of his own life, for example the worm counts may increased when he is ill, stressed, or a senior. 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, we recommend a fecal exam on every horse at least once annually. This is the only way to determine the effectiveness of a parasite control program and to detect the development of resistant parasites.

Before you deworm, please collect a fecal sample; simply put 1-2 fresh fecal balls in a zip lock bag, label it with your horse’s name, and deliver the sample to us. We prefer to have you give us a fecal sample at your horse’s wellness visit, so we can discuss management with you at that time. Alternatively, you can drop the sample in the medication pickup box on our porch, but please call us to let us know you have done so! You can store a fresh sample up to 12 hours if you keep it refrigerated. At our clinic, we do this testing in-house. The sample must be taken at least 8 weeks after the most recent deworming, or 12 weeks after deworming with a Quest compound. Otherwise there is still residual effect from the prior deworming, that can falsely effect the egg count results.

We will perform a fecal egg count on these samples and determine which horses are low (< 200 epg), moderate (200-500 epg) or heavy (>500 epg) shedders. The specific parasite species are identified and from this, a determination can be made of the most effective deworming compound. We then have you deworm the horses with the appropriate compound. Two weeks later, fecal samples should be taken again to perform a Fecal Egg count reduction test. This is a retest of a manure sample 10-14 days after worming with the recommended compound, to determine what percentage the parasite eggs were reduced. We consider a dewormer to be effective if the postdeworming sample has been decreased by more than 95%. If you have resistant parasites, in the follow up sample the egg counts often will not have gone down at all! We will then create a customized approach that is tailored to your situation. Horses are dewormed with the appropriate compound based on their Fecal Egg Count category.Testing is recommended at least annually, the cost of testing should be offset by big savings in the purchase of deworming compounds.

1. Currently used deworming compounds can be divided into three basic groups effective against worms other than tapeworms and one compound specific to tapeworms:
Benzimidazole Products. The oldest class and that with the highest resistance in small strongyles. Examples include Oxibendazole (Anthelcide EQ) Fenbendazole (Panacur, SafeGuard).
2. Pyrantel Products. Less resistance but a rapidly growing problem. Examples include Strongid paste, Strongid C, Rotectrin 2. These have some effect against Tapeworms.
3. Ivermectin/ Moxidectin. Two related compounds. We are seeing the beginnings of resistance problems but these are still very effective against a wide variety of parasites. These products kill Bots in the stomach. Moxidectin is the newest deworming compound, has the fewest resistance problems, and kills encysted small strongyles.
4. Praziquantel. This compound is specific to Tapeworms, which are not killed by the above compounds. Praziquantel is added to one of the others to kill tapeworms.


  • Any deworming today should be based on fecal exam results.
  • It is best to dose before feeding because horses with feed in their mouths can more easily spit out the paste.
  • For Quest products, the dose must be calculated based on the horse’s weight, as overdose is possible. We do not recommend Quest dewormers for youngsters, minis, or any horse that is otherwise ill or debilitated
  • Tapeworm eggs do not float in fecal solution like other species, so they are hard to diagnose. Since they have been implicated in a severe type of colic, we recommend treating against them once yearly in all horses. Products with Praziquantel are available and have excellent effectiveness against tapeworms and should be used as directed.
  • Alternative dewormers like diatomaceous earth: I have personally seen horses on these products that still have a high parasite load. As of now, there is little published evidence for their effectiveness.
  • Remove bot eggs regularly from the horse’s hair coat to prevent ingestion. Bots generally do not cause serious disease but if they can be removed it means less will be ingested.
  • Foals should be dewormed the first time at 4 weeks of age, then every 4 to 6 weeks until they are a year old with a compound that kills ascarids (oxibendazole). After that, they are monitored with fecal egg counts and treated as adults.
  • Pregnant mares should be dewormed with a safe product a few weeks prior to foaling to decrease the foal’s exposure to parasites, unless they are ill or have complicated pregnancies.

Another important consideration is pasture management. Since parasites are primarily transferred through manure, good stable and pasture management is also key. With this in mind, we suggest the following:

  • Pick up and dispose of manure from stabled horses on a frequent basis.
  • When possible, use a feeder for hay and grain rather than feeding on the ground.
  • If you have the space for it, consider dragging and then RESTING a pasture for about a month during the summer, to allow parasite eggs to dry out and die
  • While eggs may be slow to develop to infective stages in cold weather, they often survive, awaiting the right conditions. Many parasite eggs survive on snow and ice for the winter and may resume their life cycle in the spring.
  • Spreading manure on pastures without first composting it will spread parasite eggs on the pasture and can lead to heavy pasture contamination and re-infestation.
  • Rotate pastures by allowing other livestock, such as sheep or cattle to graze them. This interrupts the life cycles of equine parasites.
  • Keep the number of horses per acre to a minimum to prevent overgrazing and reduce the fecal contamination per acre, or use rotational grazing.
  • Any manure you collect should be brought as far away from your barn and pastures as you can manage it, and then allowed to compost until well broken down.

In the past, it was taken for granted that frequent rotational deworming was the best way to reduce parasite resistance. While rotation has its place in the new paradigm, we are learning that there is more to it than that. Parasite resistance is a real and growing threat. Also remember, that parasite resistance is an ever changing problem; it seems that no matter what we do, the parasites find a new way to adapt to it! Resistance is a problem that veterinarians and horse owners need to work together to manage. Resistance is inevitable but our goal in changing the paradigm now is to slow the problem, and extend the period of effectiveness of our currently effective compounds.