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You are deworming your horse wrong.

How do I know this? I don’t even know how you’re deworming your horses, but chances are if you are reading this, it’s true. New deworming recommendations were released in 2013 by the American Association of Equine Practitioners (AAEP), and they contain guidelines that are vastly different from what the equine industry has been doing for the past four decades. During this time, the ideal way to deworm horses was thought to be “Rotational Deworming,” or switching up the drug used each time you deworm, and deworming roughly 6-8 weeks. Odds are, this is most likely what you are doing. We all have for the past 48 years. Unfortunately, this has placed us in a very scary position. The parasites are becoming resistant to all of our deworming medications, and there are no new drugs being researched at this time. Other species, such as goats, are in an even worse position than we are now. Whole farms are finding that NO medications work on their goats. This is terrifying, as the parasites that target goats cause more than just gastrointestinal problems – they more or less suck their blood, causing life-threatening anemia. On a farm with complete drug resistance, these goats will die from their parasite problems, no question about it. If horse owners and managers do not change what they are doing now, we will doubtlessly be in the same situation with the next couple decades.

Hopefully I have scared you at this point. If not, please Google “parasites in horses” and look at the pictures. If nothing else, I hope you are at least a little disgusted, and perhaps that you were eating while reading this. You’re welcome!

“So,” you ask, “how do I deworm my horse properly then?” Well, it’s a little more complicated than just deworming differently. We need to stop treating horses as a herd, and actually treat them like the individuals they are, like in human medicine and small animal medicine. When was the last time you dewormed yourself? (Hopefully it hasn’t been recently!) Do you just randomly deworm your dogs and cats? No, you get your vet to do a fecal, and you base how you treat your animal on those results. This is also the way we need to start treating our horses. Not all horses need to be dewormed all the time. Some do. The only way to know the difference is to perform a fecal exam.

Before we dive into the nitty gritty of fecals and deworming, let’s cover some terminology. Bear with me....this might be brutal.

  • Shedders/Shedding: This has nothing to do with hair, but rather refers to how many parasite eggs an individual animal releases in their feces. Adult worms (and other life stages) exist inside the animal, and to secure their continued existence as a species, they lay eggs, which are released into the intestinal tract and are passed into the environment in the manure. Egg shedding has been shown through multiple studies to be specific to the individual animal, in other words, it doesn’t matter how infested your horse is, it is a function of his own body that determines how many eggs make it into his manure. This most likely is dependent on the immune system of each individual. Studies have also shown that shedding status (how few or many eggs they shed) does not tend to change during an animal’s lifetime. This is helpful, as you do not need to continue to do many fecals throughout their lifetime. One properly executed determination usually does it! Based on the results of a fecal egg count, we place each individual into a shedding category of low, medium, or high.

  • Fecal egg count (FEC): A fecal egg count examines a manure sample to determine the number of strongyle eggs per gram, and therefore is considered a quantitative test. This is important, as you need an true egg count to properly classify your horse. Qualitative fecal exams exist also, and are generally less expensive, but do not provide the most useful information.

  • Fecal egg count resistance test (FECRT): This is the way we figure out if a dewormer is no longer considered effective in a particular group of animals. This method requires performing a FEC, deworming with one drug, then performing a second FEC later. The egg count should be reduced by at least 95% to be considered effective. Anything lower than that number is indicating growing or complete drug resistance. Ideally, you would periodically determine that each drug you are using is still working well. Also note, that unlike a FEC which looks at each horse's status, resistance is specific to the parasites themselves, therefore, it is not necessary to test each horse as a herd of horses will be sharing the same population of parasites.

  • Egg Reappearance Period (ERP): This is the amount of time after deworming that it takes for parasites to be found again in the fecal exams. Each deworming medication has a different ERP, and as resistance begins to develop, the ERP begins to decrease. It is important to know that since the introduction of all of the currently available deworming medications, all of their ERPs have decreased. As their ERPs continue to decrease, none of them will work on any horses. Scary stuff.

Okay, apparently I haven’t bored you to death yet. Thanks for sticking with me. So, let’s discuss the parasites themselves so you better understand why we are recommending these new changes:

Large Strongyles

These used to be the big bad guys back in the 60’s, causing the most problems in horse. Parasitologists and drug companies worked hard to develop dewormers and recommendations to combat large strongyles. And you know what? They did a great job! These are no longer a major issue in our horse population as a whole. However, you know that fantastic deworming protocol they developed? It’s more commonly known as “Rotational Deworming,” and is very much the protocol still used by horse owners today. In other words, most owners are still fighting parasites which are no longer an issue. We’re not going to talk much more about these, other to mention that these can migrate outside of the intestinal tract, causing some pretty serious and potentially life-threatening collateral damage. Most reasonable deworming programs take care of these.

Small Strongyles

These are our actual main problem today. The FECs that I mentioned before provide counts on how many strongyle eggs are present in a sample. Large and small strongyles cannot be differentiated on a FEC, but the large strongyle population is so low now that it doesn’t matter. Small strongyles cause problems not only on the inside of the intestinal tract - they also burrow into the lining of the intestinal tract itself, causing inflammation that decreases the animal’s ability to absorb food properly. This ability to “encyst” is special to the small strongyles, makes it harder to kill them with drugs, and needs to be addressed properly in the horse’s deworming protocol.


Tapeworms are not a huge problem in horse, but some schools of thought believe that, especially in large numbers, they can contribute to intestinal issues such as colic. Note that the research to reinforce this idea is not abundant. With that said, you still should try to keep your horses tapeworm free. This is very easy to do, and they only need to be dewormed once a year against tapeworms, but you need to understand that only one drug kills tapeworms, praziquantel. If you are not using a dewormer once a year containing praziquantel (Equimax, Quest Plus, or Zimectrin Gold), you are not killing tapeworms.


If you’ve ever seen a live ascarid in person, you most likely have not yet recovered from the emotional trauma. These worms are the creepiest parasites that horses deal with (personal opinion), and can cause considerable medical issues in horses infested with them. Young horses are the main population affected by ascarids, and if not dewormed properly, these can directly kill them. Horses that are suspected to be at high risk for containing ascarids need to be dewormed carefully, as large die-offs of this worm often lead to a huge glob of dead/dying roundworms getting stuck in a section of the intestinal tract. These impactions often can only be fixed through surgery. As a whole, though, foals that are properly dewormed during their first year will not have ascarid issues.


Think itchy butts…you know, that horse who will find anything butt-height to scratch himself on. These worms are not deadly, but do cause aesthetic issues on the hind end. And lots and lots of itching. If needed, there is a pretty easy diagnostic test to find these, but most horses that are routinely dewormed properly do not have them.


Bots show up in many places on and in the horse. Those white eggs you see on your horse’s hair coat in the fall? Bots. I have seen many a writhing pink bot larva in horses’ mouths while doing dentals in the fall. Their journey’s end is the stomach, where they are not super damaging, but in large numbers can cause inflammation that can make the horse uncomfortable. Once again, with proper deworming, you don’t need to worry about these, although cleaning the bot eggs off their coat will decrease the amount they consume.

Those are your major players in current equine parasitology. Now, I’m going to be realistic. Unless your horse lives in a sterile bubble, he’s going to be exposed to all of these. They are all going to get inside of him. They are in every speck of dirt (that’s the technical term) that horses live on. So, since this is the case, we need to be realistic with the goals of our deworming program for each farm. First of all, understand that your goal is NOT to eliminate these parasites from your farm. This is not possible, no matter how good you are. Your horse will always be exposed to worms and will always have them inside of him. As long as we keep the numbers low, though, your horse will have no medical issues with these parasites. The sooner you accept this, the happier you will be, especially if you border on OCD like me.

With that said, our goals for parasite control fit into three categories:

  • Maintain healthy animals and prevent clinical illness.

  • Use changes in the environment (some of which we can control) to minimize each animal’s exposure to parasites.

  • Maintain drugs that work.

That’s it. These are completely reasonable and accomplishable goals. If we try to completely eliminate parasites, our drugs won’t work in the future. All of these goals must be accomplished to move forward in a responsible manner and protect not only our current population of horses, but future generations.

I mentioned fecal egg counts (FECs) before, and these are integral in our attempt to increase our

level of responsibility in deworming our horses. As I said, each horse sheds differently than his neighbor. Interestingly, though, when you look at an entire herd of horses, only 20-30% of that herd sheds 80% of the eggs. Think about that. That means that 75% of each herd sheds only 20% of the eggs on each farm. Enter, “Strategic Deworming.” Strategic deworming exploits these huge differences in numbers, takes a well-informed look at the shedding status of each animal, at the life cycles of each parasite, and at the environment in which each horse lives, and forms a specific protocol for each horse. By treating each horse, parasite, and community individually, ALL shedding categories can be dewormed less frequently. The low shedders are dewormed only twice a year, while the high shedders are dewormed four times a year. Sitting down and considering all these factors actually allows us to deworm everyone less, and get away with it! What does this actually mean? You are treating your horse in a more medically responsible manner, AND you are saving money while doing it. Win-win situation, huh?

I bet you’re ready to run out and grab a big bag of manure now, but it’s not that easy. We can’t just do these fecals whenever. During certain times of year, the parasite loads on the pasture are much lower, and may skew these FEC results. We recommend doing these exams outside of the summer months in our area, as the high temperatures in the summer kill many of the parasites in the environment. Moreover, you can't just randomly grab a fecal sample during the rest of the months. We need to do this in a more controlled manner to get an interpretable result. That brings us back to egg reappearance periods (ERPs). Since each drug has a different ERP, you need to understand what the ERP is for that drug, and to NOT get a fecal during that time, as there should be no eggs shedding during this period if the dewormer worked. When you are preparing to grab your fecal, consult with your veterinarian as to when to do it, so you get good results. As it is spring right now, we are recommending that our clients deworm with moxidection (Quest), which has an ERP of 10-12 weeks, therefore they need to wait 10-12 weeks after deworming to bring their fecal to us. Once this happens, we can classify their horse into a shedding category, and will recommend a specific deworming schedule for each horse. Depending on your area, these schedules may be different, so please rely on your veterinarian to provide you with the best schedule for your horse. As an example, though, we recommend our low shedders to be dewormed with Quest in May and Equimax in November. The Quest gets the encysted small strongyles that become active in the spring, while the Equimax gets the tapeworms and stomach bots that become active in the fall. There’s not a big need to deworm in the sultry summer months here, as we are not the only ones who can barely survive the heat…the parasites don’t either! Deworming during these months is a waste of money for adult horses.

We have many clients each year that agree that horses are dewormed too often, but fall back instead on “natural” or alternative treatments (see note at end). As happy and green as these treatments sound, they do not work. Efficacy of non-FDA approved medications for deworming has never been proved in formal, controlled studies. These products exist on store shelves because they exploit the terminology of “drugs” versus non-drugs. “Drugs” must be thoroughly (and usually expensively) tested and proved to work before they are permitted to be sold to the public. Non-drugs, also known as “neutroceuticals” do not have to undergo any of this testing. I could package jars of dirt from my backyard and sell it as a miracle dewormer, and it is completely legal. Ethical? No, but legal? Yes. Bascically, manufacturers of neutroceuticals can claim anything they want, and they make billions of dollars every year based on false claims. Note that neutroceuticals are not just animal products. These exist in human form, also. If it does not say “FDA approved” on the packaging, it’s a neutroceutical. Think about that before the next time you drop $50 on that bottle of weight loss pills. Trust me, if there was a safe, easy, natural dewormer for horses, we would already be using it. We've definitely done our reading on this, and I swear we're not protecting its identity to make you spend more money. If you want a really natural way to control parasites, keep your horse's environment clean. That's pretty safe for your horse. I'll talk about that more later.

When deworming your horses, you also need to know that age is a very important consideration. As foals have no immune defense developed against these parasites yet, they do need to be dewormed four times within their first year. I like to do my first deworming at 2-3 months with fenbendazole, which has a little better efficacy right now than the other drugs when killing ascarids. After that, I deworm every 3 months for the rest of the first year, being certain to use a product containing praziquantel on the third or fourth deworming. It may also be useful to check a FEC around 6 months or so to make sure the foal does not have a particularly high level of ascarids (and therefore should be treated more carefully).

Another important part of a parasite control program is environmental control. You can place a horse on the best deworming protocol ever, based perfectly on their needs, but if that horse lives in a manure pit, what difference does it make? Therefore, paying attention to the environment that the horse lives in is a huge part of minimizing exposure. There are many, many things you can do to help. Not everyone can do every item on this list, but the more you can do, the less likely they are to run into parasite issues.

  • No manure = No worms. Clean your horses’ stalls frequently. Remove all manure from their pastures, and place it in a pile that they cannot access. Dragging pastures is not optimal for parasite control, but if you are going to do so, only do it during the very hot seasons when the temperature has a reasonable chance to kill the worms that you are spreading. If you absolutely insist on spreading manure on fields, you can compost it before doing so. Composting for an adequate period of time does produce temperatures inside the pile high enough to kill the parasites.

  • Pasture rotation. Not everyone has vast tracts of land to work with, but you may still be able to pull off pasture rotation. Ideally, each herd would rotate between 3 pastures over the course of a year, leaving each pasture vacant for 6-9 months at a time. This time period exploits the parasites’ life cycles, resulting in killing them off before they can get back in an animal.

  • Rotate pastures with food or fiber animals. Equine strongyles are very species-specific. If your horse is going into a pasture that has had cows or goats for a year, there will be very few strongyles for them to eat.

  • Don’t treat during months when parasites don’t survive well in the environment. As mentioned before, this is a waste of money, as your horse will not be consuming many parasites anyway. Furthermore, the parasites that they are consuming are going to be the hardiest of of the bunch. This is a great way to encourage resistance to drugs - expose only the body builders of the parasite world, and watch resistance happen!

  • Keep stocking density low. Which pasture is going to have a higher concentration of parasite eggs? A ten acre pasture with 1 horse, or a 1 acre pasture with 10 horses? Obviously the latter. Lower stocking density waters down the supply of parasite eggs to consume.

  • Biosecurity includes parasites. Biosecurity is a big word for decreasing the spread of disease from one animal to another. This is often applied to vaccine-controlled diseases like influenza, but is also important when it comes to parasite control. If you have a new horse coming to your farm, you should immediately deworm it with a larvicidal drug (one dose of moxidectin or a five-day protocol of fenbendazole will kill encysted strongyle larvae), and as soon as possible determine that horse’s egg shedding status.

  • Every 3 years, test for drug resistance. This is where you utilize the fecal egg count reduction test (FECRT). An easy way to do this is to pick one of your highest shedders, and test one drug per year on them. If the egg count does not reduce by 95% on the second fecal, you have a problem.

If your mind is not officially blown at this point, congratulations! These are absolutely revolutionary changes to the status quo of equine deworming, but we must change what we are currently doing if we don’t want to be in an irreparable position in a couple decades. Denial won’t work. Parasite resistance is real, and it is coming. Change what you are doing now!



Many of these products claim to "work mechanically, not chemically, to microscopically abrade the outer coating of parasites, causing dehydration, or loss of body fluid, which is fatal." (That was a quote from the label of Worm Free Naturally). If this is true, you probably don't want to feed it to your horse, as it will probably abrade your horse's intestines, too. Unfortunately this is not an uncommonly claimed mode of action for natural products. Sigh... Just use real drugs, but use them responsibly.

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