How to Deworm a Horse

Co-authored by Pippa Elliott, MRCVS

Last Updated: October 21, 2021

If you notice symptoms of pain or discomfort in your horse, you may need to check for worms. Worms, internal parasites, can cause colic, diarrhea, weight loss, ulcers, mouth sores, or pot belly. Fortunately, a variety of treatment options exist. If you suspect worms are the cause, get your veterinarian involved so you can get effective treatment for your horse.

Method 1

Method 1 of 3:

Preparing to Deworm Your Horse

1
Determine if your horse needs deworming. Since this will completely depend on your horse, you'll need to submit a feces sample to your vet. The vet will run a fecal egg count test to find out if your horse has worms and what kind they are. This allows your vet to choose the most effective dewormer for that particular infestation and how often your horse needs to be dewormed.^{[1]
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- For example, a horse with a heavy worm burden may need monthly deworming, but one with few worms could be done quarterly or even twice a year.^{[2]
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2
Talk with the veterinarian about deworming resistance. Deworming your horse unnecessarily can lead to an increased resistance to the worms. For this reason, your vet may recommend rotating deworming drugs (giving four to six different dewormers) before starting back at the beginning treatment. Since some horses can develop a natural resistance to worms, they may not need deworming at all which is why a fecal analysis is important.^{[3]
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- If your horse has a heavy worm burden, a daily dewormer might be help prevent heavy worm infestation between regular deworming. But, daily dewormers shouldn't be considered a complete deworming strategy on their own since their effectiveness is limited against certain parasites and total deworming treatment is required before starting them.
3
Discuss medications. Your veterinarian will talk with you about deworming medications. These include fenbendazole, ivermectin, moxidectin, pyrantel, and praziquantel. Since each drug has a subtly different spectrum of activity, your vet will decide what medication will work best for your horse.^{[4]
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Research source} The following are drug classes to consider:^{[5]
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- Benzimidazoles (such as fenbendazole) can kill nematode eggs, unlike other drug classes, and are available as granules, pastes, and suspension formulas. These medications are usually given several days in a row.
- Macrocyclic lactones (such as ivermectin and moxidectin) are some of the most effective medications and can, in addition, kill external parasites like lice and mites. They do take 3 to 4 days before treatment is effective.
- Isoquinoline-pyrazines (such as praziquantel) are only effective against cestodes (in horses, Anoplocephala spp., also known as tapeworms.) In the United States, this drug is only available in combination with macrocyclic lactones.
4
Correctly dose your horse. You'll need to know your horse's weight since the medication is dosed by body weight and prescribed by your vet. Treatment will be ineffective if not enough medication is given, which can also encourage drug resistance. The majority of dewormers are very safe, so it is better to overestimate the amount rather than underestimate, especially if your horse dislikes dewormers and is inclined to spit them out. Plan to give extra in anticipation of the horse spitting part of the dose out.^{[6]
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- Be aware that many dosing syringes of horse dewormer contain enough for a 1,200 lb horse. If you have a large horse, you may need two syringes. Some veterinarians suggest adding on the amount required for another 250lbs (one mark on the dosing syringe) regardless of your horse's size.^{[7]
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- To find out your horse's weight, buy a measuring tape from a feed merchant or tack store. Measure around the horse's chest and see how the weight corresponds on a scale that gives an estimate of the horse's weight.
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Method 2

Method 2 of 3:

Deworming With a Dosing Syringe

1
Get the horse and medication ready. Your horse's mouth should be empty, with no wads of grass or straw in it. These could make it easier for the horse to spit the medicated paste out. Your horse should also wear a halter which will give you something to hold onto when administering the dewormer. Have the syringe ready by removing the cap and dialing the dosing ring back to the appropriate mark on the barrel of the syringe.^{[8]
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- You may want to feed and water your horse before giving the medication, since the taste of dewormers might keep your horse from eating or drinking for a while. This is especially important if your horse has exercised and is hotter or thirstier than usual.^{[9]
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2
Insert the syringe. Aim the nozzle onto your horse's tongue by lifting the lips and angling it towards the back of the mouth. The best place to place the syringe is the gap between the incisors and canines at the front of the mouth and the molars at the back.^{[10]
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Research source} Make sure you're able to hold the plunger in such a way that you can easily press the plunger.
- Do not worry too much about the exact placement of the nozzle. As long as it is inside the mouth, chances are the dewormer will coat either the tongue, teeth, or lining of the cheek and most of it will be swallowed.^{[11]
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3
Administer the dewormer. Press the plunger down to squeeze the medication into your horse's mouth. Be quick so as not to upset your horse. Once all the medication is inserted, remove the plunger and hold your horse's head up for a few seconds. This will keep him from spitting the dewormer paste out.^{[12]
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- If your horse routinely spits out the medication, try inserting smaller amounts at a time or mix the paste into his food.
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Method 3

Method 3 of 3:

Deworming With a Pellet Dewormer

1
Buy a pellet dewormer. Pellet dewormers are mixed with your horse's grain. Ask your veterinarian to recommend a particular pellet dewormer brand and dosage. The package you choose should also say how often to feed your horse, based on weight.
- Write down a deworming treatment schedule as recommended by your vet. You'll need to feed your horse the right dose of pellet dewormer for as long as suggested.
2
Measure the pellets. Use the scoop that's included in the package to measure out the correct dose of pellet dewormer. Mix this into your horse's grain feed. Only mix enough for a single feeding, not the entire day.^{[13]
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Research source} Avoid mixing the pellets into treats rather than grain.
- Grain ensures that the whole dosage is consumed, while your horse could pick around the pellets or refuse them altogether if you include them with treats.
3
Feed your horse the pellets. Offer your horse the grain feed mixed with dewormer pellets. Make sure he finishes the food, so you can be sure that all the medication was eaten. If your horse won't eat all of the pellets and grain, add a little liquid molasses to encourage him to finish it.^{[14]
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- If you plan on feeding your horse hay, be sure to wait until after he's eaten all of his grain feed with the pellets mixed in.
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Tips

Although not nearly as common as plungers or pellets, deworming vaccines are also available. Only a vet or experienced horseman or horsewoman should give the vaccine, inserting it where directed.

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Be prepared for severe "pull-back reactions" that some horses have when faced with unfamiliar movement or materials the head. A severe pull-back can often cause damage to equipment or injury to the horse, especially if the horse is tied.

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Warnings

Horses are big and can react violently to uncomfortable situations. If you are unsure of yourself, ask the vet to show you how to administer the dewormer.

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