horse getting deworming medicine

Horse Deworming Schedule: An Introductory Guide for Equestrians

Reviewed by Dr. David J. V. Drum DVM MS

Horse deworming is not the most exciting part of owning a horse, but it is a regular and crucial aspect of keeping your horse healthy. All horses are susceptible to parasites from grazing and require treatment, but not every horse has the same deworming schedule. It’s important to have a conversation with a veterinarian to determine the best treatment plan for your horse, but first, we’ll help you understand what a horse deworming schedule looks like and how to incorporate it into your routine.  

What does it mean to deworm a horse?

It’s completely normal for a horse to have worms or parasites it ingests while grazing, but they must be kept under control so your horse stays healthy. Every horse needs to be dewormed on a routine basis with an antiparasitic drug called an anthelmintic, commonly referred to as a dewormer. Parasites are harmless to a horse in small populations, but dewormers keep them from growing beyond a healthy level and harming your horse. Consult your vet to help you deworm strategically and on a consistent schedule for the most effective results and to prevent the parasites from developing resistance to treatments. 

Why deworm a horse?

If you don’t deworm your horse on schedule with the recommended drugs, parasites will grow to harmful levels that can damage the intestine and blood vessels and cause colic, anemia, ulcers, weight loss, and even death in some cases. Depending on the type of parasite, your horse may or may not show symptoms, so deworming treatments must be administered even if your horse seems healthy. 

Common types of parasites in horses

1. Bloodworms (large strongyles)

A horse picks up strongyles or bloodworms by eating contaminated pasture. These parasites are some of the most harmful to horses. They’re called bloodworms because they feed on a horse’s blood, causing anemia and weakness. Strongyles also migrate through the horse’s arteries and large intestine, which leads to colic. If the strongyles block a horse’s arteries, the intestine could lose part of its blood supply. Different drugs are administered to treat larvae than adult bloodworms–larvae are treated with ivermectin and moxidectin, and adults are treated with oxibendazole, fenbendazole, or pyrantel pamoate.

2. Small strongyles

Small strongyles are the most common internal parasite in horses and are picked up from contaminated pasture grass like large strongyles. They live in the large intestine and feed on the lining. The larvae encyst in the intestinal wall and develop into adults in the spring that require different treatment than larvae. These worms cause gastrointestinal issues, like ulcers, diarrhea, and weight loss. 

3. Roundworms (ascarids) 

Ascarids or roundworms are more common in young or old horses with lower immunity. As larvae, they migrate from the trachea and live in the small intestine. Some may even migrate to the lungs and cause respiratory issues. Large numbers of worms can cause an obstruction of the intenstines. Roundworms can lead to death if left untreated in young horses. 

4. Bot flies 

Bot flies lay eggs on a horse’s coat that cause yellow spots on the horse’s legs. The eggs are irritating to horses, but when they lick their legs, the saliva activates the eggs. The eggs molt into larvae in the horse’s mouth and damage the gum and tongue. Once they migrate to the stomach, they cause gastric ulcers. 

5. Pinworms

Pinworms are most common in younger horses. They’re difficult to treat and irritating but not as dangerous as other parasites. They lay eggs on outside the anus and irritate the horse. The horse can hurt himself from over-itching. 

5. Tapeworms

Tapeworms aren’t as common as other parasites, but they are hard to prevent because they live on mites that are too tiny for horses to see while grazing. Horses ingest the mites, and the larvae grow into adult worms inside their intestines. Tapeworms suction to the wall of the intestine and may rob the horse of essential nutrients. They can also cause colic that may become severe. 

7. Threadworms

Threadworms are rare today but are usually ingested by foals younger than six months old through the mare’s milk. They live in the small intestine and can cause swelling of the small intestine, mild to severe diarrhea, dehydration, poor digestion, and poor growth. 

How to deworm a horse

1. Conduct a fecal egg count test.

You or a veterinarian can conduct fecal parasite check along with a fecal egg count (FEC) test to determine what kinds and how many parasites your horse may have so you can treat your horse most effectively. The fecal egg count is the number of eggs found per gram of a horse’s feces. The number can correlate to how many adult parasites of a certain species are present laying eggs in the horse’s gut. Grazing always introduces the potential for a horse to ingest parasites, so your horse should be tested at least once a year. It’s also a good idea to test your horse again 2 weeks after deworming to make sure the number of parasites is reduced. Your vet may also want to collect blood samples to determine if the horse has developed anemia. 

2. Consult a veterinarian to develop a treatment plan. 

There’s no standard deworming plan that works for all horses. A veterinarian can help you choose the best fit for a horse based on its age, breed, pasture, stocking density, and FEC or blood test results. The vet will recommend the best dewormers for your horse and help you develop a deworming schedule. 

Several classes of drugs are used to treat parasites:

  • Benzimadoles consist of fenbendazole or oxibendazole and are used to treat large and small strongyles, ascarids, and pinworms
  • Pyrimidines consist of pyrantel used to treat large and small strongyles, ascarids, and pinworms
  • Macrocyclic lactones consist of ivermectin or moxidectin used to treat large and small strongyles, ascarids, pinworms, and bots
  • Tapeworms are treated once a year with praziquantel. 

A vet should guide you in choosing the best dewormers and when to administer them. A fecal egg count or blood test after treatment can determine if the dewormer was effective. He or she may recommend treating your horse with an alternate dewormer every other year to prevent drug resistance.

3. Administer the dewormer. 

Dewormers come as pastes or pelleted feed. Pelleted feed dewormers are easier to administer, whiles pastes are a little more challenging. You feed your horse the paste through a syringe in the side of his mouth. If your horse doesn’t corporate, you can hide the paste in a treat, but you’ll need to watch carefully to make sure he doesn’t spit it out and that he eats all of it. Your horse’s weight and the type of dewormer used determine the proper dosage.

4. Maintain good pasture hygiene and management.  

Deworming treatments are most effective when combined with well-managed pasture since the pasture is where your horses get infested. Here are a few tips for good pasture hygiene. 

  1. Try not to feed your horse off the ground of a stall. The floor could be soiled and an easy source for infestation. 
  2. Remove droppings from the paddock once a day or a few times per week and regularly mow your pasture.
  3. Rotate paddocks and give them time to rest to prevent overgrazing. Overgrazing increases the risk of your horses consuming contaminated grass because there’s no time for manure to break down.  
  4. If you get a new horse, deworm it and keep it in a separate paddock from the other horses for a few days to avoid infesting the others. 

When to deworm a horse

Spring and late fall are the best times of year to deworm because that’s when horses are most likely to be infected. How frequently to deworm a horse also depends on the horse’s age and shedding rate. Also, if you own only one or a few horses, they will need to be dewormed less frequently than at a stable with a lot of horses in close quarters. 

Foals, yearlings, pregnant mares, and older horses with weakened immunity all need to be dewormed differently than adult horses. 

  • Foals should be treated at 2 months, 5 months, 9 months, and 12 months for different parasites. 
  • Yearlings receive 3–4 treatments a year for the first 2 years of life with dewormers depending on the fecal egg count. 
  • Pregnant mares should be dewormed before they foal and 24 hours after the birth.

An FEC test will also give your horse’s shedding rate based on the eggs excreted per gram of feces (EPG) and guide you in developing a deworming schedule. High shedders are horses that shed a lot of eggs in their manure. These horses typically makeup 20% of horses on a farm and will most likely infect others and must be dewormed more frequently than moderate or low shedders. FECs don’t reveal every type of parasite, so it’s important that all types of shredders have a deworming schedule. 

To make sure you follow the best practices for deworming, consult the AAEP Internal Parasite Guidelines

Creating a horse deworming schedule

To determine the best horse deworming schedule you need to know which parasites your horse needs to be treated for and whether it’s a high, medium, or low shedder. Consult a veterinarian before planning your deworming schedule to make sure horses receive the most effective treatment and dosage. Deworming too much can lead to resistance–your vet can help you find the right treatment balance based on your individual horse’s needs. A deworming schedule varies based on vet recommendations and will resemble the plan below. 

Example deworming schedule for adult horses 

Low shredders (EPG 0–199): 1–2 treatments per year

Moderate shredders (EPG 200–500): 2–3 treatments per year

High shredders (EPG >500): 3–4 treatments per year

*Please note this is an example deworming schedule. Consult your vet for a schedule customized to your horse’s needs.

Shredder Time to treat Drug Type  Trade Name Parasites 
Low, Moderate, High Spring  Ivermectin/ Moxidectin  Equimax®, Zimecterin, Quest®, Rotectin®, IverCare® Large and small strongyles, ascarids, pinworms, bots
Low, Moderate, High Summer Pyrantel Pyrantel Paste®, Exodus®, Strongid® Large and small strongyles, ascarids, pinworms
Moderate, High Early fall  1. Ivermectin2. Fenbendazole/ Oxibendazole 1. Exodus®, Strongid®2. Panacur®, Safe Guard® 1. Ascarids, pinworms2. Large and small strongyles, ascarids, pinworms
Low, Moderate, High  Late fall/ early winter  1. Ivermectin/ Moxidectin 2. Praziquantel  1. Equimax®, Zimecterin, Quest®, Rotectin®, IverCare®2. Quest Plus®, Zimecterin ® 1. Large and small strongyles, ascarids, pinworms, bots2. Tapeworms

How much does it cost to deworm a horse?

The cost of deworming depends on your horse’s treatment plan and the dewormers recommended by the vet. Deworming costs are highest for high shredders that require more intensive treatment. Dewormers generally cost $5–$30, but you also need to factor in the costs of the FEC tests and vet visits. 

Are there natural ways to deworm a horse?

Deworming drugs and good pasture hygiene are the best ways to deworm a horse. There’s no proven way to kill parasites without them. Deworming drugs keep your horse healthy, and choosing not to use them puts your horse at risk. The risk of harmful amounts of parasites is higher in domesticated horses than in the wild, and we must care for our horses responsibly. Horses will always have some parasites–they’re only harmful at certain levels, which is why we use FEC tests and levels of shedding to determine dosage and frequency for the best results. 

Deworming is just one way to keep your horse happy and healthy. Learn what to feed your horse with the best health benefits or how to make horse treats your horse will love

Parasites aren’t the only irritating and potentially harmful pests your horse encounters. Horse flies are another nuisance you can get rid of with a horse fly spray from our list of 6 of the best horse fly sprays that really work. You can also learn how to help your horse face the heat with our guide to caring for your horse on a hot day.

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