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Deworming Decoded

As equestrians, we want our horses to be as happy and healthy as possible. In order for our horses to thrive, we provide them with ample amounts of clean water, high quality forage and grain, and work with a team of equine health care professionals like veterinarians, farriers, massage therapists, and chiropractors. 

Another crucial piece of the horse health puzzle is to control the amount of internal parasites that live within your horse’s digestive system. Deworming is the process of using low levels of antiparasitic medications to eliminate these potentially harmful internal parasites. The more you know about internal parasites in horses, the more apparent it becomes that they must be properly controlled in order to have a healthy horse.

Keep reading to learn how worms may affect your horse and how you and your veterinarian can work together to determine which deworming products may be appropriate for your horse.

Weapons of Mass Destruction

If parasitic worms reach high levels in your horse’s system, they can wreak havoc on the health of your horse. Often a dull coat is the first sign that a parasitic infestation is brewing. As worms begin to reproduce rapidly in your horse’s system, the symptoms may become increasingly severe. 

Weakness, diarrhea, anemia and colic may signal that your horse has high levels of Large Strongyles in his system. Small Strongyles can cause diarrhea and malnutrition, which in turn may cause drastic weight loss in your horse. When ingested, Bots can lead to inflammation and ulceration of the stomach, while a poor body condition, anemia and continued weight loss may be indicative of Tapeworms.

According to Dr. Katy Raynor of the New England Equine Medical and Surgical Center, a large amount of Small Strongyles and/or Tapeworms in a horse’s system is a common cause of colic. 

Know Your Enemy

We tend to use the broad term “worms” to describe a wide variety of internal parasites. Let’s take a look at some of the most frequently seen worms in the Northeast to assist you in understanding their lifecycle and how they affect your horse:

  • Small Strongyles are some of the most common parasites to be found in horses. Once Small Strongyle larvae are ingested by your horse, they travel through their digestive system; they then burrow into the walls of your horse’s large intestine and cecum. The larvae stay in this encrusted state typically for a few months, but sometimes as long as three years, until the weather conditions are appropriate for them to continue maturing and finally lay more eggs which are excreted in your horse’s manure. Once the eggs have exited the body, they hatch into larvae which attach to grass and thus the cycle continues.
  • Less common but more dangerous than their smaller counterparts, Large Strongyles are introduced to your horse’s digestive system when your horse eats grass that has been infected with Large Strongyle larvae. The larvae then travel through the digestive system until they reach the cecum and colon, and then they invade the blood vessels, which compromises the gut health of your horse.
  • During the warm summer months, Bot Flies lay their bright yellow eggs on your horse, frequently on their lower legs. Horses then scratch at the eggs with their teeth, and the eggs enter the digestive system. The bots then mature in the stomach, where they may result in inflammation of the stomach lining or ulcers. In addition to using dewormer to control bots internally, you can utilize a bot knife to remove the eggs from your horse’s legs and body and prevent your horse from ingesting them.
  • Slightly different than the tapeworms that infect other mammals, equine Tapeworms require a host to enter the horse’s system. Tapeworm eggs are eaten by forage mites, which in turn are eaten by your horse. Once inside your horse’s body, the eggs hatch and mature. They then attach themselves to the lining of your horse’s intestine and absorb nutrients while damaging the surrounding intestinal tissue.

Other parasites that are controlled by a regular deworming practice include Ascardids, Neck Threadworms, Intestinal Threadworms, Hairworms, Large Roundworms, Small Roundworms, Pinworms, Lungworms, and Haboronema (which cause summer sores).

Invasion Tactics

It certainly isn’t difficult for worms to make their way into our horses’ digestive systems. Parasitic worms grow and reproduce in the digestive tract of horses. Eggs are then excreted through manure and onto the grass which your horse (and his pasture mates) will eat. Horses ingest grass which has been contaminated by these eggs, and the cycle continues. Your horse can contract worms from their own manure or the manure of other horses in the barn. 

An easy way to help your horse maintain a healthy amount of parasitic worms in their digestive system is to keep stalls and pastures clean and pick up manure as quickly as possible. Pasture rotation also aids in keeping the worm population to a minimum. As an added level of support and hygiene, we recommend feeding your horse’s hay in an area where he does not tend to defecate.

It is important to note that horses have had worms for millions of years. The goal of any deworming program is not to completely eradicate the worm population in a horse’s digestive system; that simply isn’t feasible or healthy for your horse. Instead, the aim is to control the worm population and keep it at a low level that allows your horse to thrive.

Plan of Attack

When it comes to deworming your horse, donkey, or mule, there a number of different tools at your disposal. It is essential that you begin with a Fecal Egg Count (FEC) to determine what type of parasites your horse has contracted and how many they are presenting with. After reading the results of your horse’s FEC, your veterinarian will help you to develop a treatment program for your horse. Learn more about Fecal Egg Counts and what they mean by referring to our blog post Understanding Deworming.

If your horse is a “low-shedder” (a horse that excretes low numbers of worm eggs in their manure), you may only need to deworm your horse once or twice a year. Horses that are considered “high-shedders” will require additional rounds of deworming and monitoring throughout the year.

The type of parasite found in your horse’s stool feces will determine which type of deworming product you choose to use with your horse. Small Strongyles and Tapeworms are not able to be monitored with an FEC, so it is essential to treat for them a few times a year regardless of the results of the test. According to Dr. Raynor, currently, the best choices to control Small Strongyles are Moxidectin based products (Quest Gel Horse Dewormer)  and high doses of Fenbendazole (Panacur PowerPac). When it comes to treating Tapeworms, Praziquantel is the most effective ingredient. Typically Praziquantel is not the main ingredient in a deworming process, but is a supplementary ingredient in many including Zimecterin Paste Dewormer and Zoetis Quest Plus Gel Dewormer.

Ivermectin based products  (Farnam IverCare Paste Dewormer and Durvet Apple Flavored Ivermectin Paste) are very broad in their approach, aiding in the elimination process of Large Strongyles, Roundworms, Pinworms, Hairworms, Large-Mouth Stomach Worms, Bots, Lungworms, Intestinal Threadworms, Neck Threadworms, Small Strongyles, and Summer Sores. They are an excellent choice for horses that considered “high-shedders.” Some parasites are developing a resistance to this classification of medications, so it is recommended that “low-shedders” are treated with alternative products.

To effectively treat Ascarids, Pyrantel (Durvet Pyrantel Paste and Strongid Paste Dewormer) and Oxibendazole (Zoetis Anthelcide EQ Paste Broad-Spectrum Equine Paste Dewormer) are among the recommended deworming products.

Stay Vigilant

The key to parasite control in your horse is understanding that it is a constant process. It is essential that you monitor your horse for horse for symptoms of internal parasites and continuing to run Fecal Egg Counts on their manure. Horses that are high-shedders may require up to four treatments of dewormer per year, while horses that have low amounts of eggs in their manure may only need to be treated one or twice per year.

We advise you to work closely with your veterinarian to develop a deworming program for your horse. It is important to realize that there is no “one-size-fits-all” parasite control program. Together, you and your veterinarian can look at your horse and his individual situation to make a plan. While deworming pastes are the most popular, your vet may recommend a liquid or pelleted deworming product in certain environments.

Our experienced sales staff at The Cheshire Horse is always available to answer any questions that you may have regarding any of our deworming products.


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