Deworming Your Horse, When & Why

Internal parasites (worms) can have serious negative impacts on the health and wellbeing of your horses. They can compete with the horse for any digested food or often feed off the horse itself. Most concerning is some internal parasites can cause internal organ damage as they migrate within the horse’s body. There are also some instances of horses suffering from impaction colic due to too many worms in their digestive tract.

Thankfully, there are dewormers, or what are also known as anthelmintics, available to treat your horses. However, due to increases in parasitic resistance to some types of dewormers, recommendations on how you deworm your horse have changed quite significantly.

How Can Internal Parasites Harm My Horse?

A horse with a heavy parasitic infection will just look unwell. They tend to lose weight, have poor coats, and may suffer from lethargy (low energy). Here are some other ways internal parasites can affect horse health:

  • Digestive upset. Depending on the type of internal parasitic infection, some reside in the horse’s gastrointestinal tract. Here, they can cause irritation, inflammation, and even damage to the digestive system. This can lead to symptoms of diarrhea, colic, weight loss, and poor nutrient absorption.
  • Nutritional deficiencies. Some parasites will consume nutrients (feed) within the horse’s digestive system. This can lead to the horse not receiving all its nutrients from its diet. This can lead to a weakened immune system and the horse will be susceptible to other illnesses.
  • Colic. Severe infestations of some internal parasites can lead to colic. This usually happens when there are so many worms within the digestive tract, they lead to an impaction colic. It can even lead to death in severe cases.
  • Anemia. A decrease in red blood cells in the blood stream is known as anemia. Some types of internal parasites feed on the horse’s blood. This leads to weakness, lethargy, and poor exercise tolerance.
  • Respiratory issues. Some parasites, as part of their life cycle, will migrate to a horse’s lungs. This can lead to respiratory issues, like coughing and nasal discharge.
  • Decreased performance. In competition horses, with all the above issues caused by a heavy parasitic load, horses will have decreased energy levels, poor body condition, and just an overall lack of being well.

There are other impacts on horses, but the takeaway message is, internal parasites can be quite damaging to a horse’s health and wellbeing. Even in severe cases of infection, they can lead to death.

Types of Internal Parasites that Infect Horses

There are many different types of internal parasites that infect horses. Typically, horses will pick up parasites in their larval stage by ingesting parasites as they graze or eat other feed contaminated by other infected horses. The most common internal parasites seen in horses include:

  • Ascarids (roundworms). Once ingested, they migrate from the intestines to the liver. The ascarids will then eventually migrate to the lungs, are coughed up, and then swallowed to be returned to the horse’s digestive tract. Impacts are often observed more in younger horses, as mature horses will develop immunity to ascarids.
  • Strongyles (large and small). These are also called bloodworms. Large strongyles will migrate throughout a horse’s body. The more concerning small strongyles can become “encysted” in the horses intestinal lining and are resistant to deworming. Both can be quite damaging to a horse’s digestive tract.
  • These reside within the horse’s digestive tract. They can irritate the horse and induce tail itching when females migrate outside the digestive tract and lay their eggs around the horse’s anus.
  • The larvae first develop in mites. Once horses ingest these mites, the tapeworms live within the digestive tract. With a heavy infestation, tapeworms can be concerning with impaction colic.
  • Stomach bots. The bot fly will lay its eggs on the horse’s legs. When grooming, the horse will ingest the eggs and the bot larvae hook themselves to the stomach lining of the horse. Then, up to a year later, they will detach and pass through the horse’s feces.
  • These are primarily a concern with foals. They can lead to diarrhea and digestive upset.
  • Lungworms. These can specifically lead to respiratory tract issues and can lead to bronchitis or pneumonia. Donkeys are primarily carriers that can infect horses if mixed.

Treating Your Horses for Internal Parasites

For many years, it was typically recommended to deworm your horses on a regular schedule based on the seasonal trends of each type of parasite. It was even trendy for a time to feed a daily dewormer to horses to limit infection. However, parasitic resistance has become a worrying trend for horses throughout the world.

Resistance to deworming has most been observed in small strongyles. They are showing resistance to popular dewormers, like ivermectin and moxidectin. To help explain how resistance can develop, as an example, let’s say a product was 95% effective against small strongyles. Thus, when treated, 95% of the parasites are killed off. However, the remaining 5% could be resistant and survive the treatment. As we use the dewormer over and over, the 5% population that is resistant proliferates and grows to 6%, then 7%, and so on. Soon, the dewormer is only 90% effective, and over time, becomes even less effective. This phenomenon is now being observed around the world with many dewormers and across different types of internal parasites.

Resistance to dewormers has now completely altered how equine health professionals are recommending treating your horse. It is inevitable that your horse will become infected with internal parasites. To treat them, it is now heavily recommended that your horse has a fecal egg float first to determine which parasites they are infected with. Once that is determined, use the appropriate deworming product to treat your horse. A fecal egg float is a simple test using a fresh manure sample from your horse. You can speak with your veterinarian about how best to collect a manure sample and test it for parasites.

The following is a list of common horse deworming products available to you and the parasites they treat.

  • Avermectin. The active ingredient is ivermectin and is used to treat against ascarids, bots, strongyles, pinworms, threadworms, and lungworms.
  • Benzimidazole. The active ingredient is fenbendazole and oxibendazole. It is used to treat ascarids, bots, pinworms, and strongyles.
  • Milbemycin. The active ingredient is moxidectin. It is used to treat ascarids, bots, pinworms, and strongyles.
  • Pyrimidine. The active ingredient is pyrantel pamoate and pyrantel tartrate. It is used to treat ascarids, bots, lungworms, strongyles, and threadworms.
  • Isquinoline-pyrozines. The active ingredient is praziquantel. It is used to treat all the above-mentioned worms plus tapeworms.

It is worth noting that there are differences when it comes to treating horses for internal parasites compared to developing an effective strategy for limiting external parasites (flies). For example, Essential K® with Fly Control is not an anthelmintic or meant to treat worms. Rather, it has a larvicide that passes through the horse and into their manure. This limits the ability of nuisance flies to develop and has no impact on internal parasites or their development of resistance to dewormers.

Take Home Message

Another key aspect of parasite control is to have a solid pasture and manure management plan. If stalled, keeping the stalls clean and composting manure is a must. Together, good management can help reduce the parasitic load to your horses. It is always recommended to discuss your internal parasite plan with your veterinarian and seek their advice. They can help identify which parasites are a problem in your area and how best to treat your horses.


Article By: Chris Mortensen, Ph.D.
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