Throughout their lives, horses will pick up internal parasites (worms) that are detrimental to their overall health.

Internal parasites can also have major impacts on a horse’s nutritional status. They can compete with the horse for any ingested food, as well as feeding off the horse itself. Because of this, deworming has become a standard management practice. Deworming is simply the term used to treat horses with anthelmintics (dewormers), which will kill off any internal parasitic infections. It is also worth noting that not all dewormers work against all worms. Instead, there are different classes of dewormers that treat against certain internal parasites.

A horse with a significant parasitic infection will just look unwell. They tend to lose weight, have poor coats, and may suffer from lethargy (low energy). Horses with intestinal parasites may also cough, rub their tails, have diarrhea, and could even suffer bouts of colic. All in all, internal parasites pose a significant health risk to our horses.

There are many different types of internal parasites that infect horses. Typically, horses will pick up parasites in their larval stage by ingesting the 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, on to the lungs, then are coughed up and end up back into the horse’s digestive tract. There is more concern with younger horses, as mature horses develop immunity.
  • Strongyles (large and small). These are also called bloodworms. Large strongyles migrate throughout a horse’s body. Small strongyles are a major concern as they become “encysted” in the horses intestinal lining, and are resistant to deworming. Both can be very damaging to a horse’s digestive tract.
  • Pinworms. These reside within the digestive tract and cause tail itching when female pinworms emerge to lay eggs around the anus.
  • Tapeworms. First, larvae develop into mites that horses will ingest. Tapeworms then live within the digestive tract of the horse, losing its segments in feces to reproduce. These pose major concerns related to colic.
  • Stomach bots. Bot flies lay eggs on the horse’s legs, which the horse will ingest when grooming itself. The bot larvae will hook themselves to the stomach lining of the horse for up to a year, until it detaches and passes through feces.

For many years, it was typically recommended to deworm your horses on a regular schedule based on seasonal trends of certain parasites. It has even become trendy for some owners to feed their horses daily dewormers to protect their animals. However, parasites have become resistant to certain dewormers over time. This has completely altered how we now view deworming our horses.

Internal parasites, such as small strongyles, are now showing resistance to popular dewormers, such as ivermectin and moxidectin. To explain this—let’s say, for example, a product was 95% effective against small strongyles and, therefore, was used. The dewormer kills off 95% of the worms. However, the remaining 5% could be considered “resistant” and survive. In this example, these resistant small strongyles continue their life cycle and reproduce. Over time, as we use that dewormer over and over, the resistant small strongyles become more and more common. Thus, eventually, that particular dewormer is now no longer effective in reducing the small strongyle population. This exact scenario is being observed around the world with a number of our popular deworming products and across different internal parasites.

It is inevitable that your horse will become infected with internal parasites. Yet, since the internal parasites that infect horses are now becoming more resistant to even our most popular dewormers, it is now recommended that owners have a fecal egg float done to determine which internal parasites have infected their animals. A fecal egg float is a simple test of a sample of fresh manure from the horse, which is diluted and used to count the parasitic eggs within the animal. This will help determine the parasitic load within your horse and then determine how best to treat them. This also is helping to keep our most effective dewormers working against the most detrimental parasites.

The following is a list of common horse dewormers and the parasites they treat:

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

While not included here, another critical aspect of internal parasite control is a solid pasture and manure management plan. Clean stalls and dragging of pastures to break up fecal balls exposing parasites to the environment are important. If you have any concerns with your deworming strategy for your horses, you should always consult with your veterinarian.

Chris J. Mortensen, Ph.D.