Internal parasites– or worms- are a common concern for many horse owners. They can cause inflammation, immune dysfunction, and gastrointestinal disease.

In a parasitic relationship, the parasite lives on or inside another organism (host), relying on it for nourishment, shelter, and reproduction, while causing harm or damage to the host. The host-parasite relationship has developed over a millennia, with the parasite and host species co-evolving together.

All horses are susceptible to parasites and infections can lead to a number of negative health consequences. Signs of equine internal parasites include weight loss, colic, ill thrift, poor coat quality, and lethargy.

The level of parasitic infection for individual horses will depend on a variety of factors. These include feeding and grazing conditions, pasture management, and deworming practices. [1]

If you suspect your horse has a high parasite load, consult with your veterinarian to receive a diagnosis and treatment options. Making changes to your horse’s feeding program and management can help to reduce the risk of internal parasites.

Common Equine Internal Parasites

How Horses Get Internal Parasites

All grazing animals have an intestinal parasite load. Genetic and environmental factors influence which animals develop clinical disease from high parasite burdens.

Intestinal parasites are transmitted from horse-to-horse via the fecal-oral route. Adult parasites reproduce in the GI tract and the eggs are passed in the manure and contaminate the surrounding environment. Another horse grazing in area will ingest the eggs or larvae, and the parasite will mature into its adult life stage internally within the host.

Under natural conditions where horses are allowed to roam freely, internal parasites rarely cause clinical disease. This is because horses live a nomadic lifestyle and graze over large areas. In this environment, they don’t eat near their piles of manure, which would be contaminated with parasite eggs.

However, the confinement of horses to small pastures has led to higher parasite burdens as parasite egg populations become concentrated in small areas.

Unless something is done to interrupt this cycle, parasites will continue to reproduce, infecting any horse that grazes in the pasture.

High parasite loads are more common in young horses because their GI immunity is still developing. Adult horses do gain some immunity to certain types of parasites, however some adult horses remain susceptible to high parasite burdens throughout life.

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Types of Internal Parasites Affecting Horses

Over 150 species of internal parasites can affect the horse. Today, the most clinically relevant species include small strongyles, roundworms, and tapeworms.

Large Strongyles (Strongylus vulgaris)

Historically, large strongyles were considered the most dangerous internal parasite for horses. Part of their lifecycle involved migrating through the mesenteric artery, the main blood supply to the bowel. This resulted in signifiant damage and impaired blood flow to the affected gut- termed “verminous arteritis”. In severe cases, horses died of complications related to colic.

The good news is that since the 1970s, frequent deworming practices have greatly reduced the prevalence of this parasite. [2] As a result, large strongyles are rarely found in domestic horses today.

Small Strongyles (Cyathostomin spp.)

Small strongyles are the most common internal parasite in horses today. [3] Once the larvae of this parasite are ingested, they burrow into the gut lining of the large colon where they continue to develop. This life stage refers to the “encysted strongyle” population. At some point, the larvae emerge as mature adults.

Encysted strongyles can be dangerous to the horse when large numbers of encysted larvae emerge all at once. This is known as larval cyathostominosis. Most horses do not show clinical signs of infection, however larval cyathostominosis has been reported to be fatal in 50% of all cases. [4]

Young horses between the age of 1-4 are the most sensitive to strongyles since they are still developing immunity to this parasite. [5]

Ascarids (Parascaris equorum)

Roundworms, also known as ascarids, can grow to be 50 cm in length and are especially dangerous to foals and young horses.

Once ingested, roundworm larvae move through the gut wall to the liver and then the lungs where they are coughed up and swallowed. The larvae mature into adults in the intestine.

Horses with high roundworm loads may develop a cough or nasal discharge while the larvae move through the lungs. They may also show signs of weight loss, pot belly, and diarrhea when ascarid burdens are high. In severe cases of ascarid infestation, the adult parasites can actually obstruct the small intestine, causing colic. [6] [7]

Tapeworms (Anoplocephala perfoliata)

Tapeworms are another large and potentially dangerous species of internal parasite, growing up to 20 cm in length.

They are transmitted through an intermediate host, the orbatid mite. This mite is found in grass, harvested forage, and even grain.

When the mite is ingested by a horse, the tapeworm larvae develop into adult tapeworms. Within 6-10 weeks, the females shed proglottids, or egg cases, to be passed in the manure.

Tapeworms gather in clusters at the junction between the small and large intestine- the ileocecal junction- where they can disrupt GI motility and lead to colic. [8] [9]

Young horses and those over 15 years of age appear to be most at risk of tapeworm infection.

Pinworms (Oxyuris equi)

Pinworms lay eggs around the outside of the horse’s anus, causing itching and irritation. Horses will rub against trees, stall walls, or anything they can to relieve the itching.

This frequent rubbing can lead to hair loss and create sores around the anus which may become infected.

Threadworms (Strongyloides westeri)

High threadworm infections in foals can lead to delayed growth, lethargy, diarrhea, and anemia. However, most horses develop natural immunity to threadworms by 6 months of age.

Transmission can occur via milk. Therefore, deworming the mare during pregnancy can also help to reduce threadworm transferral to the foal.

Bots (Gastrophilus intestinalis)

Botflies are a seasonal parasite, leaving eggs on the horse’s hair coat during the summer grazing months. Some of the eggs are later ingested as the horse grooms itself.

Once in the mouth, bot eggs hatch into larvae that migrate to the stomach to attach themselves to the lining and continue to develop. They will then detach and pass through the manure where they will pupate into flies.

Generally, bots are not recognized to be of clinical significance. However, it is conceivable that large numbers of botfly larvae in the stomach can lead irritation of the stomach lining or gastric impaction.

Symptoms of Parasite Infection in Horses

If horses become infected with high numbers of parasites, they may show clinical symptoms. These can include:

  • Poor body condition
  • Slow growth or ill-thrift in young horses
  • Diarrhea
  • Colic
  • Reduced exercise performance
  • Poor reproductive performance
  • Tail rubbing (with pinworms specifically)

However, it is also possible for a horse to have a high infection rate and show no clinical symptoms.

Steps to Establishing a Deworming Schedule

It’s important to note that all horses normally have some level of internal parasite infection at any given time.

The goal of deworming is not to eradicate all parasites from a particular individual, but rather to limit infections so animals remain healthy and do not develop clinical disease.

Until recently, it was recommended to keep all horses on a routine and frequent deworming schedule. Due to growing drug resistance, this approach is no longer recommended. Instead, a targeted approach should be used to treat horses with high infection rates.

The American Association of Equine Practitioners updated their Parasite Control Guidelines in 2019, the aim of which was to help veterinarians improve strategies and programs for parasite control in horses, as well as address growing drug resistance. [9]

Their recommendations were developed based on the following: [9]

  1. Today, small strongyles and tapeworms are the most clinically relevant internal parasites for adult horses, while roundworms remain the most important parasite infecting foals and weanlings.
  2. Drug resistance is highly prevalent in small strongyle and roundworm species. [10] [11]
  3. Parasite control programs should be individualized to the horse, as adult horses vary greatly in their susceptibility to infection with small strongyles and level of strongyle egg shedding.
  4. Horses less than 3 years of age are more susceptible to parasite infection and more at risk for developing disease, and therefore require special consideration.

Fecal Egg Count Test

The first step in establishing a parasite control program for your horse(s) is to have your veterinarian perform a Fecal Egg Count (FEC) on each horse. FEC’s determine how many parasite eggs per