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 | Mad Barn Canada

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 sign