Equine Gastrointestinal Parasites

More than 150 different parasite species can be found in the horse's intestine. Fortunately, only a handful create problems. Which parasites 'act up' and damage the intestine depend on the age and natural defenses of the individual horse.

Most parasites gain entry to the horse's intestinal tract by accidental swallowing since the parasites live on blades of grass in the pasture. In order for a majority of these parasites to complete their life cycle, there are two distinct phases.

The migration of worm parasites in the horse is, to a large extent, unstoppable. Virtually all horses are infected with parasites to some extent. Small numbers cause minimal damage, but larger numbers pose a risk for colic and other symptoms. The situation is different for weakened, debilitated or immunocompromised horses, which succumb to small numbers of parasites. As a rule, older horses appear to develop immunity against the common gastrointestinal parasites and tend not to be affected by parasite-related problems as commonly as younger horses.

Over the past several decades, a whole variety of ill-defined maladies have been blamed on the effect of parasitism in horses. The "diagnosis" of "migrating parasites" has been a traditional "fall-back" diagnosis for mysterious clinical problems (during the 1990s, this fashionable diagnosis was largely replaced by the diagnosis of EPM). Based on the extremely high effectiveness of some of the current deworming drugs, true parasite damage is not seen as often in horses as it used to be.

What to Watch For

Symptoms of parasite infection vary a lot depending on whether it is mild parasitism or of a more serious nature. You may see any of the following:


It is very common for adult horses to have been parasitized by several different parasite species at the same time. The important groups of equine gastrointestinal parasites are listed as follows:

Small Strongyles

Currently, the "small strongyle" group of parasitic worms represents the most important cause of gastrointestinal parasitism in North America. There are in excess of 100 different species of small strongyles that afflict horses. These miniature worms are so important, that today's modern parasite control programs and drug development efforts target small strongyles.

Some degree of small strongyle parasitism can be found in most adult horses. Unlike many of the parasites of the gastrointestinal tract of dogs and cats, the small strongyles actually cause damage by migrating inside the wall of the intestinal tract of horses. Horses acquire this type of parasite by eating pasture grass that has been contaminated by parasite larvae, which develop from eggs shed in horse manure. Pasture contamination by parasite eggs can be very severe and these eggs can survive for long periods of time on the pasture. Therefore the best way to reduce the chance of parasite problems is to combine methods that kill parasite eggs in conjunction with dewormers. Dewormers alone will not assure you that your horse will avoid parasite-related diseases.

Following ingestion of the infective larval-stage parasite at pasture, the parasite burrows into the wall of the horse's large intestine and elicits an inflammatory reaction. This inflammatory reaction, when sufficiently extensive, may interfere with the ability of the intestine to absorb nutrients and lead to colic and diarrhea. During its migration within the wall of the intestine, the parasites grow and mature.

The small strongyle parasite also has the option to enter a phase of dormancy in the intestinal wall at which time it is relatively resistant to deworming drugs.

These dormant parasites have one of the most sophisticated tricks found in nature. Not only are they resistant to drugs, but they can carefully and accurately time their awakening to coincide with optimal weather conditions for survival in the pasture. They predict the best weather for survival outside by sensing changes in the horse's circulating hormones, which change with the seasonal fluctuations in day length. This trick of nature prevents the death of eggs due to exposure to the elements, in particular very dry and warm conditions.

These parasites will also go into hibernation when the intestinal lining is already overcrowded with adult parasites. The hibernating stages, therefore, represent a reservoir of parasites that trickle down into the intestine when there is enough room, created by natural death of the adult worms, or deworming. Following adult deaths, many new mature larvae are able to "break out" into the intestinal lumen and reestablish a population of adult parasites and enable further egg production.

Clinical problems associated with the small strongyles range from the asymptomatic "carrier" to diarrhea, unthriftiness, poor growth, impaired performance and increased risk for colic.

Large Strongyles

There are only three different species of the large strongyles, the most important of which is Strongylus vulgaris. Before the advent of the new powerful deworming drugs, problems attributed to parasitism associated with Strongylus vulgaris were widespread. However, it is rare to see clinical problems due to this parasite in the modern age.

The large strongyles are recognized in the horse's manure by their red color (they contain blood); to this end, they are also commonly known as "blood worms" or "red worms." With an important single difference, the life cycle for Strongylus vulgaris is similar to that described for the small strongyles. However, instead of undergoing a period of arrested development in the lining of the large intestine, Strongylus vulgaris undergoes a migratory phase in the blood vessels that supply the intestinal tract. By migrating in the lining of the wall of this important artery, damage associated with Strongylus vulgaris includes interruption of the blood flow to the intestine.

Less commonly, larvae of large strongyles may be found in remote sites such as the brain, but, most typically, this aberrant migration is restricted to an intestinal artery and the lining of the intestinal wall. Most clinical problems associated with large strongyle parasitism include unthriftiness, diarrhea, poor growth, impaired performance, weight loss and increased risk for colic. Other problems may be reported if the parasite larvae invade other remote sites. Rarely, if parasite damage to the artery is sufficiently severe, death may occur as a result of internal hemorrhage into the abdomen.


One single tapeworm parasite that causes clinical problems in some horses is Anoplocephala perfoliata. Unlike other species, the horse tapeworm is rarely, if ever, seen by horse owners in the horse's manure. The tapeworm "segments" that contain eggs usually dissipate before they arrive in the rectum.

A substantial number of tapeworms in the horse's intestine rarely cause severe disease. Young horses are more likely to be affected with tapeworm parasitism than older horses, presumably as a result of developing immunity. Clinical problems associated with tapeworm parasitism in adult horses include unthriftiness, impaired performance, weight loss and increased risk for colic. Horses acquire Anoplocephala perfoliata by inadvertently ingesting a free-living grass mite at pasture. This mite is essential for the completion of this parasite's life cycle. Well-established old pastures are more likely to be contaminated by grass mites than young pastures.


The roundworm, or ascarid, parasite of horses is Parascaris equorum. Ascarid eggs are resistant and ubiquitous in horse environments. Virtually all young foals are exposed to roundworm parasitism. By approximately 10 months of age, the young horse has developed immunity and essentially eliminates the adult ascarids in the intestinal system.

Roundworms are rarely seen in the intestinal tract of adult horses. Prior to immunity and roundworm elimination, these parasites undergo migration from the intestine, through the liver and peritoneal cavity, then through the lungs, and return to the intestine to mature into adults and lay eggs. Eggs are passed in manure to contaminate the environment and lead to infection of other young horses.

Clinical problems associated with roundworm parasitism include unthriftiness, diarrhea, poor growth, impaired performance, anemia, weight loss and increased risk for colic. Other problems such as peritonitis, liver disease and lung inflammation (coughing) may be reported as the parasite larvae migrate through these internal organs.

Foals may be parasitized by a very large number of adult roundworms in their small intestine. An important cause of severe colic is the situation that occurs following the administration of a powerful deworming drug to foals that are carrying a heavy roundworm burden. The sudden death of large numbers of these roundworms may lead to complete blockage and even rupture of the small intestine.

Other Parasites

Horses are commonly parasitized by bot fly larvae. Throughout the summer, bot flies lay their eggs on the hair shafts of horses at pasture. These small yellow bot fly eggs can easily be seen on dark-colored horses.

The flies selectively lay their eggs on the hair coat of the fore quarters from where the horse ingests the eggs during grooming (licking) behavior. These eggs develop into larval stages in the horse's stomach and may, in sufficiently large numbers, lead to gastric irritation and mild colic or unthriftiness. Mature larvae are passed out in feces to pupate on the ground. Large numbers of larvae in the rectum (temporary attachment) have been reported to cause straining.

Bot larvae are readily killed by most new deworming drugs. With the advent of widely available endoscopic equipment (capable of visualizing the lining of the horse's stomach), veterinarians recognize that large numbers of bot larvae are often found in the stomach of adult horses. However, significant damage is rarely attributed to this parasite – its presence more importantly implies that the horses have not been recently dewormed using an effective anti-parasite drug.

Other worm parasites are sometimes identified in the adult horse stomach. These parasites are the closely related "stomach worms" (Draschia megastoma, Habronema muscae and Habronema microstoma). This group of parasites causes minimal damage to the lining of the stomach.

Use of modern deworming drugs have virtually eliminated clinical problems associated with these parasites in horses in North America. In warm weather, the larval stages of these stomach worms are sometimes transported by flies to moist areas of the horse's body. At these locations, the larvae may cause aggressive inflammatory reactions and bleeding, known as "summer sores." Typical areas of the horse's body include small wounds, the eye and tear duct, and the moist skin at the tip of the penis/prepuce.