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Equine Gastrointestinal Parasites

By: Dr. Philip Johnson

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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.

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