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Equine Gastrointestinal Parasites

By: Dr. Philip Johnson

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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.

  • Intestinal phase. In one phase, the parasite lives inside the horse and causes various degrees of intestinal damage. During this intestinal phase, adult forms of the parasites undergo sexual reproduction and produce eggs. Eggs are shed in the horse's manure onto the pasture.

  • In the other phase (the "free-living" phase), the parasite lives outside the horse in the environment, completely independent from the horse for day-to-day survival. The eggs hatch into small worm-like larvae, which pass through several life stages before they are infective to other horses. Ultimately, the pastures become contaminated by a combination of eggs and larvae, which serve as a reservoir of repeated infections.

    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:

  • Failure to thrive
  • Weight loss
  • Reduced food conversion efficiency (competition for nutrients)
  • Impaired growth and predisposition to other infectious diseases
  • Colic
  • Diarrhea
  • Emaciation or unexpected sudden death
  • Affliction of other body systems such as the liver, lungs, brain and spinal cord, skin, heart and reproductive organs


    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 (also known as cyathastomes)
  • Large strongyles (also known as "blood worms" or "red worms")
  • Tapeworm
  • Roundworm
  • Stomach worms
  • Bots

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