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Intestinal Parasitism, Growing Horses

Synonyms: Foal Internal Parasitism

Foals affected by internal parasites often appear slower growing, pot-bellied and rough coated. But seemingly healthy appearing youngsters may have a large worm load too.

The most important parasite in growing foals is the Ascarid parasite (Parascaris equorum). It is a large, pale worm that looks like a 6″-15″ long bean sprout. There are many other worm species that affect foals, but this one is the most likely to cause disease. This parasite is considered to be a growing problem due to drug resistance.

Another important parasite of younger horses is the Pinworm (Oxyuris equi). This parasite does not cause severe disease but causes intense itching around the anus and tail base. Affected foals damage the hairs at the tail base from rubbing.

The veterinary approach to parasite control in foals has changed as we have recognized resistance in worm populations. Old strategies may not work.

Parasitism in foals may be diagnosed based on their general appearance and/or treated on a preventative basis. Parasites should be assumed to exist in growing foals. Due to this, fecal testing is generally not performed as much in young foals as in adults. This is one case where de-worming without fecal testing may be the best approach to treatment.

After 1 year of age, foals are wormed according to fecal results.


  • What deworming protocol do you recommend for this foal and my others?
  • What management techniques should I employ to reduce parasitism in my young horses?

    Talk to your vet about parasite control regimens ideal for your region.

    Young horses should probably not be dewormed before 60-70 days of age, and a benzimidazole wormer should be used because there is increasing resistance to the ivermectin drug class in Ascarid parasites.

    There are many recommended management techniques to reduce parasitism. All require overall herd and facility management. Regular removal of manure from pastures and corrals will significantly reduce parasite burdens in horses. Proper composting of manure, in which the fermentation process kills parasite eggs, renders quality fertilizer that can be safely spread on pastures.

    Rotating pastures with other animals (cows, pigs or sheep) may also help reduce ingestion of parasite eggs by breaking the cyclical pattern of ingestion, infection and shedding. Likewise, annual or bi-annual cleaning of all barns, stalls, tack and equipment is good management that may help remove the more stubborn eggs that can survive for long periods awaiting a host.

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    Author: Doug Thal DVM Dipl. ABVP


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