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Equine Parasite Control

By: Dr. Philip Johnson

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Internal parasites are a major threat to the health of your horse. They can cause external damage and you may not be aware that your horse is infected. Parasites can lower your horse's resistance, deplete valuable nutrients, cause gastrointestinal upset and may lead to colic, intestinal ruptures and death.

There are more than 150 internal parasites that affect horses, but the most damaging are strongyles and ascarids (roundworms), that sometimes cause permanent damage that can profoundly affect your horse's performance.

  • Large strongyles (bloodworms) and small strongyles inflame the intestinal wall, impairing nutrient absorption and reduce feed efficiency.

  • Ascarids (roundworms) hatch in the intestines then migrate throughout the lungs before returning to the gut. Since lung tissue does not regenerate as healthy tissue, the damage is permanent.

    What To Watch For

    Many horses that have dangerous parasite levels appear to be healthy, while inside the parasites are doing their damage. Signs are occasionally present, however, especially in young horses. These include:

  • Weight loss
  • Cough and nasal discharge
  • Lethargy
  • Colic
  • Loss of appetite
  • Diarrhea
  • Dull coat
  • Tail rubbing and hair loss
  • Mouth lesions


    Fortunately, horse owners have the power to prevent parasite damage. The prevention of parasitism in the horse broadly falls into 3 categories:

  • Checking the number and type of parasites in horses through a fecal examination
  • Using drugs to kill parasites in the horse
  • Minimizing the number of parasites in the pastures, where the horse gets infected

    Common Misconceptions

    Many horse owners and veterinarians continue to adhere to old-fashioned and out-dated practices when it comes to minimizing parasite pathology in their horses. Deworming of horses should not be reserved for times when a horse appears to be thin or has been experiencing colic and diarrhea.

    In the past, the use of specific techniques to increase the effectiveness of deworming drugs was necessary because many of these original drugs were not very effective. These specific techniques included the administration of very large quantities of a drug, which was believed to be more effective because, although most of these early drugs were not absorbed from the gastrointestinal tract, it was believed that some amount of the drug might be absorbed if a very large quantity could be given.

    In order to administer these large quantities, the veterinarian had to pass a nasogastric or stomach tube and pour the required volume of deworming drug straight into the stomach through the tube. This practice was known as "tube worming" and it became very popular. At the present time, deworming drugs are extremely effective at killing parasites throughout the body and the need for "tube worming" can no longer be justified.

    Another strategy involved mixing together a number of different agents in large volume and passing them into the stomach to enhance the effectiveness of each agent. Veterinarians no longer use mixtures of weak deworming drugs in parasite control programs for horses.

    It should be noted that many horses are dewormed routinely at times when there may not be any parasites in their gastrointestinal tract to be killed; therefore, not all deworming activities are necessary. For maximal effect, deworming treatments should be timed and coordinated to be given at times that are likely to have the most effect at killing parasites in the horse. Simply treating adult horses on a regular (but unregulated) basis with deworming agents certainly does not compensate for poor horse management practices.

    It should be remembered that the most damage inflicted by the common horse worm parasites in North America is done by the larval stages of the parasites. The adult worms in the intestine certainly produce eggs that lead to pasture contamination and infection of other horses, but adult worms do not cause any appreciable damage themselves. It makes sense that deworming programs should be designed to prevent larval damage in the horse.

    Generally speaking, the tests used to determine whether a horse is currently parasitized have limitations. It should be remembered that a given horse might be very significantly parasitized but "tests" (for the presence or absence of parasites based on the presence or absence of parasite eggs) are very often negative due to long prepatent periods (several weeks). A prepatent period is the time that transpires between the ingestion of a parasite egg and the appearance of new eggs in the horse's feces. Therefore, negative fecal tests for parasites should not be interpreted to mean that there are not any larvae in the horse. Furthermore, damage inflicted by parasites may cause intestinal disturbances long after the parasites have been eliminated.

    The age of your horse is important in determining the best method for preventing parasitism. Young horses are more prone to parasitism than older adults. Adult horses may have very high worm egg counts (in feces) and contain tens of thousands of worms, but will appear in better physical shape than a similarly parasitized yearling. With age, the immune system of older horses helps prevent parasite migration and the damage inflicted by parasites in the gastrointestinal tract. Different species of parasites are important for horses less than one year of age compared with those with medical significance after the first year of life.

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