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January 23, 2018 – As pastures grow long and luxurious in the summer sun, more horses are spending time outside grazing. Internal parasites love the summer too, and warm weather can increase a horse’s chance of coming in contact with parasites. Appropriate parasite control strategies are an important component in keeping our horses healthy.

There are almost 150 different types of internal parasites that affect horses. However, according to the American Association of Equine Practitioners (AAEP), the four types of internal parasites that pose the greatest danger to horses are:

  • Large strongyles
  • Small strongyles
  • Roundworms
  • Tapeworms

Knowing the life cycle of internal parasites helps us understand control recommendations. The life cycle begins when an egg or larvae (an immature worm) is ingested by a horse. Once ingested, the larvae develop into adult worms that live in the digestive tract. Some intestinal parasites migrate out of the digestive tract during their maturation process. They pass through many different types of tissues, including distant organs, eventually returning to the digestive tract. When fully mature, the adult worms produce eggs or larvae that are shed in manure, starting the cycle again.

As described by the AAEP, internal parasites are “silent thieves and killers.”  Internal parasites compete for nutrients, impair a horse’s immune system, and their extensive migrations cause damage to other tissues. Parasites can cause colic and sometimes can be fatal. Internal parasites can be particularly harmful to foals, who are more vulnerable to diseases in general.

There are many strategies horse owners can use to help control parasites. These include both environmental control measures as well as the careful use of dewormers.

Recommended environmental control suggestions include:

  • Rotating pastures
  • Cross grazing with small ruminants when possible
  • Removing manure from pastures and avoiding overcrowding
  • Avoid housing mares and foals in the same stall year after year

Equine experts recommend that horses have a fecal examination done by a veterinarian at least once a year. Your veterinarian will examine the feces, identify the type of parasite(s) present, and an egg count if warranted. Always consult with your veterinarian before giving any dewormer. Resistance to dewormers is becoming a problem in horses, and using too much or too little dewormer, or the wrong type of dewormer, can contribute to resistance.

Morris Animal Foundation has a longstanding commitment to improving the lives of horses everywhere. For more information about our equine research programs, please visit our website,