Animal Protection Line


Animal Care


Parasite Control

Keeping internal parasites under control is an ongoing part of looking after horses. Several different kinds of worms are commonly found in horses, and unless a horse is on a proper deworming program they can cause serious internal damage before you realize what is happening. In the long run, a regular parasite control program will save you money in feed costs and veterinary bills. Parasite control is likely the second most important horse management priority, just after supplying horses with clean water and high-quality feed.

Most Common Worms in Horses in Alberta

There are many internal parasites that can afflict horses. Those commonly encountered are large strongyles, small strongyles, ascardis, pinworms, bots and tapeworms. 

Any or all of these parasites can be present in a horse at one time. Different worms harm the horse in different ways. They can damage tissues and vital organs, cause obstructions and ulcerations within the digestive tract, and cause irritation as they lay eggs.

Signs to Watch For

Contrary to what one would expect, many horses with dangerous parasite levels look perfectly healthy. It is safe to assume your horse has worms. The only way to be sure about the degree of infestation is to have your veterinarian perform a fecal laboratory examination. Some horses, particularly young ones, can show obvious signs of carrying a heavy worm load. Symptoms may include:

  • Dull, rough haircoat
  • Lethargy (decreased energy)
  • Decreased stamina
  • Loss of condition
  • Slowed growth in young horses
  • Pot belly (especially in young horses)
  • Colic
  • Diarrhea

Protection Against Worms

The best protection is to combine a deworming program with parasite management program. That way you kill the parasites already living in the horse and lessen the degree of re-infestation.


There are many safe, convenient products available today. Consult your veterinarian to set up the best program for your situation. A minimum deworming schedule is twice a year, but more frequent deworming is called for in many situations.

Deworming medication for horses comes in a variety of forms. Whatever type of medication you choose to use, it is important to follow the directions carefully and make sure the horse gets the whole dose. It’s also important to deworm all horses on the same property at the same time. Your veterinarian can help you choose the best dewormers for different times of the year.

Parasite Management Program

Chemical dewormers are only one part of an inclusive parasite control plan. Good management is essential as parasites are most frequently transferred though manure.

  • Keep the number of horses per acre to a minimum to prevent overgrazing and reduce pasture contamination with parasite eggs and larvae.
  • Pick up and dispose of manure regularly (at least twice a week, even in dirt or sand yards).
  • Do not spread manure on fields to be grazed by horses. Instead, compost it in a pile away from the pasture.
  • Mow and harrow pastures periodically to break up manure piles and expose parasite larvae to the elements.
  • Consider rotating pastures by allowing sheep or cattle to graze them, thereby interrupting the life cycles of equine parasites.
  • Keep foals and weanlings separate from yearlings and older horses to minimize the foals’ exposure to roundworms and other parasites.
  • Use a feeder for hay and grain rather than feeding on the ground.
  • Remove bot eggs regularly from the horse’s haircoat.
  • Consult your veterinarian to set up an effective deworming program for your horses and monitor its effectiveness.

External Parasites

In addition to internal parasites, there are also external parasites such as fleas, ticks and lice which can be contracted through direct contact with another horse that is already infested. Consult your veterinarian if you suspect your horse has become infested.

For more information, please visit:

4-H Guide on Horse Health (pdf)

Strategies for Effective Parasite Control (American Assoc. of Equine Practitioners)

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