Deworming Horses

If you are deworming your horses only twice per year, you may be leaving them vulnerable to the dangerous parasites that infect and breed in even the best care-for horses.
How often do you deworm your horses? Equine parasites are always in season, and produce anywhere from 5,000 to 100,000 eggs every day of the year. It can take as long as six months from infection until external signs appear, at which point internal damage may be irreversible. Establishing a regular parasite control program is as important to your horses' health as supplying them with clean, plentiful water and high quality feed.

Choose a Deworming Schedule

When developing a deworming program, you need to decide between two basic strategies: daily deworming or interval deworming.

Daily Deworming: Administer a daily dose of the deworming agent in your horse's feed. Dosing is based on your horse's weight; it is imperative that you have an accurate weight and that you stay on the program. Once or twice per year, you will need to add a botacide wormer such as ivermectin. Talk with your veterinarian if you choose daily deworming in order to create the best strategy for your horse.

Interval Deworming: This is purge deworming, designed to clear most of the parasites the horse has been exposed to with one dose of wormer paste. Timing is important and it is recommended that you deworm every 8 weeks. If you treat too early, the worms may be too immature to be affected by the dewormer. If you treat too late, the worms may have had the opportunity to produce eggs, which will infest your horse's environment. Depending on your horse's age and exposure, there may be slight variations to this schedule, so talk to your veterinarian about a specific plan.

Get Rid of Parasites Before They Become a Problem

Here are some suggestions from the American Association of Equine Practitioners:

  • Pick up and dispose of manure droppings in the pasture at least twice weekly.
  • Mow and harrow pastures regularly to break up manure piles and expose parasite eggs and larvae to the elements.
  • Rotate pastures by allowing other livestock, such as sheep or cattle, to graze them, thereby interrupting the life cycles of parasites.
  • Group horses by age to reduce exposure to certain parasites and maximize the deworming program geared to that group.
  • Keep the number of horses per acre to a minimum to prevent overgrazing and reduce the fecal contamination per acre.
  • Use a feeder for hay and grain rather than feeding on the ground.
  • Remove bot eggs quickly and regularly from the horse's haircoat to prevent ingestion.
  • Rotate deworming agents, not just brand names, to prevent chemical resistance.
  • Whatever program you choose, stick with it!

The Oregon Veterinary Medical Association is a nonprofit organization of veterinarians who are dedicated to helping people give their animals a high quality of life.

Equine Internal Parasites Power Point Slideshow