Internal parasites are silent killers. They can cause extensive internal damage, and you may not even realize your animals are heavily infected. At the very least, parasites can cause gastrointestinal irritation and unthriftiness. At its worst, parasite infestation can lead to colic, intestinal ruptures, and even death. Following are answers to common questions to help you develop a sound parasite management program for your horse.

* Can horses that appear fat, sleek, and shiny have problems with parasites?

Yes. Contrary to popular belief, many horses that have dangerous parasite levels appear to be perfectly healthy. Though the outside may look normal with absolutely no signs of infestation, on the inside, the worms are doing irreparable damage.

* What is the most important means of combating worms?

In terms of management, establishing an effective parasite control program is probably second only to supplying the horse with clean, plentiful water and high-quality feed. By utilizing this type of management with broad-spectrum deworming agents and rotating them as recommended by your veterinarian, you should be able to rid your horse of most worms.

* What worms cause the most problems?

There are more than 150 internal parasites that afflict horses, including several major species. Among the most common and troublesome are:

  • Large strongyles
  • Small strongyles
  • Ascarids (roundworms)
  • Pinworms
  • Bots
  • Tapeworms
  • Threadworms
  • Lungworms

Any or all of these parasites can be present in the horse at one time, but they may be at different stages in their life cycles. This will influence the deworming program needed.

* What is the best way to administer dewormers?

The primary ways of administering dewormers are: Oral paste syringe; nasogastric tube (tubing); or feed additive.

All three methods are effective. The key is that the deworming product must be given in the proper dose at the proper time, and that it is fully consumed and retained by the animal. Deworming pastes and feed formulations have come into widespread use because of convenience and ease of administration. They are a good choice as long as the horse ingests the entire dose. The dose must be calculated based on the horse's weight. The problem is some horses may find them unpalatable and spit them out.

* So, is tube worming the most reliable way to worm a horse?

Tube deworming, once the method of choice, is still a highly effective means of controlling parasites. The advantage of administering dewormers via a nasogastric tube is that the veterinarian can ensure the proper dose is delivered directly to the horse's stomach. The disadvantage is that it can cause the animal temporary discomfort when it is passed through the nostrils and down the esophagus into the stomach. Because of the skill required to safely insert the tube, this method of deworming should be performed only by a veterinarian.

* How can I prevent resistance to dewormers?

After a period of time, parasites develop resistance to many of the chemicals used to kill them. The deworming agents may therefore simply become ineffective. To prevent this, it's important to rotate classes of drugs used in your program. Be sure that you don't simply change brand names, however, since many products contain the same drugs but under different labels. Although some manufacturers claim certain products do not require rotation, a good safeguard is to do it anyway. With opinions varying as to how often dewormers should be rotated, it is recommended to consult your veterinarian for guidance.

* Besides chemicals, how can I prevent worms in my horse?

Chemical control is just one part of a total parasite control plan. Since parasites are primarily transferred through manure, good management is also key. You should:

  • Pick up and dispose of manure droppings on a regular basis (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 equine parasites
  • Group horses by age to reduce exposure to certain parasites and maximize a 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
  • Consult your veterinarian to set up an effective and regular deworming schedule

For more information on parasite control, please contact the American Association of Equine Practitioners for a brochure at 4075 Iron Works Pike, Lexington KY, 40511; 859/233-0147.

About the Author

American Association of Equine Practitioners

AAEP Mission: To improve the health and welfare of the horse, to further the professional development of its members, and to provide resources and leadership for the benefit of the equine industry. More information:

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