We’ve all heard about the dangers related to gastrointestinal equine parasites along with the many myths associated with horse deworming. It can be a little confusing and sometimes overwhelming to completely understand the impact of intestinal worms, the most common of equine diseases. We are commonly asked questions such as “Which dewormer should I use at what time of year?” and “Which wormers affect what parasites?” so we put together this FAQ to help you better understand the importance of horse wormers, like Strongid C and Panacur PowerPacs, and better plan an effective worming schedule.

How does a horse get worms?

Horses typically get worms when turned out with previously infected horses or when they are turned out in a contaminated pasture. In both situations, it is highly likely the horse will become infected as well. Pastures become contaminated with the eggs and larvae or parasitic worms through the manure of an infected horse’s manure which then mixes in the grass of the pasture. As your horse grazes, the eggs and larvae are ingested. A pasture can stay infected for a considerable amount of time so always keep the threat of horse worms in mind.

What parasites can affect a horse?

The four most common types of internal parasites are Strongyles, Ascarids, Tapeworms and Bots. Each species of parasite affects a horse in its own way.

  • Strongyles (blood or red worms) – Found as three different species: S. vulgaris (up to 25mm), S. edentatus (up to 40mm) and S. equinus (up to 50mm). Strongyle infection occurs by ingestion of the larvae, which begin their transformation into parasites as they travel down the animal’s intestine. The S. vulgaris can cause damage in the cranial mesenteric artery, eventually causing colic, gangrenous enteritis, or intestinal stasis and possibly rupture. The other two species are active blood feeders that can lead to anemia, weakness, emaciation and diarrhea.
  • Ascarids (roundworms) – The larva of this nasty worm start its growth in the small intestine and then migrate through the liver, the lungs and finally, the pharynx or throat where it gets swallowed again. The worm returns to the small intestine to mature and reproduce. Roundworms are an issue with younger horses up to about 15 months of age because of their lack of immunization against the worms. A small infestation will probably have a negligible impact on the horse’s health; however a heavy infection can trigger weight loss, stunt the young horse’s growth, give a rough hair coat and/or pot-bellied appearance, and cause lethargy and/or colic.
  • Tapeworms – Tapeworms take a different approach to infecting your horse. Forage mites in the grass eat tapeworm eggs; the tapeworm larvae then develop within the mites. The horse ingests the forage mites during grazing. Now that the larvae are in the horse’s gut they can develop into maturity. They adhere themselves to the gut wall at the ileo-caecal junction, thusly increasing the risk of intestinal obstruction or rupture due to inflammation at the attached site.
  • Bots – Adult flies lay yellow-colored eggs to the horse’s forelegs, chest and shoulders. As the horse grooms itself, the horse’s saliva releases the egg adhesive and the larvae then enter the mouth. Once ingested, the larvae travel and attach to the lining of the stomach when it causes irritation, digestive issues and obstruction. After 8-10 months, the larvae are passed in the *** and then burrow into the ground to pupate. They surface from the ground as adult flies and repeat the cycle.

How do I know if a horse has worms?

While a horse may appear to be in good health, it still can be infected with worms. Common signs of parasite infection in both younger and older horses include:

  • Lethargy
  • Loss of weight
  • Loss of condition
  • Diarrhea
  • Colic
  • Lack of appetite
  • Dull coat

The best method for confirming whether or not a horse has worms is to have your vet perform a fecal egg count and blood test. These tests confirm the species of parasite, provide an idea of how many adult worms are in the intestine, and give an estimate on how badly your pasture is infested. The blood test measures chemicals in the blood produced by inflammatory responses to the migration of the larvae.

Is there a way to manage worms?

There are generally three steps for effective parasite control. Always refer to a vet for the most effective program for your particular horse.

Managing the pastures – Decrease the amount of infective eggs and larvae from the pasture.

  • Remove and dispose of *** in the pasture. While time consuming and not always an easy option, doing so at least twice a week will still be effective in reducing the population of eggs and larvae. Also, mowing and harrowing the pasture exposes the larvae to predators and the elements and helps to decrease the population.
  • Pasture rotation
  • If possible, rest the pasture for at least six months
  • Keep the number of horses per acre to a minimum
  • Supply hay and/or grain in a rack rather than feeding from the ground

Monitoring fecal egg counts – Help diagnose the parasites as well as determine the effectiveness of your worming program.

Worm your horse – Giving a horse a dewormer helps remove adult worms from the intestine and reduces the chance of re-infection by decreasing the number of infective larvae in the *** and, in turn, the pasture.

Which type of wormer removes which parasites?

There are a variety of wormers available to treat a multitude of parasites.

 How often should a horse be wormed?

Traditionally, veterinarians recommend worming your horse every two months. However, there is a lively debate about the effectiveness of repeated use of the same wormers. Before beginning a worming schedule, it is wise to have a serious discussion with your vet about the best possible worming schedule for your horse. Here are some factors to consider when determining which dewormer to use:

  • Your horse’s age
  • A fecal egg count reduction test performed by your vet
  • Even if your horse is stabled, it is still susceptible to worms
  • Only use the drugs that are necessary to kill the worms that are detected in the fecal egg count reduction test. Overexposing worms to dewormers can cause them to become resistant after awhile.
  • Try to use the weather and climate to your benefit. Different climates can affect parasite reproduction which in turn reduces the frequency of deworming.
  • Be sure to follow dosage directions. If your horse does not consume the entire suggested dosage, you’re under dosing him, which is less effective on the parasites.
  • If you have any questions on designing a worming schedule, please contact your veterinarian.


Here is an example of a rotational schedule chart. *

*This is for reference only; please refer to your veterinarian for an effective wormer schedule.

Disclaimer: This article is for informational use only. Please refer to a veterinarian for any questions or concerns you may have when starting a wormer schedule or enhancing your current regime.


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