Deworming/Parasite Prevention in Horses

Deworming/parasite prevention in horses


Deworming/parasite prevention in horses


  • Parasites are of significant concern in horses. Infection with intestinal parasites may cause mild symptoms to severe, life threatening disease.
  • Parasites are generally transmitted by fecal-oral contamination. This means horses are usually infected through grazing on fecal-contaminated pasture. Some parasites are transmitted when a fly (such as the bot fly) lays eggs on the horse's coat. The horse then ingests the eggs when licking or chewing the affected area.
  • Your veterinarian can determine your horse's parasite infection status through a simple fecal exam.
  • Several different deworming products are available for parasite control and prevention in horses. Discuss a deworming program with your veterinarian who is familiar with your geographic region and your horse's environment.

What you need to know

Parasites are most commonly transmitted by ingestion when a horse is grazing on fecal-contaminated areas.Parasite eggs are shed in the feces and can survive for a lengthy amount of time in manure piles on the ground. Some parasites are spread when eggs are laid on the horse's coat, as is the case with the bot fly. Eggs are laid on the hair coat, the horse licks or chews the area and the eggs are ingested. Once swallowed, the eggs hatch and begin a lifecycle within the horse, which affects the gastrointestinal system.

Intestinal parasites are of significant concern in horses. In their natural environment, horses would graze over large amounts of space thereby reducing the risk of transmission of intestinal parasites. Most horses however are stabled among several other horses in a relatively small space. Additionally, many horses travel for competition or other activities thus increasing the chance for parasite exposure and transmission.

Different species of intestinal parasites affect different parts of the gastrointestinal system. Some parasites (or “worms”) migrate to arteries that supply blood to the gastrointestinal tract. If the worm burden is large, this may cut off blood supply to the intestines and cause life threatening colic. Other intestinal parasites migrate through the wall of the intestines, into internal organs such as the liver and lungs before completing their maturation back in the intestines. Finally, there are parasites that can burrow into the wall of the large intestine, become dormant and thus resistant to many deworming products, only to re-emerge during favorable intestinal or environmental conditions.

Foals are more susceptible to intestinal parasites than adult horses. It is important to discuss deworming recommendations for your mare and foal with your veterinarian prior to foaling. By properly deworming your mare before foaling, you may reduce the parasite exposure to the foal. Additionally, your veterinarian will be able to help you formulate a plan to protect your growing and developing foal against the harmful effects of parasitism throughout the early years of its life.

Clinical signs of parasite infection


  • Dull hair coat.
  • Failure to gain weight/poor body condition.
  • Lethargy/lack of energy.
  • “Pot belly” appearance.
  • Colic (mild to severe).
  • Diarrhea.

Many horses with significant internal parasite infections will show no outward clinical signs. While these normal appearing horses might not be outwardly suffering from being heavily parasitized, they will be shedding parasite eggs by the millions in their manure, contaminating the environment and infecting other horses which may become debilitated as a result of internal parasitism.

Treatment and Prevention

In order to determine your horse's parasite infection status, your veterinarian will obtain a fecal sample to check for evidence of parasites. Depending on the results of the Fecal Egg Count (FEC), your veterinarian can best advise you on which dewormer to use for treatment. Even if the fecal sample is negative, it is still recommended that your horse be on a twice yearly regular deworming program. Your veterinarian can recommend the best deworming program for your horse by considering individual and herd risk variables present in your horse's environment. For instance, your veterinarian will consider things such as movement of new horses onto the farm, number of horses turned out together, the size of the grazing area, and your horse's travel activity (i.e. Does your horses ever leave the farm? If so, how often and to where does your horse travel?)

Some of the most commonly used dewormers include paste or gel that is administered by mouth with a dosing syringe. These are usually given every few weeks to few months depending on the product and the horse's risk for infection. Other dewormers are intended for daily use and are administered in the feed. These products are usually in pellet form.

Some dewormers are recommended to be given at certain times of the year since they are effective against parasites in an encysted larval stage (this refers to the immature parasites that are embedded in the intestines) or tapeworms.

Anthelmintic treatment will always be less effective if there is continuous exposure to infective parasites. You can help reduce the spread of, and re-exposure to, parasites with some basic management techniques.

  • Pick the manure piles out of the pasture twice weekly, or drag the pasture with a chain harrow on a regular basis to break up manure piles (horses should not be grazed on a harrowed pasture for 2 to 4 weeks). This will expose the eggs to the UV radiation from the sun, which will effectively kill many-but not all-types of parasite eggs. Roundworm eggs are extremely resilient to environmental factors so dragging fields where young foals, weanlings and/or weanlings will graze is not recommended. When new horses arrive at the farm, keep them separated, and treat them with a dewormer prior to introduction to grazing on the pastures shared with other horses.
  • Identify high fecal egg shedding horses (these horses usually represent about 20% of any given herd) through FEC's and do not allow them to graze with (and infect) the rest of the herd.

Parasite control is an important factor in the health and wellbeing of your horse. Working with your veterinarian to identify your horse's parasite status through simple diagnostic tools, reducing environmental risk factors for re-infection and appropriately utilizing anthelmintic products based on the horse's clinical needs, you will minimize the opportunity for parasites to negatively impact your horse's life.