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Care Guide

About Care Guides[x] These care guides are written to help your clients understand common conditions, tests, and procedures, as well as to provide basic information about pet care. They are based on the most up-to-date, documented information, recommendations, and guidelines available in the United States at the time of writing. Pharmaceutical product licensing, availability, and usage recommendations are based on US product information. Use the Download Handout button to generate a PDF for printing or e-mailing to your clients.

Parasite Prevention and Deworming in Horses

    • Parasites are of significant concern in horses. Infection with intestinal parasites may cause mild clinical signs 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 eats the eggs when licking or chewing the affected area.
    • Several different deworming products are available for parasite control and prevention in horses. Discuss a deworming program with a veterinarian who is familiar with your geographic region and your horse’s environment.
    • Pasture management techniques such as picking manure piles out of the pasture twice weekly or dragging a chain harrow around the pasture to break up manure piles are helpful for reducing infection with intestinal parasites (horses should not be grazed on a harrowed pasture for 2 to 4 weeks).

    What Do I Need to Know About Parasite Prevention and Deworming in Horses?

    Parasites are most commonly acquired when a horse is grazing on fecal-contaminated areas. Parasite eggs are shed in feces and can survive for a lengthy amount of time in manure piles on the ground. Some parasites, such as the bot fly, lay eggs on the horse’s haircoat, and the horse eats the eggs when it licks or chews the area. Once swallowed, the eggs hatch and begin their lifecycle within the horse, affecting the gastrointestinal system.

    Intestinal parasites are of significant concern in horses. In a natural environment, horses would graze over large amounts of space, thereby reducing the risk of transmission of intestinal parasites. Most horses, however, are stabled with 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 (worms) migrate to arteries that supply blood to the gastrointestinal tract. If there are many worms, they may cut off the blood supply to the intestines and cause life-threatening colic. Other intestinal parasites migrate to parts of the intestines or to the stomach.

    Foals are more susceptible to intestinal parasites than adult horses are. It is important to discuss deworming recommendations for your mare and foal with your veterinarian before foaling.

    External parasites, such as lice and ticks, may also be a problem in certain regions. Ticks may transmit blood-borne parasites such as Anaplasma phagocytophilum, a parasite that most commonly causes fever and lethargy (tiredness).

    What Are Clinical Signs of Parasite Infection in Horses?

    Clinical signs of parasite infection in horses include the following:

    • Dull haircoat
    • Failure to gain weight/poor body condition
    • Lethargy/lack of energy
    • “Potbelly” appearance
    • Colic (mild to severe)
    • Diarrhea

    How Is Parasite Infection Prevented and Treated?

    Your veterinarian may take a fecal sample to check for evidence of parasites. Depending on the results of the fecal exam, your veterinarian can best advise you on which dewormer to use for treatment. Even if the fecal sample is negative for parasites, it is still recommended that your horse be on a regular deworming program. Your veterinarian can recommend the best deworming program for your horse. He or she will consider things such as movement of new horses onto the farm, the number of horses turned out together, the size of the grazing area, and your horse’s travel activity (how often and where your horse travels).

    Some of the most commonly used dewormers come as a paste or gel that is administered by mouth with a dosing syringe. These are usually given every few weeks to few months. 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 (typically November to January) because they are effective against parasites in an encysted larval stage (this refers to the immature parasites that are embedded in the intestines). Additionally, your veterinarian may recommend rotating between deworming products throughout the year to prevent parasites from developing resistance to the ingredients in a single dewormer.

    You can help prevent the spread of, and reexposure to, parasites with some basic management techniques. Pick 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 UV radiation from the sun, which kills many types of parasite eggs. 

    When new horses arrive at the farm, keep them separated and treat them with a dewormer before grazing them on pastures shared with other horses.

    Fecal exams can assess the effectiveness of your deworming program and identify horses that may need treatment for worms.