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Fun fact: Did you know that horses always have a small number of parasites in their GI tract?

It is important to regularly treat your horse for worms, as worms are one of the leading contributors to colic in horses, and can cause other medical troubles. It was once believed that horses should be treated for worms every other month, but researchers have since discovered that applying de-worming medication too frequently caused the parasites to develop a resistance to deworming medications, and therefore reducing the effectiveness of the medication.

Now, researchers and veterinarians suggest that you test your horse’s feces for the presence of parasites in the spring and the fall before deciding to treat them. One of the most common techniques is the McMaster’s Technique, which quantifies the number of eggs in the feces, often called a Fecal Egg Count, or FEG. The FEG is measured by Eggs per Gram, or EPG. This will help you and your veterinarian determine if your horse is ready to be treated, and what type of treatment and dosage they will be needing.

While it is best to consult your veterinarian before treating your horse, most adult horses need to be dewormed once or twice a year because many build up a natural resistance to worms as they age. Foals require a different set of AAEP guidelines for controlling worms. Foals should be dewormed at 2-3 months, 4-6 months, 9 months, and 12 months old. Foal deworming is often focused on specifically targeting ascarids and small strongyles. Luckily, foals usually develop a natural immunity to ascarids as they age.

If you own multiple horses, or your horse lives on a farm with other livestock, it will be most effective to treat all the animals at the same time. If only a horse is treated, they can become reinfected more quickly by exposure to other infected animals.

There are three different classes of deworming medications: benzimidazoles, pyrimidines, and macrocyclic lactones. Benzimidazoles target both long and small strongyles, ascarids and pinworms. Pyrimidines target large and small strongyles, ascarids, tapeworms and pinworms. Macrocyclic lactones target large and small strongyles, ascarids, pinworms and bots.

The Penn State’s Extension Site is one of several resources that has an excellent chart that simply breaks down the different EPG shedding tiers for treatment quantities, a simple breakdown of which drugs affect which parasites, and what branded versions of each drug type could entail.

In the world of veterinary medicine, there have been many discussions and concerns about the efficacy of the current, chemical de-worming treatments due to chemical resistance. This happens when worms survive the medicine and is usually caused by the worm’s repeated exposure to a specific kind of drug. If all parasites develop resistance to the current drug classes, there will be no effective medical treatments for parasites.

Most deworming medications come in the form of a paste, gel, oral suspension, a pill.

At BRD, we are happy to provide these deworming options. Feel free to check out our resources for benzimidazoles (fenbendazole) which is used to treat strongyles, ascarids and pinwords), pyrimidines (pyrantel which is used to treat strongyles, ascarids, pinworms and tapeworms), macrocyclic lactones (ivermectin which is used to treat strongyles, ascarids, pinworms and bots).

Have any questions? Free free to reach out, and join our newsletter for updates on horse health.

PennState Extension. (2021, November). Decoding Dewormers: Types, Resistance Concerns, and Use for Horses.

University of Minnesota Extension. (2021) Deworming your horse. Horse Health.

Burton, Erin. (n.d.) Module 3.3: Modified McMaster’s Technique. Clinical Veterinary Diagnostic Laboratory, Creative Commons Attribution-NonCommercial 4.0 International License. Accessed online, March 17, 2022 from

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