Deworming Horses

By: Kathleen Morriss, DVM

TVMA Member
Junction, TX.

Published February 2018

Internal parasites in horses cause everything from dull coats and diarrhea to severe colic. Most horse owners take preventative measures by including deworming in their horses’ health care plans. The common way to deworm a horse is with an oral paste, and there are many options available at the local feed store. Which is the best dewormer for your horse? How often should your horse be dewormed? Your veterinarian can answer these questions, and it is worth a consult to develop the most effective deworming schedule for your program.

The Rise of Resistance

Many horse owners follow a deworming schedule in which all their horses, whether they need it or not, are dewormed every three to four months. Unfortunately, this constant use of dewormers leads to resistance in the parasite population and less effective medication. When a population of parasites is exposed to a dewormer, there are some resistant parasites that survive. These resistant parasites reproduce, creating more resistant parasites. As a result, the next round of deworming is less effective, and additional resistant parasites survive. Eventually, this leads to a large population of resistant parasites that cannot be exterminated with current drugs.

The Refugia Concept

One way to combat this rise of resistance is to develop new drugs, but this is difficult and costly. The best way to manage resistance is to reduce the use of dewormers by only deworming horses that need it. Research shows that approximately 20 percent of a population of horses is responsible for shedding about 80 percent of the parasites. This means that if you have 10 horses in your herd, there are probably two of them that are naturally less resistant to internal parasites and have the highest load. As a result, these “high shedders” are constantly putting more parasites into the environment. The other eight horses are likely to be “low shedders” that have natural immunity to the parasites and do not contribute as many parasites to the environment. If we refrain from using dewormers in the horses with natural immunity, we will reduce the use of our currently available products. Overuse of a drug, whether it is an antibiotic or an antiparasitic, is one of the key contributors to the development of resistance. The parasites that live in the “low shedders” will not be exposed to the drugs and will not develop resistance. They can then breed with the populations from the horses that are treated, which will help prevent the development of resistant parasites

How to Develop Your Deworming Program

With the goal of reducing resistance, it is important to identify “high shedders.” These are the horses that will need to follow the standard program, which can help reduce shedding. Your veterinarian will use a fecal egg count test to determine how many parasite eggs per gram of feces are present from each horse. Horses that have more than 500 eggs per gram are considered “high shedders.” The horses that are identified as “low” or “moderate shedders” require less frequent deworming. Usually two times per year for low shedders is sufficient. Your veterinarian can help determine which time of year and product is appropriate based on your geographic location. Adopting a targeted deworming program will reduce the development of resistance to our current dewormers.

Special Considerations for Young Horses

Horses two years and younger require more frequent deworming since they have not yet developed adequate natural resistance and are more susceptible to roundworms, which can cause colic and respiratory disease from migrating larvae. As a result, all young horses should be dewormed every two to three months with a product effective against roundworms such as fenbendazole, marketed under the trademark name Panacur®.


With the increasing prevalence of resistance to dewormers, consider scheduling a consult with your veterinarian. Using fecal egg count tests, you can develop a modernized deworming plan for your herd. Although the initial cost is high, the savings over the long term from reducing the amount of dewormer you buy annually will repay the investment. You also will reduce the development of resistance, which will prolong the effectiveness of our current dewormers.

Kathleen Morriss, DVM, is a graduate of Auburn University College of Veterinary Medicine and lives in Junction, Texas, on her family’s ranch and does relief work at small and mixed animal practices in the Austin and San Antonio areas.

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