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A New Method for Deworming Horses

Every 3 months, you find yourself wormhole-155328_1280 staring at a shelf full of dewormers with more questions than answers. More often than not, you grab for a familiar-sounding wormer such as an ivermectin like Zimectrin®, a fenbendazole like Panacure®, or a moxidectin like Quest®. Although each of these wormers have their own purpose, few know which anthelmintic to use at what time.  It turns out, this method of routine worming has inadvertently led to problems with parasite resistance

 Resistance on the Rise

Articles released by the American Association of Equine Practitioners and University of Florida’s Large Animal Clinic report startling information about the emergence of resistant parasites. Anthelmintic resistance essentially means  parasites are able to survive a dose of a dewormer that will typically kill the given species. Furthermore, resistance is an inherited trait passed on to offspring (AAEP).

Around 40 years ago, veterinarians recommended regular deworming intervals as a way to control large strongyles—a very prominent parasite that posed a lot of problems at the time. The regular intervals essentially diminished the population by keeping larvae from growing into harmful adults. Now, after years of this method, the problem has shifted away from the less-prevalent large strongyles and toward other parasites (AAEP). In 2001, Kaplan, Klei et al. conducted a study to examine resistant parasites in Southern United States and found that small strongyles are creating a new threat to horses. They concluded small strongyles are showing resistance to fendendazole, oxibendazole, and pyrantel pamoate leaving only one anthelmintic drug class (avermectin/milbemycin) without resistance. This means that some horses (almost half of those in the study) only have one drug class left that will kill parasites.

Furthermore, small strongyles elsewhere in the world are showing resistance to avermectin/milbemycin (Kaplen, et al). The problem arises from horse owners today following parasite control recommendations from 40 years ago. If this continues, treatments will become less effective and give rise to greater problems.

Deworming Meets Modern Methods

Veterinarians are now recommending a new, method of parasite control that does not involve a “one size fits all” approach. They are shying away from using a routine calendar deworming schedule and now suggesting a more customized regime for a healthy and happy horse. Owners should treat their horses by targeting a specific parasite with its biology and lifecycle in mind using an affective anthelmintic to get the job done. This starts by having your veterinarian perform an FEC, a fecal egg count, to determine if your horse is in need of parasite control and if so, which kind of control. They can also test the horses to reveal which dewormers work and which dewormers do not work for your horse.

Most horses have low populations of worms at any given time. Although this makes many horse owners cringe, it is actually okay and makes for a healthy animal. Low levels of parasites stimulate immunity and very rarely cause any disease. Therefore, it is important to perform these tests regularly to know the healthy baseline for your horse and when populations need to be controlled.

The Fecal Egg Count Reduction Test (FECRT) can tell you if your horse’s strongyles or ascarids are resistant to a given dewormer (AAEP). A veterinarian performs a fecal egg count before deworming and then another fecal egg count 14 days after giving the dose. Those numbers are then used to calculate the change in parasites by using the FECRT equation. AAEP has guidelines of suggested levels of no resistance, susceptible to resistance, suspected resistance, and resistance. Your veterinarian may apply the result from the FECRT equation to these guidelines to get a better understanding of your horse’s parasites. If a horse has a higher than normal result, a veterinarian can provide you with the exact product and dosage to treat the problem efficiently and effectively.

Deworming Schedules for Different Horses

For those curious about a “typical” deworming schedule, Colorado State University and the Association for Equine Practitioners outline a few guidelines for adult horses, pregnant mares, and foals. After a vet performs a fecal egg count, they will know the eggs per gram of manure (EPG). Based on CSU’s Equine Recommended Deworming Schedule, adult horses will fall into either of three categories: Low Shedders (less than 200 eggs per gram), Moderate Shedders (between 200 and 500 eggs per gram), and High Shedders (greater than 500 eggs per gram). Your veterinarian can then determine if deworming is needed and the correct dosage. If deworming is needed, Colorado State University suggests these products and times based on the EPG from a fecal egg count:

Adult Horses

Shedding Rate EPG FEC Test Time Spring Summer Fall Winter


<200 Test in spring and fall Ivermectin


N/A Ivermectin

Moxidectin with praziquantel

Moderate Shedders 200-500 Test in spring and fall Ivermectin


or double-dose fenbendazole for 5 days

Pyrantel Pamoate


N/A Ivermectin w/ praziquantel

moxidectin with praziquantel

High Shedders >500 Test in spring and fall and look for resistance Ivermectin


Or double-dose of fenbendazole for 5 days

Pyrantel pamoate


Ivermectin with praziquantel

moxidectin with praziquantel

Pyrantel pamoate




Pregnant Mares

For pregnant mares, Colorado State suggests deworming the same as an adult horse. However, they suggest giving her ivermectin with praziquantel or moxidectin with praziquantel when vaccinating before foaling.


Many horse owners have questions about if, and when they should deworm foals. The American Association for Equine Practitioners notes that horses under the age of 3 should have a different parasite program because they are at a higher risk for infestation. Colorado State University recommends deworming foals as follows:

2 Months Ivermectin
4 Months Oxibendazole—Pyrantel pamoate at 5 months
6 Months Moxidectin with Praziquantel
8 Months Pyrantel pamoate—Fendendazole at 9 months
10 Months Ivermectin
12 Months Fenbendazole

It is important to note that dosage will depend on a couple factors. A horse’s susceptibility to parasites varies depending on: their lifestyle, living arrangements, resistance to anthelmintics, time of year, weight, age, level of egg shedding, etc. (AAEP).  Therefore, make a point to discuss a plan with your veterinarian.

The cost of a FEC test varies by vet but should only cost around the same price as a tube of dewormer. During your next check-up, be sure to do your part in preventing parasite resistance and ask your vet about a fecal egg count test. Together we can make healthier and happier horses.




Works Cited

Kaplan, Ray M. These Ain’t Your Father’s Parasites: Dewormer Resistance and New Strategies for Parasite Control in Horses (n.d.): n. pag. Large Animal Clinical Services University of Florida. Web.

American Association of Equine Practitioners AAEP Parasite Control Guidelines (n.d.): n. pag. AAEP Parasite Control Subcommittee of the AAEP Infectious Disease Committee Web.

Colorado State University (n.d.):n. pag. Equine Recommended Deworming Schedule CSU Web
Creative Commons License
A New Method for Deworming Horses by Summit Equine Nutrition is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.

Published: Mar 30, 2016
Last Modified: 
May 9, 2024 @ 9:31 pm

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