Mythbuster Monday: Deworm Horses Every 4 to 6 Weeks

On Mythbuster Monday, we tackle a variety of equestrian myths to either bust or confirm. Today’s discussion: Should horses be dewormed every four to six weeks?

It’s Mythbuster Monday, where Horse Nation dives into different equestrian myths and provides research-based evidence to either bust or confirm those myths. Today’s topic: Do horses need to be dewormed every four to six weeks? Will deworming a horse too often cause resistance to the treatment? Is there a more effective schedule to deworming your horse? Read further to find out!

Myth:  Horses should be dewormed every four to six weeks

Myth or Fact: Myth


Horses can get worms if turned out with horses that are or were infected by the parasites. In some instances, these parasites can live in the pastures for years.

Worms are excreted through manure and the manure then mixes with the grass in the pasture. Other horses then graze this area and become infected. Once infected, treatment is needed to kill the parasites inside the horse.

Deworming is an antiparasitic treatment done to destroy, inhibit and remove internal parasites in horses. At some point in their lifetime, domesticated horses will have some level of parasite load. Dewormers are given to avoid the health issues that coincide with large parasite numbers.

There are many different types of dewormers that can be used for treatment, so obtaining the correct product for the removal of parasites is imperative.

Parasite resistance to the medication is a growing concern for management. Some parasites have become highly resistant to the drug making it more difficult to rid them.

Resistance to the drugs is becoming more frequent due to how often they are being exposed to the medication. So how do horse owners prevent resistance to the product?


Red Hills Veterinary Hospital states that the common practice used to be to deworm each horse ever two months. However, this has lead to parasite resistance. They now recommend performing fecal egg counts (FEC) in the spring and fall and treating according to the results.


Chosen Valley Vet also goes off of FECs, but depending on the count depends how many times per year the horse is dewormed. Horses with FECs of less than 200 should be dewormed in the spring and fall. Horses with a FEC between 200 – 500 should be dewormed in the spring, late summer and early winter. High shedders or horses with an egg count greater than 500 should be dewormed in the spring, summer, fall and winter.


Dr. Martin Nilsen of Colorado State University’s Veterinary Teaching Hospital also encourages horse owners to deworm their horses based on fecal egg counts. He makes the suggestion of checking treatment efficacy with FECs after administering the dewormer. Obtaining an initial fecal egg count and then seeing the reduction after treatment will aid in finding an effective product for an individual farm.


The Vermont and New Hampshire Vet Clinic states that the effectiveness of dewormers and parasite resistance are concerning topics in equine medicine. Due to the outdated method of deworming horses every four to six weeks, there are cases of strongyles being resistant to many commonly used deworming products. In addition to the resistance to the medication, no new drugs are hitting the market which has changed the recommendation of how to treat the parasites.


After diving into the research, it appears that parasite resistance to deworming products has pushed for a change in treatment. Rather than deworming horses routinely every four to six weeks, most veterinarians are now stating to get a fecal egg count first before deworming. This aids in utilizing the right deworming product and helps avoid unnecessary medication administration.

Do you have an equine myth you’d like us to tackle? If so, send it our way! Email your suggestions to [email protected]. Put Mythbuster Monday in your subject line.