It’s important to stay on top of deworming your horse, expert says


Horses and parasites

URBANA, Ill. — It’s important for horse owners to stay abreast of the latest recommendations for parasite control. Their horse’s health and the effectiveness of anti-parasite medication may depend upon it, according to experts at the University of Illinois Veterinary Teaching Hospital in Urbana.

Back in the 1970s, an insidious equine parasite, the large strongyle, was a big problem, says equine veterinarian Scott Austin. To combat it, equine health experts introduced the interval program, in which all horses on the farm were treated with a rotation of dewormers on a set schedule.

And, thanks to the effectiveness of the interval program, large strongyles, also known as bloodworms, are now very rare.

New approach

Today the most common equine parasite is the small strongyle (Cyathostome), according to Austin. Interval deworming programs have made little impact on the prevalence of this parasite in horse populations, so a new approach to controlling parasites must be adopted by horse owners.

“The University of Illinois equine primary care service recommends a targeted deworming program,” says Austin, who is board certified in equine internal medicine.

“This approach involves identifying the horses with parasites by counting parasite eggs in feces, and then tailoring treatment of each horse on the basis of their level of egg shedding.”

Austin says that individual immunity to parasites is a more important in determining whether an individual horse sheds parasite eggs than is the frequency of deworming treatments.

It is estimated that 20 percent of the horses in a herd account for 80 percent of the eggs shed onto pastures. Up to 50 percent of the horses shed few or no parasites, whether or not they are dewormed, he says.


The dark side of giving dewormers to all horses at a set interval, regardless of the level of parasitism, is widespread resistance to previously effective deworming products. Strains of parasites with resistance to the avermectins, such as Ivermectin and Moxidectin, have recently emerged. Resistance is of great concern because no new classes of anthelmintics that are effective against horse parasites have been identified.

“We must preserve the effectiveness of the drugs that are available,” says Austin. With this goal in mind, Austin recommends using fecal egg counts to determine the level of egg production by an individual horse.


The benefits are twofold: first, fecal egg counts allow you to tailor the treatment for each horse based on the level of egg shedding, thus reducing the overall amount of product used. Secondly, by closely monitoring effectiveness of treatments, you will be able to tell when a particular deworming agent has lost its effectiveness on your farm.

Egg counts are performed by veterinary staff. The proper procedure for collecting fecal samples for analysis is to scoop two fresh fecal balls from the center of the manure pile into a Ziploc bag that has been labeled with the date, owner, and horse’s name. Squeeze the air out of the bag before sealing, and place the bag in the refrigerator if the egg count will not be done immediately.

Egg counts must be done at an appropriate interval after the administration of the last dewormer, so ask your veterinarian to recommend when that will be.


After a horse is 4 years of age, it may be classified into one of three categories based on egg production and should be dewormed at an appropriate frequency to minimize the effects of the parasites.

Guidelines are: low shedder — deworm twice a year; moderate shedder — deworm three times a year; high shedder — deworm four times a year.

“Horses between 1 and 4 years of age, which are developing immunity to parasites, should be managed as if they are high shedders regardless of fecal egg counts,” advises Austin.

A second fecal egg count should be completed two weeks after the horse was dewormed. At this time, if the egg count has not been reduced by at least 90 percent, that means “parasites are resistant to that particular group of products and that product should no longer be used in this herd,” says Austin.

For more information about parasite control, consult your local equine veterinarian.


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