Your farm horse might be resistant to certain parasites. Have a fecal egg count reduction test performed on your horse to see if its deworming medication is working.

When it comes to keeping your farm horse healthy or parasite-free, the answer no longer comes from a simple round of deworming medication or, for that matter, a regular deworming rotation schedule.

“We know that not all horses carry parasite burdens large enough to be of clinical concern. We know that repeated use of anthelmintics [broad-spectrum dewormers] causes selection for resistance. And we know that parasites are not transmitted year-round,” says Bob Storey, lab manager at the University of Georgia College of Veterinary Medicine’s Department of Infectious Diseases. “So, with all that we do know, we should be utilizing a treatment and prevention regimen that is based on need rather than the calendar or tradition.”

To keep your horse parasite-free, work with your livestock or equine veterinarian to determine the treatment method that will work best in your region, on your farm and with your horse. Before you go into that meeting, make sure you have all the information you need.

What parasites affect my horse?
Some parasites that affect your horse depend on the area of the country where your farm is located. Parasites include nematodes, arthropods and tapeworms. To determine what kind of parasites infect your horse, perform a fecal egg count reduction test (FECRT).

However, some parasites in your horse aren’t cause for concern. Your vet can explain which ones and why.

“The most pathogenic parasites, that is the ones capable of causing the most damage including potential death of the horse, are rare now in herds that have been dewormed quite often,” says Gene Lyons, a parasitologist at the University of Kentucky’s Equine Research Center. “The remaining strongyles—the small strongyles that are found in almost all horses—can occasionally cause clinical problems, but usually they don’t produce observable negative effects.”

What deworming medications are available to treat my horse?
The main categories of medications available in the U.S. to treat parasites are benzimidazoles and pyrimidines, which treat nematodes; macrocyclic lactones, which treat nematodes and arthropods; and prazinoisoquinolines, which treat tapeworms.

“Years ago, when there were so many more classes of anthelmintics, and effective ones, on the market, they could be dependent on providing excellent control of parasites,” Lyons says. “Now with such few compounds … selective use of them is imperative.” 

When is my horse most at risk for getting parasites?
Seasons impact the parasites are infecting your horse. Small redworms, a type of nemopod, for example, are transmitted between late fall and early spring in the South, while in the North, they are transmitted between late spring and early fall, Storey says.

Strongyle worm larvae also resist cold and freeze-thaw cycles, says Mary Rossano, Assistant Professor in the Department of Animal Sciences at the University of Kentucky.

Is my current deworming treatment working?
If you notice your horse showing symptoms of parasite infestation, such as a poor coat and weight loss, work with your veterinarian to figure out if it is a problem with your parasite-control regimen or caused by an illness.
Parasitologists advise that you regularly perform a FECRT on your herd, and ask for the results of the egg count in grams.

“In a perfectly healthy, well-cared-for group of horses, you will have a distribution where approximately one-third of the horses will be shedding the most eggs,” Storey says. After performing the fecal egg count on each horse over a period of about six months, you can determine which horses are low, medium and high shedders.

If you think your dewormer isn’t working properly, perform the test, implement your parasite treatment, then perform the test 10 to 14 days later to compare results.

“If the dewormer that you used is effective, the post-treatment number will be less than the pre-treatment number by at least 95 percent,” Storey says. “If not, then your dewormer is not fully effective against your population of parasites.”

What are my other options to control parasites?
Figuring out the best method to keep your horse parasite-free can be a challenge. Rossano offers other techniques you can implement on your farm to limit the spread of parasites.

One option is pasture-dragging to break up manure piles where parasites reside. Do this in the hottest part of the summer, Rossano says, when parasites die the fastest. Don’t drag pastures and spread out the manure during other times of the year—that will only spread the parasites to other parts of the field. Keep the horses off pasture for one to two weeks after doing this.

You can also implement pasture rotations, if your farm is large enough to do so. Make sure younger horses follow the older ones in the rotation, because younger are more at risk for contracting parasites and older horses shed less eggs. Or alternate horses with other livestock on a pasture.

Finally, remove manure from horses stalls, and if feasible, the pasture, and compost it in a way that is hot enough to kill the parasites.

“Removing manure from pastures has been shown to be at least as effective as chemical deworming for controlling strongyles,” Rossano says.