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Here’s the Deal With Dewormers…

Most horse owners know why deworming is an essential component in their health regimen. Horses are exposed to a number of parasites, some of which can stealthily wreak havoc inside an already delicate gastrointestinal system. The word itself is enough to send shivers up your spine, even though many parasites are harmless and some are even good. And we thank you for your dirty work, good parasites, but your noxious cousins gotta go.

For many years, horses were treated with a “one-size-fits-all” worming approach, rotating between a little of this every couple months, followed by a little of that, then back to this and one more shot of that, to fend off all types of harmful parasites. This worked well enough until the parasites began developing resistance to standard treatments, because that’s what parasites do. It’s practically in their job description.

Turns out, what’s good for one horse is not necessarily good for the whole herd. That’s why it’s so important to administer a fecal egg count test before you begin your yearly treatment program. While not a “be all end all”, it can help determine exactly what type of dewormer each horse needs and how often. Horses with a fecal egg count of 200 or less are classified as low shedders, 200-500 are moderate shedders, and anything more than that are high shedders.

Does it cost a little extra? Sure, and it only tells part of the story, but studies show that as much as 80% of colic cases are caused by parasites, so it’s money and time well spent.

Most veterinarians recommend deworming healthy adult horses at least twice a year. Low shedders can usually get by with this, but the moderate to high shedders need several treatments throughout the year. Spring is the best time to employ dewormers containing moxidectin and ivermectin. Also be sure to include praziquantel for tapeworm control. Some products, like Quest Plus®, contain both.

Know your enemies

Strongyles (aka bloodworms). The most common equine parasites in the ecosystem come in two distinct sizes (large and small), and are potentially the most hazardous to adult horses.
Weapons of choice: Ivermectin and moxidectin for large strongyles. Moxidectin and fenbendazole for the small variety.

Ascarids (aka roundworms). Pose the biggest threat to foals, weanlings and yearlings.
Weapons of choice: benzimidazole/oxibendazole, pyrantel pamoate

Tapeworms. These nasty freeloaders can grow up to three inches and are resistant to some treatments.
Weapons of choice: Praziquantel

Bots. These little flies glue tiny eggs to a horse’s hair where they can be ingested or burrow their way inside.
Weapons of choice: Moxidectin


Bottom line

Different horses have different deworming needs.

Have your vet perform a fecal egg count test this spring and work out the best treatment option for your horses. The test won’t tell the whole story, but it’s a good place to start, especially in the absence of clear symptoms (weight loss, colic, lethargy, diarrhea, poor coat, etc.).

Other factors like age, location and living arrangement will dictate the course of treatment.

Keep your pastures and stalls clean and consider fly/insect control products.


All content is for informational purposes only. Contact your local veterinarian if you have any questions regarding the health of your animals.


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