Parasites present a pesky problem for horses and their owners alike. A year-round rotation of dewormers that alternated every other month was once considered enough to keep your horses protected. However, there is an increasing trend of drug resistance among parasites, making them harder to manage through traditional strategies. Your horse’s parasites are changing; your deworming protocols need to keep up.

Thanks to parasite control guidelines from the American Association of Equine Practitioners and a growing amount of independent research, we know individualized, age-appropriate and seasonal treatment plans are the most successful. Rather than frequent, calendar-based deworming treatments, effective and properly timed treatments are best for keeping your horse and your wallet happy.

Effective active ingredients

For effective treatment, it is vital horse owners know which parasite(s) their horse is infected with and which deworming product effectively treats the parasite. Your veterinarian is the best person to provide this information.

The key parasites in adult horses, small strongyles, are universally present in all grazing horses. The primary concern with small strongyles occurs during the encysted stage of their life cycle, when the parasite burrows into the walls of the lower intestines. Inflammation results when the parasite emerges from this cyst stage.

“Moxidectin is the only deworming active ingredient that has been shown to be effective in treating encysted small strongyles, which are strongyles in the larval stage, in a single dose,” says Dr. Kenton Morgan, DVM, Equine Technical Services, Zoetis.

There is a five consecutive days dosing regimen that claims to be effective against encysted small strongyles, but recent data indicates there is growing small strongyle resistance to fenbendazole.

It is also important to keep in mind that, while the available deworming products may all look different, many brands contain the same active, parasite-killing ingredient. So, even though the products have different names, you may be using the same active ingredient, and overexposure to the same active ingredient could be leading to parasite resistance. That’s why it’s important to read the packaging and look for the active ingredient, often found in parenthesis below the brand name.

Parasite seasonality

Deworming treatments must also be properly timed during the year, corresponding with your horse’s parasite burden and the parasites’ cycles of transmission, typically spring and fall.

Ascarids, also known as roundworms, are hardy parasites resistant to environmental influences. They are the key parasite of concern in young horses, as older horses develop an immunity to them. However, in foals, ascarids can cause poor growth, airway inflammation and small intestinal impactions.

“There are three products that perform best when targeting ascarids,” Morgan says. “The active ingredients to look for are pyrantel pamoate, oxibendazole or fenbendazole.”

Engage your veterinarian

The most important tool in your parasite arsenal is your veterinarian. Veterinarians can perform a fecal egg count test to determine which horses in your herd need treatment and which don’t. Once you have a baseline established, they can work with you to develop an Individualized Deworming program.

“All horses do not carry the same parasite burden. Usually, in a herd, 80% of the parasite burden is hosted by 20% to 30% of the horses,” Morgan says. “High strongyle egg shedders are responsible for the majority of parasite transmission.”

Your veterinarian can also test your horse or herd to see if the dewormer you used was effective. A fecal egg count reduction test will show if your horses have parasites that have become resistant to a specific active ingredient. This test is done after your horse or herd has been treated. Your veterinarian can then help you select additional treatment options, if necessary.

Consult your veterinarian for assistance in the diagnosis, treatment and control of parasitism. Visit QuestHorse.com to learn more about Individualized Deworming treatment plans for your horse.

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