Here’s the bad news: worms are becoming resistant to our efforts to kill them, thanks in part to traditional deworming protocols that called for dosing horses every two months. That was the thinking when the first broad-spectrum dewormers (also called anthelmintics) hit the market back in the 1960s, but over the decades that protocol helped the worms become less sensitive to the medicine. “And there are no new dewormers available for immediate release,” said Dr. Jacquelene Pasko, a field care associate at Hagyard Equine Medical Institute, US Equestrian’s Official Equine Pharmacy and Veterinary Service Provider.

So veterinarians and horse owners today use a different strategy to maximize dewormers’ effectiveness and keep potentially harmful internal parasites at bay. Here’s what you need to know:

The target.

“People used to target for the large strongyles, and that’s kind of how we got into the two-month rotational deworming cycle that we now have to move away from,” said Pasko. “Those large strongyles are still a concern if they’re in a horse in large numbers, but today we have them pretty much under control. So the ones that we’re targeting now are the small strongyles and the tapeworms in our adult horses.”

Your horse’s immune system is your ally.

“Horses have some natural immunity to worms,” said Pasko. “Different horses are going to have different immune status, just like they would to anything else their immune system is exposed to. We’ve found that in a horse that doesn’t need deworming frequently, you don’t see any additional health benefits by deworming him more often than he needs. The only thing that does is help accelerate the process of dewormer resistance.”

Resistance risk factors.

Frequently deworming a horse that doesn’t need it is one. Another: only using the same type of dewormer every time. “Different worms are sensitive to different dewormers,” said Pasko. “So if you use the same type of dewormer over and over again, you’re not affecting the worms that aren’t sensitive to it.” Worse, you’re actually encouraging the worms that are sensitive to it to become resistant.

“The worms are going to try to survive,” Pasko explained. “So the ones that have genes that allow them to expel the dewormer or be insensitive to it are the ones that are going to be left, so effectively you’d be selecting for those resistant worms. You want a worm population that has sensitive worms so that when you use a dewormer you actually get an effect.”

Monitor your horse’s fecal egg count.

“For adult horses, we recommend doing a fecal egg count both in the spring and in the fall,” said Pasko. “We’re looking at the amount of eggs that horse is shedding, and the fecal egg count the best tool we have to evaluate that so we can tailor the deworming to the horse’s specific needs. We use that to determine whether a horse is a high, low, or moderate shedder, based on the number of eggs we count per gram of feces, and we base our deworming recommendations on those categories.”

Pasko recommends taking a fecal egg count in the spring and fall, when worms tend to shed more eggs. Different dewormers have different lengths of effectiveness, but, in general, you want to make sure to take the fecal egg count when the most recent dewormer you’ve given is just past its window of effectiveness—your vet can help with the proper timing, which will help you get an accurate picture of how many eggs your horse is shedding.

To test for resistance to a specific dewormer type, consider taking a fecal egg reduction count (FERC), too.

“For that, you take your initial fecal egg count, then deworm the horse with the appropriate dewormer, and then a couple of weeks later you take a second fecal egg count (the FERC),” explained Pasko. “You’re looking to compare the number of eggs in the first fecal egg count versus the amount in the second. Your vet will be looking for a certain percentage of decrease in those numbers, and if the count doesn’t reach that percentage, then there’s a suspicion that you might have some resistance to that dewormer. That might influence the dewormer choices you make.”

Testing multiple horses in a herd—provided the horses have been on the property for some time and haven’t recently shipped in from elsewhere—can provide a good general picture of how widespread any resistance might be, Pasko added. Be sure to include the highest egg-shedders to get a clearer picture of dewormers’ effectiveness.

Your horse needs a tailored deworming schedule.

Your vet will also consider the climate where your horse lives when making a deworming recommendation, but broadly speaking, Pasko says, “Low shedders really only need to be dewormed once or twice a year, both for strongyles and tapeworms. For moderate shedders, we usually recommend those twice-yearly dewormings as well as one or two additional dewormings in spring and fall, because those are the times when the worms are shedding the most eggs. For the high shedders, you have the basic twice-a-year deworming for strongyles and tapeworms, and then we’d recommend two to three additional dewormings divided up between the spring and the fall, depending on how the seasons progress where the horse is located.”

Mix it up.

Vary the types of dewormers, but consult with your veterinarian, who can help you determine when to use what. “Different dewormers work for strongyles than on tapeworms, so what you use depends on what you’re targeting,” said Pasko. “The twice-a-year dewormings are targeting strongyles and tapeworms, and the additional deworming targets strongyles.”

“Deworm” your pastures, too.

The goal of a good deworming protocol isn’t only to deworm your horse. It’s also to decrease environmental contamination. “We’re really trying to decrease egg-shedding into the environment, where it can then be picked up by other horses,” Pasko said.

Where practical, periodically removing manure from pastures and turnout areas where horses graze is one idea. Controlling pasture population density is also helpful: a crowded pasture means more manure in more places where horses graze, and that’s how worms spread. If you spread manure on your grazing fields, Pasko says, be sure the manure is composted first.

“Composting manure properly should take care of any larvae that was in that manure,” she said. “But when you spread uncomposted manure that has live larvae in it, you’re just contaminating your pastures more.”

 

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