Practical Tips to Prevent Parasite Resistance in Horses

Scientists have reported widespread parasite resistance against every anthelmintic drug class on the U.S. market.

Scientists have reported widespread parasite resistance—internal parasites’ ability to survive deworming—against every anthelmintic drug class on the U.S. market. No new anthelmintics have been introduced for horses since the 1980s, and none appear to be coming down the pipeline. Therefore, if we want to keep our horses healthy and prevent illness associated with high worm burdens, we need to take a more natural approach to parasite control.

During the University of Maryland Extension’s healthy horse-keeping webinar series, agriculture educator Maegan Perdue described a variety of steps owners can take to do just that.

“It’s important to understand parasite resistance so we can use our tools the correct way and not abuse them and cause the problem to get worse,” she said.

Parasite resistance occurs, she said, when deworming programs aren’t effective—and even under the best management practices, no dewormer is 100% effective. “Resistant parasites that survive treatment are going to pass their genetics to the next generation, and a lot of these parasites can make thousands of eggs a day,” she said. “Just a few surviving can have bad implications on your program at home.”

When to Deworm

Historically, parasitologists recommended deworming horses every two months, because they noticed eggs returning two months after initial treatments. “Our first dewormers weren’t around very long before we had resistance to them,” said Perdue. “It was first documented in the 1960s. Widespread resistance has since been documented in all drug classes on the market.”

To slow the development of resistance, veterinarians now recommend taking an integrated approach to parasite management, using strategic deworming.

“You don’t want to go to the feed store and buy six tubes of dewormer and deworm every horse in your pasture,” said Perdue. “Only deworm the ones that need it.”

In doing so, you also help bolster a population of parasites known as refugia—those that were not exposed to the dewormer and are still susceptible to it—so they can pass on their genetics.

So how do you determine which horses need to be dewormed? Through fecal eggs counts (FECs). “We rely on these in equines to determine if they have high parasite loads,” said Perdue.

Your veterinarian can collect and run fecals, or you can collect samples yourself and send them to a private lab. If you choose the latter route, “share the report with your vet so you can discuss who needs to be dewormed, when, and with what type of drug,” she said.

In addition, fecal egg count reduction tests (FECRTs) can help you determine the level of resistance on your farm and whether the product you’re using is effective: “So take fecal egg counts, deworm, do another fecal egg count 14 days later, and see if there’s a significant reduction,” said Perdue. “That means the drug is working.”

Susceptible classes of horses she recommends paying close attention to include foals (who are developing their immune systems), seniors (who are more vulnerable to disease), and working horses (their bodies are under more stress than the pasture pet’s).

“Work with your veterinarian,” she urged, “who can help you determine if and when to deworm.”

Environmental Controls

When it comes to preventing parasite resistance, you can only control what you can control, said Perdue, which includes how you manage your pastures and how clean you keep them. She listed smart practices property owners can employ in the fight against resistance:

Quarantine new horses “When you get a new animal, you are going to get its parasites too,” said Perdue, “So you’ll want to isolate those animals until you know what you’re dealing with and get it under control.”

Don’t, however, isolate them on pastures your other horses will be grazing—try a drylot or sand arena instead. This quarantine period is also a good time to perform an FECRT to find out what type of resistance is coming in with that animal, she said. Keep new horses separate long enough for the eggs to pass through their systems (as determined by the FECRT).

Reduce chances of horses picking up parasites The only way to truly achieve this is by preventing your horses from grazing—keeping them in stalls or paddocks with no grass. Otherwise, Perdue recommends:

  • Removing manure piles before eggs hatch;
  • Composting manure at temperatures above 104 degrees F to destroy eggs and larvae;
  • Ensuring feed and water sources are not contaminated with manure; and
  • Feeding off the ground, such as with hay feeders.

Keep pastures healthy 

To mitigate parasites in a grazing system, Perdue advises:

  • Keeping grass taller than 3 inches, because infective larvae typically travel only 2-3 inches up a blade of grass. This is not only healthier for the grass but horses also have less chance of ingesting infective larvae, she said.
  • Using proper stocking rates (one horse per 1-2 acres) to help keep pastures at appropriate heights and reduce horses’ chances of grazing around manure piles containing infective larvae.
  • Renovating pastures for better forage selection, which can lead to less overgrazing of desirable forages. “If your pasture has spots with nice grass and spots with weeds, all the horses are going to go for the nice grass,” she said. Till the land before you reseed it, because some larvae get buried and can’t survive tillage.

Implement pasture rotation This practice allows pastures to rest, grow, and get back to the height we need them, said Perdue, while also disrupting the parasite life cycle. Horses must be moved before they graze grass too short, however. “Get down to 3 inches and move them,” she said. “This encourages more uniform grazing and prevents overgrazing of certain areas.”

Implement multispecies grazing Turn your horses out with a species (e.g., ruminants) that doesn’t share the same parasites. Thanks to their complimentary grazing habits, the goats or sheep or cattle will eat the weeds and undesirable forages that horses won’t, said Perdue. Just make sure everyone plays nice, she warned. You can also rotate species through pastures, starting with the most susceptible group on the cleanest pasture first, she added.

Drag pastures Dragging breaks up manure balls and exposes larvae to hot, dry conditions they can’t survive. As an added bonus, said Perdue, it distributes nutrients from manure that might contribute to a more productive pasture. She cautioned that dragging can make parasite problems worse if breaking up the fecal balls spreads eggs and larvae over the entire pasture or if environmental conditions (warm, cloudy, moist vs. really hot) are favorable for larvae to survive. After dragging, remove horses and rest the pasture for the season.

Keep clean pastures clean A fairly new recommendation, said Perdue, is to avoid placing horses that have just been dewormed on clean pastures. “If we take an animal, treat it, and move it to clean pasture, all the eggs it’s shedding are coming from parasites that are resistant to the dewormer we just used,” she explained. “So the only thing being deposited on it are resistant parasite genetics.”


Livamol with BioWorma is a new product on the U.S. market designed to eliminate parasites. It’s not a dewormer but a fungus (Duddington flagrans) you put in your horse’s feed daily that traps and kills nematode larvae. Because it’s dosed by animal weight, this product can be expensive, Perdue said, costing about $4 per 1,000-pound horse per day.

“According to its website and the research done so far, it can reduce parasite larvae on pasture by an average of 84% in horses,” she said. “If you have a really contaminated pasture, this can help you out.”


Horses are always going to have parasites. Just because they do, however, isn’t reason alone to deworm them. “We want to deworm them because they need it,” said Perdue. “So if their fecal egg count is super high and we want to reduce that potential for pasture contamination, then we need to deworm them.”

Otherwise, we’re simply perpetuating parasite resistance against the few effective deworming drugs we have available. Management tactics such as keeping pastures healthy, cleaning up manure, and grazing other species can help, said Perdue, when used together to control parasites.

Work with your veterinarian to put a good protocol in place and make sure you’re using an effective product in the right dose, at the right time.






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