Equine Parasite Resistance

By Lisa Kemp
(my website for link: www.KempEquine.com)

It’s good to deworm your horse regularly, rotate medications, and eliminate as many worms as possible, right? It’s what we’ve done for decades. Now, it’s looking like that formula is no longer the answer. And it’s becoming a global issue, rooted in our own backyards and pasture lands. So what’s a responsible horse owner to do? Read on for the newest wisdom and guidance on the topic of equine parasite management.

The Worm Docs
Equine parasitologists are finding that parasite resistance to anthelmintics, drugs that expel parasitic worms (helminths), has increased since the 1950s (CHECK). They’re sounding the alarm now because of the dearth of effective deworming drugs entering the marketplace, meaning we need to make sure what we have access to still works.

They’re also working to find solutions. Recent parasitology gatherings and conferences have focused on topics such as quantifying parasite resistance and developing sustainable practices. Eight presentations from a March, 2009, gathering at the University of Kentucky Gluck Equine Research Center have been recorded and archived at the website for The Horse magazine, and together provide an overview of the issues and proposed solutions.

The upcoming World Association for the Advancement of Veterinary Parasitology (WAAVP) conference, scheduled this month, is titled ‘Parasites in a Changing Landscape.’ While the topic matter is broader than just equine, a quick scan of the scientific program shows that parasite resistance to anthelmintics, and how to not only manage but optimize deworming using fecal egg counts and sustainable practices, is a hot topic in the industry.

It’s About Time
Since the 1980s, a variety of over-the-counter paste wormers have made it easy to deworm horses according to the calendar. Squeeze, squirt, swallow…and another worm bites the dust. But that easy-peasy approach and sometimes prophylactic use of medication has fostered an environment in which the parasites left behind are resistant to deworming medication, and they’re the ones passing on genes to future generations of parasites.

One possible indicator of anthelmintic resistance appears to be a decreased time interval between deworming and the recurrence of parasite eggs in the feces, referred to as an egg reappearance period or ERP. A 2008 study showed a shortened lifespan and maturation cycle in small strongyles following ivermectin treatment; the parasites returned more quickly than when ivermectin was first commercially available. The concern is that lower ERPs might indicate a resistant parasite population.

Experts differ in their opinion of just what constitutes a resistant population, whether it’s up to 25% of a population showing signs of resistance, or if it’s any indication of resistance at all. What they do agree on is that it’s irreversible.

Let’s Talk About Worms
“When ivermectin came out about thirty years ago, Strongylus vulgaris was the parasite we were targeting most; it did a large amount of damage to both young and adult horses,” says Dr. John Byrd of Mahomet, Ill.-based Horsemen’s Laboratory (CHECK business name, website, location, etc. – DVM, other letters?). “Ivermectin killed adult strongylus vulgaris , but also a high percentage of stronglus vulgaris larvae circulating through the horse. We never had a product that effective before,” says Byrd. “Due to ivermectin, we rarely see Strongylus vulgaris anymore, yet we are still deworming our horses as if it’s the big problem it was.”

Historically, these large strongyle larvae, or bloodworms, were the origin of significant health issues in adult horse populations where they had gained a foothold. Migrating through a horse’s organs as well as its intestinal tract and arterial system (CHECK), S. vulgaris have caused thromboembolism, colic, and death. They were the heavy hitters of the equine parasite world. (CHECK – correct to say?)

As for other parasites, they cause varying amounts of harm. Small strongyles are pesky and can bring about diarrhea and weight loss, but don’t seem to cause the more serious health issues their larger brethren do. Various types of tapeworms, long thought not to affect horses since segments dissolve in the large intestine and aren’t typically visible in manure, can contribute to a range of problems including ileal impaction or spasmodic (gas) colic.

Dr. Hoyt Cheramie, DVM, MS, DACVS, Veterinary Professional Services, Merial Limited (CHECK title, letters, company name) says that ascarids (Parascaris equorum), also referred to as roundworms, are a serious issue for foals, and it’s the eggs that are infective. “After 10 days of being passed, ascarid eggs have infective larvae in them, and they can be picked up from almost any surface. Plus, they survive for an amazingly long time,” says Cheramie. The ascarids travel through a foal’s lungs, causing damage that can limit functionality and health. Fortunately, adult horses are generally resistant to ascarids.

Addressing parasites effectively has a direct effect on the health of the horse; the question now is, what’s the best methodology?

The Refuge of Refugia
Enter the concept of refugia, essentially a way of keeping the parasite gene pool sensitive to anthelmintic drugs. If an entire parasite population is killed off, the only worms remaining are those that survived the dose, and are resistant to the medication. If all you have are resistant parasites, their stronger genes are going to be passed on.

However, if there are sensitive parasites that escaped the deworming medication, then you have a better chance of maintaining anthelmintic sensitivity in your parasite population, allowing you to keep it from getting out of hand and harming your horses. “Until recently, if we found any eggs at all, we recommended deworming the horse. Following the Gluck symposium, we advise the owner that it’s better to leave a few strongyles behind, and try to explain why,” says Byrd.

Maintaining refugia in your parasite population is like working with the worms rather than decimating them. Your best chances for success involve a combined methodology of smart pasture management; selective, customized use of dewormers; and ongoing assessment of parasite numbers and each horse’s ‘shedding’ tendency through use of fecal egg counts.

A New Parasite Paradigm
If you’ve not heard of ‘shedding’ before, it’s an important concept in the new strategy for controlling worms. Although it’s not clear why, some horses will simply shed more eggs in their manure than others, leading to more opportunities for re-infection for the entire herd.

By deworming high shedders more frequently, you’ll minimize the numbers of eggs going into the environment. However, to compensate for that additional deworming and to promote refugia, you should deworm low-shedding horses less frequently, sometimes as little as once or twice per year.

“We don’t see horses getting dewormed less frequently dying from parasite infestations as we dis with S vulgaris. The parasites we deal with don’t intend to kill their host; they would then limit their lifespan. It’s not in the best interest of the parasite to severely damage the host,” says Cheramie. He points out that understanding how parasites operate, and working with them to limit transmission and maintain refugia while preventing clinical disease, are the current strategies being embraced globally.

“One theory is that there’s a baseline level of parasitism in humans helps to modulate the immune system, something that’s been suggested due to the lower level of allergies and immune related diseases in undevelped countries than in the U.S.,” says Cheramie. “Whether or not we can extrapolate that to the horse remains to be seen, but there’s now some thinking that the goal of parasite control shouldn’t be to have zero parasites, but instead to control and limit the transmission of parasites across equine populations as much as possible.”

The New Formula for Success
The new strategy means a little more work, a bit more thought put into the how and the why of deworming. It’s no longer total annihilation of parasites, but sustainable management of environment and practices. It’s a recipe to protect the effectiveness of deworming drugs we already have, so that we can continue to use them into the future.

Sidebar: Parasite Protocols for Horse Owners:

Dr. Byrd and Dr. Cheramie offered some hands-on, useful tips for horse owners to put into immediate practice:

Environment Management:
Did you know that in early morning hours a worm larvae will crawl to the top of a blade of grass, then descend to the roots once the temperature heats up? Or that a single dewdrop can harbor over 100 strongyle larvae? To manage a horse’s environment in order to minimize parasite exposure, it’s important to understand a parasite’s lifecycle and habits, and take them into account as you design your environment management strategies:
– Remove manure from a pasture at least twice weekly, before larvae become infective.
– Dragging a pasture is OK, but do it during the hottest part of the day when exposed worm eggs and larvae are more likely to die off, not in the cooler morning and evening.
– Keep grass mowed short; it provides fewer places for worms to hide.
– Dry lots and stalls are less likely to be a problem, provided manure is removed regularly and the area is kept clean.
– Areas where foals and weanlings are kept can be hosed down to remove eggs, although it’s unlikely to kill them.
– When possible, an open grassy area is a better foaling option for mares than a dirt-packed stall floor with bedding on top. If a stall is necessary, power-wash the walls, floor and mats in between horses to minimize ascarid egg populations.

Parasite Management:
Counting parasite eggs in manure is something we’d gotten away from as over-the-counter dewormers became widely used. Now, as we need to be more selective and targeted in our anthelmintic use, it’s both recommended and makes sense to know what, where, and in whom the target is.
– Have your veterinarian perform fecal egg counts for your horses before they’re dewormed, and keep track of the numbers over time to get a good picture of what’s happening. Or, use an Internet service like Horsemen’s Laboratory if you want to collect your own samples and mail them in for evaluation.
– Deworm high-shedding horses more often, low-shedding horses less frequently.
– Use a dewormer that’s suited to your parasite population. Consult your veterinarian to determine if a rotational cycle of medications is needed or warranted.
– Isolate new horses. Perform fecal egg counts and deworm appropriately before introducing them to your established herd.


Dr. John Byrd – Horsemen’s Lab:


The Horse
– Link to replay of Q&A (text) from ‘Ask the Vet Live’
– Links to 8 video webinar “Horse Courses’ from the Spring 2009 series at University of KY Gluck Center:

Fort Dodge Animal Health PDF – ‘Stop the Madness’ 4 page PDF and ‘Worm Smart’ chart:

Scientific program PDF for World Association for the Advancement of Veterinary Parasitology (WAAVP) 2009 conference