Researchers continue to study internal parasites and their evolution, with a focus on preventing resistance to deworming drugs

Mark Twain probably wasn’t talking about equine parasites when he said, “It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.”

Still, few sentiments ring truer when it comes to the rapidly changing world of parasite control in horses.

It wasn’t long ago we knew regular rotational deworming as the best way keep internal worms at bay. While that was true to the best of our knowledge at the time, it was what we didn’t know—and what researchers sought to find out—that’s led to our current understanding of parasites and how to manage them.

But even that is ever-evolving.

“It’s important to keep researching parasites because all horses are constantly exposed to them, they can cause significant disease, and, as living biological organisms, they keep evolving and changing,” says Martin K. Nielsen, DVM, DVSc, PhD, Dipl. EVPC, ACVM, the Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky Gluck Equine Research Center, in Lexington.

So, he and other researchers carry out new studies to advance their knowledge about equine parasites, the drugs that help manage them, ways to control them sans medications, and more. Here’s what some of that recent research has shown us.

Key Focus: Anthelmintic ­Resistance

One of the main topics scientists are focusing on is anthelmintic resistance (AR), which occurs when parasites survive deworming treatment and then pass that resistance on to subsequent generations. And, says Ray Kaplan, DVM, PhD, Dipl. ACVM, EVPC, it’s a problem all horse owners should understand.

A bit of background: While dewormers are likely some of the most common treatments owners administer to horses, parasites typically don’t cause serious disease in adult horses.

“That’s why my colleagues and I recommend that most adult horses don’t need to be treated very often,” says Kaplan, the senior associate dean at the St. George’s University School of Veterinary Medicine, in Grenada, West Indies. “Rather than traditional rotational deworming, base anthelmintic treatments on individual horses’ needs determined via fecal egg counts (FEC) and fecal egg count reduction tests (­FECRTs).

“If you give a treatment that doesn’t work well and is not needed, you’re just wasting time and money,” he continues. “But the other thing that happens is you’re making resistance worse by giving unneeded ­treatments.”

This could have serious consequences, especially for the population most susceptible to parasitic disease: foals.

“Parasites are a much greater concern in young horses less than a year old,” Kaplan says. “The worse resistance gets because we are treating adult horses that don’t need to be treated, the more at risk foals become to infection with parasites that won’t respond to the dewormers.

“Failure to test for drug resistance can permit resistance to worsen without any awareness; this can eventually lead to potentially catastrophic parasitic disease without the ability to treat it,” he says.

Vets and researchers have long known AR is a growing problem. But to get a better idea of where we currently stand, Nielsen recently conducted a literature review.

Ultimately, he confirmed it’s a problem that’s progressing rapidly with new reports being published regularly.

For instance, in addition to the more well-documented resistance to benzimidazoles and pyrimidines, researchers have found small strongyles (also called cyathostomins) in the United States and Brazil are exhibiting resistance to macrocyclic lactones (ivermectin and moxidectin); this means strongyles now exhibit resistance to all three of the available anthelmintic classes. Nielsen described documented resistance in all small strongyle life stages to the larvicidal fenbendazole treatment, a benzimidazole that has been used to kill encysted larvae in intestinal walls. And in recent years scientists have documented resistance to macrocyclic lactones in ascarids (Parascaris spp) and pinworms (Oxyuris equi).

“No new anthelmintic classes with new modes of action have been introduced for equine usage during the past 40 years, and no such new products are expected in the foreseeable future,” Nielsen said in his literature review.

Shortened egg reappearance periods and anthelmintic resistance

One way to monitor anthelmintic performance is to look at egg reappearance periods (ERPs), or the time between an effective deworming and when the horse begins shedding a significant number of parasite eggs again. Nielsen says researchers have observed ERPs getting shorter in recent years and, “as illustrated in our recent paper, reduced ERPs are associated with significant loss of parasite control.”

That study, he says, is the first to quantify the potential consequences of shortened ERPs in strongyles. He and colleagues evaluated strongylid ERPs following ­treatment with ivermectin and moxidectin using research-based computer simulations.

“The ERPs of ivermectin and moxidectin were originally reported in the eight-to-10 and 12-to-16-week ranges, respectively, but several recent studies have found them to be around four to five weeks for both (active ingredients),” Nielsen says in the study.

Ultimately, the researchers determined the contracting ERPs resulted in significant increases—100 to 300% for ivermectin and “a few hundred to several thousand percent” for moxidectin—in all the parameters they measured: The number of eggs in feces, the number of infective larvae on pasture, and the number of encysted larvae in horses.

“These findings are particularly concerning if horse owners and veterinarians do not monitor ERP and, therefore, operate under a false assumption of anthelmintic performance,” the researchers said in the study.

Emerging anthelmintic resistance in tapeworms

While AR is well-­documented in many equine parasites, this phenomenon hadn’t been reported in the most common tapeworm affecting horses, Anoplocephala perfoliata.

However, Nielsen says, throughout 2022 several vets reported to him that the two available drugs used to eliminate A. perfoliata eggs—praziquantel and pyrantel ­pamoate—were showing instances of becoming less effective. So he carried out a study that confirmed both products showed reduced efficiency in eliminating A. perfoliata eggs in Thoroughbred yearlings and mares.

“These findings sharply contrast data from historic field efficacy studies conducted for both (dewormers) and raise concern about anthelmintic resistance having possibly developed,” Nielsen says in the study, which underscores the need for sustainable alternate control options.

New Guidelines for Diagnosing Anthelmintic Resistance

“The only way to tell if you have drug-resistant parasites in your horses and on your farm is to test with the fecal egg count reduction tests,” says Kaplan.

However, he adds, if not conducted and interpreted correctly, FECRTs can yield misleading results. That’s why he, Nielsen, and colleagues recently published guidelines on how to use FECRTs to diagnose AR in several species, including horses. The full guidelines are available in the study, accessible at tinyurl.com/yppndber.

Previous FECRT guidelines focused on sheep and weren’t appropriate for horses.

“The guidelines were designed after performing lots of mathematical exercises to figure out how many horses are needed, and how many worm eggs need to be counted during the fecal egg counts, to have a high probability that the tests are going to produce an accurate, conclusive result rather than just ‘a result,’ ” Kaplan says.

“If you only have one, two, or three horses … we won’t necessarily be following the guidelines, (which require a minimum of five horses), but we should still do the tests,” he continues. “We just have to interpret those results a little bit more carefully.”

For instance, he says, an owner testing ivermectin efficacy in a single horse might end up with zero eggs in the post-treatment sample. This works out to a 100% reduction in eggs but might not be an accurate result.

Kaplan says when getting such a result the drug might very well have yielded high efficacy. “However, it may not have,” he adds. “You may have only had an 80% or 90% reduction in eggs, but just by chance you didn’t see any eggs in the post-­treatment test. That’s a possibility, especially if the horse did not have a lot of eggs prior to treatment.”

“The more horses we test, the more we can eliminate that possibility,” he says. “If we tested five horses with ivermectin and then counted the sufficient numbers of eggs in the guidelines, we can be very confident that if we see 100% reduction in that, that the drug really was highly effective.”

The ‘Why’ Behind Horse Owners’ Parasite Control Programs

Researchers in Ireland recently assessed horse owners’ opinions on parasite control practices and AR and, more importantly, why they hold those views.

While turning a field study derailed by COVID-19 into a desk research project, Nicola Walshe, MVB, MRCVS, PhD, Cert. AVP, reviewed the literature regarding horse owner parasite control practices and came across two studies, published about 20 years apart, involving surveys of Thoroughbred owners’ parasite control programs in Ireland and views on AR.

“When I looked at the results, they were very similar,” says Walshe, an assistant professor in equine clinical studies at the University College Dublin School of Veterinary Medicine. “As a member of both the academic and clinical communities, it struck me that we had 20 years to change people’s minds on overreliance on therapeutics to their approach to anthelmintic resistance, their approach to diagnostics and pasture management. And nothing changed.”

So she decided to work with a group of social scientists to try to find out why things haven’t changed.

The team found most owners indicated their parasite control programs relied “almost exclusively” on dewormers.

“I expected the overreliance on ­therapeutics, but I did find it interesting that every time we asked people what their parasite control program was, the first thing they gave us was their anthelmintic treatment regime,” she says. “That was often the only thing until I probed them with other ­questions.”

And even though past research has shown removing manure from pastures is a highly effective parasite control measure, most owners indicated it was too labor-intensive and “seemed unwilling to consider it as part of a parasite control program,” the team said.

Most owners did not obtain parasite control advice from their veterinarians for multiple reasons, including cost and because the farm hadn’t done so in the past. Some, however, worked with their veterinarians on FECs. The owners who still refrained from sampling and testing said they did so because the diagnostics were expensive and labor-intensive, especially if horses aren’t stabled.

Walshe says she was particularly interested in what she learned about owners’ views on AR: “People recognized and knew a bit about it. They recognized it as an industry-level threat, but they couldn’t tell me how they would identify it on their own farm.”

All in all, helping owners gain a better understanding of AR, how to detect and manage it, and ways to manage parasites sans anthelmintics could help change the tides of relying on tradition to dictate parasite control measures.

“That’s why we feel it is crucial to find ways to effect meaningful behavioral change regarding parasite control,” she says. “We do feel the resistance problem is a lot larger than we know.”

Simplifying Fecal Egg Count Tests

Traditionally, conducting FECs was a time-consuming and labor-intensive process for veterinarians and researchers. However, several studies over the past few years have focused on a new approach that not only makes it easier to carry out such tests but also reduces variability in results, Nielsen says.

He and colleagues developed and, in 2016, published a study validating the first automated FEC test, called the Parasight System. Since then, he says, multiple similar systems have been developed and are commercially available in several countries.

The general concept behind these tests is similar: A computer or smartphone program or app identifies and counts parasite eggs in images of manure samples and uses a process called deep learning to essentially act as a human brain would, learning from the images it has evaluated to become more effective at processing them in the future. This makes results more consistent and can help eliminate the inevitable risk of human error.

“The biggest advantage to this artificial intelligence is that it removes a lot of the variability we see between counts, which has been both a source of frustration and a challenge when trying to interpret results,” Nielsen says.

All these systems make FECs “quicker and easier for anyone to do, almost anywhere,” he says, which he hopes will remove—or at least reduce—the barrier many owners and veterinarians still see when considering such testing.

DNA Sequencing the ‘Nemabiome’

Finally, Nielsen describes two recent studies in which he worked with a team from the University of Calgary to map the DNA of the different species of small strongyles that can affect horses.

It’s a similar process to that used with the microbiome, he says, which involves sequencing all the bacteria present in any kind of biological sample. Instead, the team is sequencing the nematode DNA—hence the “nemabiome” moniker—found in samples from a horse’s feces or intestinal tract.

“There are 40 different small strongyle species described in horses, and it’s not unusual to see between 15 and 25 different species co-infecting the horse at the same time,” Nielsen says. “And that’s just the small strongyles. Then you have the large strongyles and other categories of parasites on top of that.

“For a long time, really, we’ve just been talking about small strongyles without differentiating what species we’re talking about or what the species composition is,” he continues. “And that’s simply because we haven’t had this tool to investigate it.”

Nielsen says future investigations will likely reveal important information that could help in the fight against AR.

“Does the composition of species (in a horse) have anything to do with drug resistance? Is it certain species that contribute to drug resistance?” he says. “We’re only beginning to scratch that surface right now, which is very interesting. We’ve had a couple of publications now, and there’s a lot more work to be done.”

Future Research

Researchers have made great strides in understanding the parasites that impact our horses and how to best manage them, but more work remains.

Nielsen says he and colleagues are in the process of updating the American Association of Equine Practitioners’ Parasite Control Guidelines, which offer research-based recommendations for horse owners and veterinarians alike.

But, he adds, “We need new dewormers, and we need them now. We also need a better and more refined understanding of parasite biology. And we need to learn more about how to integrate nonmedical approaches into parasite control programs.”

Walshe adds that observations from veterinarians in the field will continue to drive future research topics. “We are certainly seeing, anecdotally, an increase in parasite disease on this side of the world, maybe because of resistance issues, perhaps it’s the much wetter, warmer conditions, or people are just getting better at identifying it,” she says. “There’s no research done on that one yet, so I couldn’t say it 100%, but certainly the perception is out there.”