The future of parasite control

A leading equine parasitologist describes the challenges we face in the race to protect horses from worms.
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A leading equine parasitologist describes the challenges we face in the race to protect horses from worms.

It’s not that often in horsekeeping that a widespread, commonly accepted management practice is upended and dismantled in a matter of a few years, but that’s just what happened with parasite control. For decades, the prevailing wisdom was that horses needed to be dewormed based on the calendar---every eight weeks faithfully, rotating through chemical classes. But a growing body of evidence began to show that calendar-
based parasite control was leading to a widespread resistance to deworming chemicals among parasites

We parasitologists saw the signs of this looming disaster early and confirmed our observations via extensive studies. We also proposed a more selective deworming method that could slow the development of resistance considerably (see “Surveillance Deworming Basics,” page 50). And by around 2009, equine veterinarians were receiving word about the changes necessary to protect future generations of horses from “super parasites” resistant to all dewormers. And the veterinarians, in turn, educated their clients. Mission accomplished? Far from it.

Much work remains to be done if we are going to protect horses across the globe from damage inflicted by internal parasites. Of course we must continue education efforts regarding deworming schedules, but that won’t be enough. We need to keep searching for other methods of parasite control, both in traditional locations and in new areas that may, at first, seem a little strange. We need to embrace new technologies and even utilize social-media solutions to jump-start these efforts. The changes we’ve made so far are good, but they aren’t enough, and we need to stay vigilant in our efforts to protect horses right now and years from now.

Changing minds,one at a time

By now, virtually every equine veterinarian in this country knows that regularly scheduled, across-the-board deworming is a bad idea. And, I know, many horsepeople do as well. But how many people have acted on this information and changed their approach to parasite control? Not nearly enough.

We recently surveyed managers of large Thoroughbred farms here in Kentucky. We found that, despite knowing of the risks associated with scheduled deworming, only about 25 percent of respondents had adopted the new surveillance-based recommendations. The other 75 percent are still doing it the old way.

This surprised me. I thought we’d be better off by now, and I hope that we might be elsewhere. Farm managers in Kentucky are dealing with a lot of yearlings and young stock who don’t always fit easily into the “test and select” approach to deworming. On some of those farms, scheduled deworming may still be the best choice. But that same study also found that the managers were treating very frequently, even more frequently than we recommend without a targeted approach. In fact, they were using lots of drugs that we know don’t work anymore. The managers don’t appear to be making much of an effort, at all, to change the way they deworm, and a majority declared they wanted some sort of assurance that the new approaches would be successful before they would consider any change.

This study is still our only published evidence of how people are utilizing the information we’ve sent out, and it’s a bit disheartening.

On the positive side, my impressions from talking to average owners are that the message is getting through. The majority I’ve spoken to (and I’ll admit I mostly speak to owners who are very informed about parasite control) are adopting the new recommendations and take pride in keeping abreast of the latest thinking on deworming.

In fact, I am currently involved in a project in Pennsylvania, where the extension service received a three-year grant to disseminate information about surveillance-based parasite-control programs. The group reached out to me to help them implement a plan, and I was happy to travel to educate extension officers in various regions around the state on how to do egg counts and direct horse owners toward the best practices. This initiative and energy will change a lot of things in that state for the better. I think that could be a fabulous model for other states. If Pennsylvania can do it, why not Kentucky? Why not every state?

As far as veterinarians go, I think they are getting on board. Traditionally in this country, veterinarians haven’t been very involved in parasite management, but an owner needs a veterinarian’s help to implement a surveillance program. I see the dynamic working this way: Owners come to the veterinarians asking them to help facilitate the change. And we all know horse owners can be persistent, so veterinarians who are reluctant to embrace change aren’t always happy with me. But if it’s facilitating change, I’m OK with that. The veterinarians who do embrace the change can always come to me for assistance and advice.

For many practitioners, getting involved in surveillance deworming is a marketing decision. How do they offer this service without straining other resources? I gave a talk on equine parasite control at the American Association of Equine Practitioners Convention last winter. We had about 300 people in the room. The speaker right after me spoke about using a wellness program in his practice, where horse owners subscribe to a plan that calls for a practitioner to make regular visits to a healthy horse to direct preventive care. A parasite surveillance program fits perfectly into this sort of system, with a veterinarian doing fecal egg counts and recommending treatments as part of the set fee. This could be a business model for other practices. The trouble is that many practices don’t have the wellness emphasis. They are known as orthopedic specialists or reproduction experts. Parasite diagnostics may not fit into their business plan or expertise. There’s nothing wrong with that, but other practices may choose to specialize in parasite control as part of their business plan.

So, all in all, I think we’ve made some progress. A lot of people are buying into the concept of surveillance deworming, and more and more will every year. At the same time, we still have a long way to go.

The search for new solutions

Even if everyone in the world suddenly embraced surveillance deworming, we’d still have the problem of resistance. You can’t reverse it; you can only slow it.

Here’s a quick overview of how resistance develops: Individual worms that are resistant to a particular chemical class occur spontaneously in a population, as a random genetic mutation. Resistant worms are fairly rare and would remain so, but too frequent and indiscriminate deworming gives them a significant advantage by killing off their competition. After a treatment, a proportionally larger population of invulnerable worms remains to continue their life cycle, passing their resistant genes further into the farm’s parasite population. The more frequently a horse is dewormed with the
resistance-prone chemical, the more quickly this process occurs.

Right now, there are three major classes of equine anthelmintic drugs: benzimidazoles, pyrimidines and macrocyclic lactones (which include ivermectin and moxidectin). Understanding how they work on different internal parasites is crucial. Fortunately, all of these drug classes work against large strongyles. As a result, large strongyle infection is very rare in a healthy equine population and not a very big threat today.

Here’s the not-so-good news: The greatest parasitic threat to adult horses is posed by cyathostomes (small strongyles), which are now, by and large, resistant to two of the three drug classes. Benzimidazoles don’t work against these parasites on a large majority of farms. Pyrimidines are slightly more effective against small strongyles, but not by much. Both of these drugs still work against ascarids, an important parasite in young horses.

So that means the macrocyclic lactones---ivermectin and moxidectin---are left holding the fort against strongyles, but there’s trouble ahead. Studies have shown that the time it takes for eggs to reappear after macrocyclic lactones is shrinking. It used to be we didn’t see eggs until six to eight weeks after a deworming with ivermectin. Now they are popping up within four weeks. Moxidectin used to get us 12 to 16 weeks before eggs showed up again; now we are down to five in some areas. Macrocyclic lactones, I should add, are not effective against ascarids.

If this sounds confusing, it’s because it can be. We have a tendency to simplify and talk about “parasites” in general. But the specific worms you are talking about matter. No single drug takes out everything anymore. If you don’t select carefully, you can get it wrong. That either accelerates the resistance process or leaves your horse unprotected, or both. All of this isn’t a Kentucky trend; it’s being scientifically demonstrated virtually everywhere, around the globe.

As I mentioned before, you cannot undo this process. Once a chemical does not work against a worm population in a geographic area, it won’t ever work there again. A well-executed surveillance-based deworming program can slow the spread of resistance but cannot reverse it. What’s the solution, then? Ultimately, we need a new class of chemical dewormer.

Where will these new agents come from? Well, the pharmaceutical industry is hard at work researching alternatives. However, in the past 10 years we’ve seen only three new anthelmintic classes arise, and none of them are for horses. There are a few reasons for that. For starters, it is expensive to develop a drug for an exclusively equine market. From a sheer dollars-and-cents standpoint,recouping money from research and development is much easier and faster if you’re selling for small animals or livestock rather than horses. So why not piggyback horses onto a small ruminant product? It’s not always that easy.

Take for instance, a new drug named derquantel, the first commercial product in a new class of anthelmintic for use in sheep, an animal that faces a resistance crisis even more dire than horses do. It appears to be quite effective. It also turned out to be toxic to horses, so there goes that idea.

There is another recently developed and very effective sheep dewormer called monepantel, but I have yet to see any data regarding its efficacy in horses. I’m not sure it’s even been tested yet in horses. It’s also possible that an effective dose in horses would be prohibitively expensive; drugs don’t always scale up well for larger animals. Do I think pharmaceutical companies should spare no expense and dedicate themselves to developing a new equine anthelmintic? Of course, but the reality is that might not be feasible.

Meanwhile, clinicians in Australia and New Zealand are trying a different approach, again with sheep. They are trying combination deworming, using two different chemicals at the same time. Right now they are using various dewormers together, and it seems to be more effective than either one alone. And it might extend the efficacy of both of these products. What’s still unclear is how sustainable this approach would be and, of course, we don’t know if it will translate to horses at all. But it’s an interesting idea and something we are currently studying in our research herds here at the university.

A new source of research support: Crowdfunding

A famous parasitologist once said, “Out in nature, there is a very efficient dewormer that hasn’t been discovered.” With the challenges we currently face, I think it’s time we---all of us---actively go looking. And last year we launched a truly innovative program to facilitate that search.

The work began when researchers identified a crystal protein produced by the bacterium Bacillus thuringiensis that was capable of killing certain intestinal worms without any adverse effect on the mammalian host. It makes sense that a naturally occurring bacterium may kill parasites to gain a competitive advantage in nature.

We’ve been very designer-drug focused for a long time, but nature can also do a very good job of keeping parasites in check. I’m not saying we need to return to a time when a dose of tobacco was considered good deworming, but we can’t lose sight of the fact that plant and microbe derivatives often make for effective treatments. In fact, ivermectin is produced by a fungus.

This extremely important work quickly became a collaborative effort, with different institutions and experts from various specialties working together. And part of that effort has been crowdfunding. Essentially, we used the Internet and social media to ask regular people to donate money to fund preliminary research into the potential of certain bacteria to kill equine parasites. (To learn more about our ongoing efforts go to

I know a few traditionalists in academia raised their eyebrows and scoffed at the idea of asking the public to fund research, but it worked. Horse owners are well-informed and curious. And they want to be involved. Our appeals were heard, and we raised enough money to perform some preliminary studies that have produced encourag-ing results.

In these studies, we were looking at the parasite-killing properties of the crystal proteins the bacteria secrete. We also specifically wanted to examine their effect on parasite strains that are resistant to two of the three currently available drug classes. Our early data has shown that these proteins not only kill drug-naïve parasites but also drug-resistant strains in a laboratory setting. This is incredibly encouraging.

With this data in hand, we’ve applied for a research grant through the U.S. Department of Agriculture. We should be hearing soon if we’ve gotten funded, which will allow us to move forward with and begin testing the efficacy of these proteins in actual horses over the course of five years. If it works, this could be the answer to many of our problems.

Although much work remains to be done, I’m optimistic that we will ultimately be successful in encouraging the widespread adoption of optimal parasite-control practices. Many people have already embraced surveillance deworming and are happy with the results. What’s more, as they help spread the word, I find that I don’t have to do a whole lot of convincing when talking to horse owners about the changes we must make to safeguard our ability to control parasites well into the future. Horse owners are smart, proactive people who want to do all they can to protect their horses. We still have a ways to go, but I think we can get there.

This article first appeared in EQUUS issue #456, September 2015.