Parasite control pioneer

Three decades ago, a California veterinarian started offering fecal egg count tests to horse owners. Now the rest of the industry is catching up.
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It’s one thing to be on the leading edge of change, it’s another being 30 years ahead of your time. For one equine practitioner, a realization about the nature of traditional deworming practices and a crusade to change them put him decades ahead of the rest of the horse industry.

The story begins in 1991, when John Byrd, DVM, was an equine veterinarian with a solo practice in Southern California. Like most veterinarians, Byrd spent most of his days doing lameness exams, colic calls and suturing the occasional wound. Also like most veterinarians in the United States, he didn’t administer deworming treatments to his patients—his clients did that themselves—but he’d offer advice about timing and products when asked.

However, when horse owners began asking him about a newly introduced daily deworming product, Byrd began to wonder about this rote approach to parasite control. “People would ask, ‘Should I give this to my horse between paste treatments?’ And my first thought was how could these horses possibly still have worms to treat?”

How often is deworming needed?

Byrd didn’t have to wonder for long. “I decided I’d check and see if the horses in my practice actually needed more deworming.” To do this, he used fecal egg counts—laboratory tests used to determine the parasite burden in an animal. For the test, small manure samples are placed under a microscope. Then a veterinarian or laboratory technician literally counts the number of eggs in a designated area of the slide. The number of eggs identified reflects the types and estimated populations of mature internal parasites inside the horse.

In the early days of parasite control, conscientious owners faithfully followed the eight-week deworming schedule recommended by the manufacturers. (Adobe Stock)

Counting parasite eggs

“I knew how to do fecal egg tests,” says Byrd. “We learned how to do them in veterinary school. I’d done them in small animals, but I’d never actually done one on a horse.” Undeterred, he ordered the necessary materials and began collecting and analyzing manure samples from horses in his practice.

What Byrd learned flew in the face of standard deworming recommendations. “Egg counts in horses can identify adult roundworms and strongyles in the digestive tract. Those are the parasites that most commonly cause problems in horses and the ones we worry about. When I started testing horses in my practice, I discovered that most of these horses didn’t have any significant evidence of these parasites at all, even weeks after a treatment. Not only did these horses not need more deworming, but they also didn’t need as much as they were already getting.”

Byrd didn’t realize it at the time, but he had made a discovery that would not be recognized by the wider horse industry for another two decades.

A brief history of deworming

The challenge of parasite control is not a new one. Archeological evidence of equine parasites has been found in Roman ruins. The earliest horsekeeping texts refer to an array of treatments, ranging from feeding chicken eggs to mercury. These treatments likely accomplished very little, and many were likely harmful to the horses.

John Byrd, DVM,

By the 1900s, scientific research into internal parasites had begun and more “modern” recommendations for parasite control emerged. These treatments, such as administering quarts of turpentine or linseed oil via nasogastric tube, weren’t much better than the ancient practices.

First generation

The first generation of parasite-control medications were introduced in the 1950s. These included phenothiazine, an antipsychotic medication found to be effective against strongyles, and piperazine, the first broad-spectrum dewormer that targeted several species of parasites. While better than earlier treatments, these toxic drugs could only be safely administered by a veterinarian via nasogastric tubing.

Some companies provided stickers that could be placed on the calendar to ensure a dose was never forgotten. Freed of parasite burdens by medications that were not otherwise toxic, horses suddenly could live healthier, longer lives. With the support of these modern anthelmintics along with modern veterinary care, more and more horses began living well into their 20s.

Modern anthelmintics

The deworming agents we know today were developed in the 1960s and were, without a doubt, a boon to equine health. Safe and effective, these products were also easy for owners to administer themselves. Conscientious owners faithfully followed the eight-week dosing schedule recommended by the manufacturers.

Equine parasites were not vanquished, however. They simply adapted. As early as the 1960s, evidence started to accumulate that some of the most popular deworming chemicals were becoming less effective. By the early 2000s, leading parasitologists were sounding the alarm about a phenomenon known as resistance (see “How Resistance Develops” below).

Rise of the “super worms”

By deworming so frequently, the equine community was unwittingly creating a generation of “super worms” immune to the effects of parasite-control products. Rotating chemical classes of dewormers between treatments could, in theory, slow this process but the frequency with which people were treating every horse on a property was still putting intense selection pressures on parasite populations.

This wasn’t just a theory, either. Research from around the globe was confirming that certain deworming chemicals were less effective in horses than they had been just a few years earlier. Some chemicals were completely ineffective. Eventually, researchers documented resistance to every available treatment. Without a significant change in fundamental practices, the equine industry might lose its only weapon against internal parasites.

A more targeted approach

Parasitologists are a solutions-oriented group, so they didn’t just wring their hands over the situation. Almost as soon as increased resistance was identified among equine parasites, the same experts began recommending changes in deworming protocols to slow that process while still protecting horses.

This innovation, now known as surveillance deworming, utilizes regular fecal egg counts to identify the specific horses in a herd who are shedding significant numbers of parasite eggs. Then only those horses are dewormed with appropriate products on a schedule dictated by their individual response to treatment. Follow-up testing after treatments would then identify agents that are ineffective on a property.


This sample invoice illustrates how Horsemen’s Laboratory reports the results of the fecal egg count tests it conducts for clients.

With surveillance deworming, owners could treat their horses less often, which would potentially save money while also helping to preserve the efficacy of anthelmintics for future generations of horses. On paper, it seems like a system that horse people would immediately embrace. The reality, however, is that very, very few people did.

The benefits

Undeterred, parasitologists continued to work hard to explain the benefits of surveillance deworming to the public. They brought their message straight to owners through lay publications and as guest speakers at local equestrian gatherings. To reach veterinarians—who might then convince their clients to change their deworming habits—parasitologists published dozens of scientific papers and spoke at professional meetings and conventions. The science was settled and the need for a change was urgent. Yet, nothing changed.

According to the most recent United States Department of Agriculture’s National Animal Health Monitoring System study (conducted in 2015), only 5.5 percent of farms with horses age 4 and older tested manure for parasite eggs and then based their deworming practices on the results. This means that, even when presented with a better way to deworm, nearly 95 percent of people caring for mature horses stick with an outdated, more expensive method that becomes less and less effective over time.

But Byrd already knew that.

Hurdles to overcome

Byrd started Horsemen’s Laboratory in 1992, beginning work that continues to this day—performing fecal egg counts on manure samples either sent directly from owners, or through their veterinarians or other organizations. The lab, which is now based in Illinois, then returns a full report on the status of each horse. The company has never sold any deworming products or worked in conjunction with any entity that does.

Byrd soon learned that enthusiasm for the idea of testing didn’t translate into action. “I started buying ads in local horse publications, offering the service. People would call, very excited, and I’d send a kit. And then I’d never hear from them again. I’d go to horse fairs and give out free kits. People would say, ‘This is great,’ and put the kit in their bag and never send a sample. It was very frustrating.”

When Byrd inquired, owners gave various reasons for their reluctance to test their horse’s manure prior to making deworming decisions. “Some people said their veterinarian told them it was impossible to send samples in the mail and get an accurate reading,” he says. “The worry was that the eggs would hatch, leading to inaccurate readings. We developed a method of packing the samples tightly into a container, limiting exposure to oxygen, which eliminated that problem.”

Pennywise pound foolish deworming

For other owners, perceived cost was the barrier. “People see testing as an additional expense, but what they aren’t considering is how much they might save by not buying dewormers every other month. Looking at our historical data—and we’ve analyzed over 160,000 samples since we started—of those only 21 percent of samples indicate the need for deworming. That means there’s a lot of money spent on unnecessary treatment.”

A low shedder may need deworming only once a year, adds Byrd. “We recommend ivermectin or moxidectin once a year for all horses,” he says. “You’ll need to also treat for tapeworms, which doesn’t always show up in our McMaster’s fecal egg counts. But even with that, you’re still going to save money compared to deworming every eight weeks. And you’ll also be helping prevent resistance.”

Are all worms harmful?

Another early hurdle Horsemen’s Laboratory confronted was an unease owners had with results that showed any evidence of parasites, says Byrd. “People don’t want their horse to have any parasites at all. They get really squeamish about the idea of a single worm in their horse, but horses are going to have some parasites—that’s completely normal.”

Results of fecal egg count tests—from Horsemen’s Laboratory or other laboratories—are never going to show a “zero parasites” result. That’s because FECs do not quantify the adult worm population; FECs test for eggs produced by that population. “Clients will sometimes ask, ‘Can you tell me how many worms are in my horse?’ No, we can’t, and any company who says they can isn’t answering that question honestly,” explains Byrd. “What a laboratory can do is identify a fecal egg level that reflect a parasite burden experts recommend is appropriate to treat.”

The highest stakes

In the early 2000s, as the mounting scientific evidence of resistance to de-worming chemicals became impossible to ignore or refute, Byrd also began educating clients on the importance of FEC to confirm that treatments they were giving were working.

“We always want to do a two-week follow up test to make sure the products we recommend are still working,” says Byrd. If a product is still effective on a property, the egg count in the same horse will drop by 90 percent two weeks after the treatment. If it doesn’t, you know that product no longer works. And there’s no sense in using a product that doesn’t work. Sometimes people ask if they should increase the dose if the first doesn’t work. But if the parasites aren’t sensitive to the manufacture’s recommended dose, more isn’t going to change that.”

A loyal following

Through Byrd’s persistence, Horsemen’s Laboratory slowly developed a base of loyal clients who recognized the value of periodic fecal egg counts.

“I had one client with one horse who we ran samples on for 28 years,” says Byrd. “We did 69 samples on that horse in his lifetime and only nine ever came back positive. That horse was managed well but most likely was also genetically immune to internal parasites. How many times was he unnecessarily dewormed in his lifetime without testing? Not only would that have wasted money, but it would have contributed to resistance on that property.”

What’s next?

But the wave of surveillance-deworming converts has yet to appear. “I’m surprised people have been so slow to change, especially when the research about resistance is irrefutable,” says Byrd. “There is so much at stake. I spoke at a meeting [at the Food and Drug Administration] a few years ago. There were veterinarians discussing resistance in sheep and goats in New Zealand. In some places they can no longer raise sheep and goats on particular pastures because of resistant parasites. If they turned them out [on those pastures], the sheep and goats would all pick up resistant parasites. And many of the young die. The horse industry could be in the very same situation if we aren’t careful. From an environmental standpoint, and even an ethical standpoint, fecal egg counts are the right thing to be doing, no matter which lab does them.”

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