Combat Equine Parasite Resistance

As internal equine parasites grow defiant in the face of traditional deworming tactics, it may be time to change your battle plan.
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It's not news that the internal parasites, or worms, that affect our horses are growing resistant to the drugs we use to battle them. In fact, the phenomenon was first noticed in the 1960s, says Martin Nielsen, DVM, PhD, ?Diplomate of the European Veterinary Parasitology College and an assistant professor of equine parasitology at the University of Kentucky's Maxwell H. Gluck Equine Research Center.

Choose an appropriate dewormer for your horse based on both his age and the type of worms he harbors. | ? Dusty Perin

Choose an appropriate dewormer for your horse based on both his age and the type of worms he harbors. | ? Dusty Perin

In the 1980s, though, we thought we had worms on the run. Horse owners had a double-whammy defense: rotational deworming strategies that systematically swapped drugs to nail any parasites left behind by the former treatment and the introduction of ivermectin?a new deworming agent with no resistance issues.

But today the tide has turned. Rotation plans aren't doing what they're supposed to do, says Dr. Nielsen. There is evidence that ivermectin isn't quite the unstoppable powerhouse it once was. And there are no new ?deworming drugs on the horizon.

In short, today "resistance is likely present in all managed horse herds worldwide," according to the ?Handbook of Equine Parasite Control written by Dr. Nielsen and Craig R. Reinemeyer, DVM, PhD, Dipl. ACVM, founder of East Tennessee Clinical Research.

Suddenly our horses seem more vulnerable than ever to the potential damage caused by internal parasites. That includes everything from generally poor condition and weight loss to potentially more serious, life-threatening problems like colic. Even a horse who appears healthy could actually be a ticking time bomb?loaded with ?internal parasites that could pose a hazard if he experiences stress, illness or nutritional deficiencies. He also could be a risk to other horses who might be more susceptible to the parasites he releases, or sheds.

So what's a responsible horse owner to do?

Take the Sustainable Approach

The solution, says Dr. Nielsen, is an evidence-based deworming program that uses routine monitoring to reduce the intensity of treatment by addressing individual horses based on their specific needs. The "evidence-based" part refers to using fecal egg counts to monitor both the presence of internal parasites and the effectiveness of deworming agents. Fecal egg counts literally evaluate the number of worm eggs in your horse's manure, an indication of how many adult worms he is hosting (see "Get a Fecal Egg Count," below).

This is what Dr. Nielsen calls a "sustainable approach." The idea is that reducing unnecessary and ineffective treatments will decrease problems with resistance. Therefore, the strategy will continue to be effective over time.

This approach is, by nature, individualized to each horse, so it can't be summarized in a tidy, color-coded chart like the old rotational plan. But, says Dr. Nielsen, "There are still general recommendations we can make." Here are his big three:

1. Evaluate your dewormer. "Everyone, no matter what [drugs] they are treating with, should evaluate the efficacy of treatment once a year," says Dr. Nielsen. A lot of horses can tolerate a large parasite burden without showing any overt health symptoms, he notes. So eyeballing your equine isn't an effective way to judge if your dewormer works. Instead, do a fecal egg count.
Dr. Nielsen recommends doing one count at the time of treatment and another 14 days later. Ideally, you want to see a reduction of at least 90 percent on the follow-up count. If you don't and you're sure your horse got a full dose of the drug, your dewormer probably isn't working, and you should treat again using a different class of drug (see "Worms vs. Weapons" below).

2. Administer an annual treatment. "We recommend that every horse gets a basic foundation of one [deworming] treatment per year at the end of grazing season," says Dr. Nielsen. Treatment at this time, he explains, will interrupt the lifecycle of key parasites, including large and small strongyles, tapeworms and even bots. Plus, after this deworming, your horse won't typically pick up additional parasites until the next grazing season begins.

Note: In the Handbook, the authors suggest using a blood test to determine the presence of tapeworms, whose eggs don't ?appear in the manure. If it's positive, your fall treatment should combine tapeworm treatment with a broader-spectrum drug (see "Worms vs. Weapons" below).

3. Target and treat problems. Building on the first two steps, says Dr. Nielsen, do an additional egg count in the spring to target horses who are shedding a high number of worm eggs in their manure. Then give just those animals another dose of dewormer. Regardless of whether these horses appear unhealthy, they can pump up the parasite infection pressure on that farm, says Dr. Nielsen, which puts every horse at risk.

Some studies have shown that 20 percent of a typical herd is responsible for as much as 80 percent of the group's parasite load. So chances are good that not every horse on the property will need this second treatment.

Over time you'll be able to identify horses who are perennially high egg shedders and those who are consistently low. Treating the high shedders with a daily feed-through dewormer (like Strongid C/C 2X or Equi Aid SW/CW with pyrantel tartrate) may be ?advisable.

Dr. Nielsen, who moved to the United States from Denmark just over a year ago, says this approach has become the norm in his native country. "The level of deworming is much lower in Denmark, and they run a lot of egg counts," he says. In fact, he adds, "In Denmark and in an increasing number of European countries, anthelmintics [dewormers] must be prescribed, and the veterinarian must have diagnostics before prescribing."

Rules for Young Horses
The 1-2-3 punch described above is a guideline for average adult horses, says Dr. Nielsen. But young horses need a slight twist on the plan. Dr. Nielsen recommends four deworming treatments during a foal's first year:

1. Age 2-3 months. At this age, foals tend to be highly susceptible to ascarid infection?rarely an issue in mature horses. Ascarids left unfettered can cause respiratory problems and impaction colic that can require surgery and hospitalization. Dr. Nielsen recommends using a benzimidazole dewormer at this stage.

2. Weaning. At this age, start with a fecal egg count to both assess the level and identify the type of parasites your foal may harbor. Then treat with the most appropriate drug (see "Worms vs. Weapons," page 50).

3 & 4. Yearling. First, coordinate a treatment with spring deworming for any adult horses on the property. Then administer another treatment midway through grazing season. In both cases, use egg counts to pick an appropriate drug.

From yearling age until maturity at 4 or 5, young horses may continue to be more susceptible to parasite infections than adults. While egg counts and an individualized approach should still be the rule, a starting guideline is to deworm before, halfway through and at the end of grazing season.

Farm-Management Strategies
Of course, battling internal parasites isn't all about drug warfare. Proper farm-?management practices can go a long way toward reducing the number of parasites on the farm and thus decreasing the number of parasite eggs to which horses are ?exposed. Here are a few tried-and-true tactics from the Handbook:

  • Restrict pasture access for horses who are known high egg shedders.
  • Regularly clean manure from stalls, ?paddocks and pastures. Manure should be ?removed before eggs develop into infective larvae?approximately every week ?during the grazing season.
  • Quarantine new horses because the stress of moving can cause a spurt of egg-shedding. Administer a deworming treatment on arrival and follow up in 14 days to ensure the fecal egg count has ?decreased to acceptable levels before turning the horse out with your existing herd.
  • Avoid overgrazing and keep a close eye on pasture density. A general rule of thumb is one horse per 1-2 acres of land, but in reality it depends on multiple factors, including the size of the horses and quality of the forage.

The Bottom Line
Deworming may have become more complicated over the past few years, but it still isn't rocket science. By changing old-school practices to embrace the notion of a personalized approach, every individual as well as horses on the whole stand a better chance of a healthy life.

This article originally appeared in the February 2013 issue of Practical Horseman magazine.

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