An Equine Deworming Strategy to Prevent Parasite Resistance to Medications

Smart strategies help defeat the growing problem of drug resistance.

Worms that can fight off the effects of the chemicals you rely on to protect your horse? Superworms? It sounds like a nightmare. But it’s becoming a reality: Already some dangerous equine parasites have found ways to resist widely used deworming medications, and the problem is spreading.

Luckily, people are smarter than worms. Here’s how one farm beat resistant worms and how you can ?apply the same strategy to protect your horse from this threat.

Several years ago, Dr. Dianne Little and other researchers from the North Carolina State University College of Veterinary Medicine began following horses on a breeding farm in central North Carolina. For years the farm had dewormed horses every eight weeks, switching drugs each time from fenbendazole to pyrantel to ivermectin. Yet high levels of worm eggs?specifically, small strongyle (cyathostome) eggs?were turning up in manure just a few weeks after treatment. In addition, some weanlings were showing signs of a disease condition called larval cyathostomiasis, which develops when small strongyle larvae become encysted in the gut wall.

Further checks showed that the worms were resistant to two of the three broad classes of deworming medications: benzimidazoles (such as fenbendazole) and pyrantel salts (pyrantel pamoate or pyrantel tartrate). Resistance develops when a few worms survive treatment and pass the traits that helped them survive to their offspring. On this farm, only the third class of dewormers?macrocyclic ?lactones (ivermectin and moxidectin)?worked against small strongyles, currently the most widespread and dangerous equine internal parasite.

The NCSU veterinarians wondered if they were watching that last bastion fall. Ivermectin was still killing worms on this farm, but the interval it took for eggs to ?return to high levels after treatment was shrinking?from eight weeks to three or four weeks and even two weeks in some horses. Shorter egg reappearance time is thought to be an early sign that resistance is developing. Were the worms on this farm turning into ?superworms, unscathed by all available ?medications?

To get a handle on the problem, veterinarians mapped out a two-part plan. First, the farm switched from rotational deworming to a strategic program, treating each horse only when its fecal count rose above target levels (200 eggs per gram for mares and 100 epg for foals). Horses aren’t equally susceptible to parasites and produce variable amounts of eggs in their manure. On a typical farm, 20 percent of the horses shed 80 percent of the worm eggs. The idea was to identify the high shedders, treat them effectively and manage the others with fewer treatments. Reducing treatments would save money and might help reduce the chance that worms would develop resistance.

Second, the veterinarians recommended changes in pasture management. Strongyle eggs are passed in manure. Larvae hatch in the field and are picked up by grazing horses. Reducing the number of horses grazing each pasture, rotating and resting pastures to interrupt worm life cycles and stopping the practice of spreading uncomposted manure on fields could reduce reinfection.

These steps weren’t taken right away, and problems continued. But about a year after new managers took over and made the changes, egg reappearance times began to increase. Ivermectin, it turned out, still worked against the worms. The problem had been rapid reinfection from pasture that was heavily populated with worms due to poor management.

Winning Strategy
Veterinarians 1, worms 0?but this farm’s experience with resistant worms was far from unique, Dr. Little notes. In some areas, 90 percent of farms have benzimidazole-resistant stron?gyles; 20 to 40 percent have pyrantel-resistant worms. Indiscriminate dosing, she and other researchers say, encourages resistance to develop. That concern applies to daily as well as interval deworming medication.

Low daily doses of pyrantel tartrate, for example, kill strongyle larvae soon after they enter the horse. But what if some survive the frequent exposure to a low dose? There may then be selection for worms resistant to higher doses of drugs in the pyrantel class. Dr. Little found evidence of this at a boarding stable where daily pyrantel tartrate was no longer controlling strongyles; resistance to pyrantel pamoate was also identified.

Many researchers believe it’s just a matter of time until worms develop ?resistance to all three classes of dewormers. “No new drug classes are in the pipeline, so veterinarians and horse owners need to work together to extend the life of the drugs we have,” Dr. Little says. For that, think strategically.

Parasites are a herd problem, Dr. Little says, so a good plan covers all the horses on the property, whether that’s your backyard or a large boarding stable. The number and age of the horses, the amount of pasture they have and your geographic location are all factors. Frequent trips to shows and new horses coming onto the property can increase ?exposure risks.

Your veterinarian can help you devise an appropriate strategy. The first step is to do fecal egg counts?ideally, on all horses on the property, on the same day, so you can identify the horses that shed large numbers of eggs. (The box “Egg-Count Basics” below explains how.)

If your farm is typical, some horses will have higher counts than others. They’re most likely the high shedders; further tests can confirm that. These horses contribute more to the herd problem, and they may be at risk for larval cyathostomiasis, especially if they’re young or very old. You should also alert your veterinarian to any horse that has recurrent colic, diarrhea, poor body condition or a rough coat. These signs can appear in horses with high levels of encysted small strongyles, even if fecal egg counts are low. So these horses may need additional monitoring and treatment. (For tips on how to manage your horse’s deworming program in a boarding situation where the whole barn is not using the program, see box “What If You Board?” at the end.)

Once you’ve assessed the problem, reach for your weapons: medication and management.

Strategic Medication
Use medication to eliminate worms without overtreating your horses. Here’s how:

Get the dose right. It’s based on weight. Use a weight tape to get an estimate. Be sure your horse swallows all the medication.

Check effects. A fecal egg count 10 to 14 days after deworming will tell you if your medication is working. The count should drop by 95 percent. If it doesn’t (or if you have a high initial egg count in the face of daily deworming), something’s not right. Your horse may not have ?received a full dose, or you may be dealing with resistant worms.

Test and re-treat. Repeat fecal counts in six to eight weeks (or at the time you might expect to deworm again, depending on the product you used), and treat only horses with egg counts above 200. Initially, you may need to run farm-wide fecal counts every couple of months. Once you’ve identified the high shedders, less susceptible horses may need to be checked and dewormed only a couple of times a year.

Cover all bases. Egg counts mainly tell you about small strongyles?the major threat but not the only one. Even if your horse’s count never rises above 200, you need to consider:

  • Tapeworms: They’re hard to ?detect. If they’re a problem in your area, treat every six or 12 months with praziquantel (sold in paste combos with ivermectin or moxidectin) or a double dose of a pyrantel pamoate paste.
  • Large strongyles: Once a leading cause of devastating colic, they’ve been largely wiped out by modern dewormers. But on a program with less frequent, targeted treatments, there’s a chance they could return. Treatment every six to 12 months with ivermectin will keep them at bay.
  • Ascarids: These worms, which can be picked up year-round, mainly affect young horses. Deworming every six to eight weeks from ages 8 weeks to 2 years will keep numbers from building up. All three classes of drugs work, although there have been reports of ascarids resistant to ivermectin.

These steps should also protect your horse against many other internal parasites (such as pinworms), so you won’t need to do anything special for them. For example, a fall treatment with praziquantel and ivermectin or moxidectin will control tapeworms, pinworms, bots and strongyles in one dose.

Strategic Management

  • Use management to cut the risk that worms will reinfect your horse. Different parasites have different life ?cycles, but most spread their eggs or larvae through manure. How you handle the poop, then, makes a big difference.
  • Pick up manure in paddocks at least twice a week.
  • Put hay and grain in feeders, not on the ground. Don’t allow manure to contaminate water troughs.
  • Don’t overstock pastures. Good stocking rates vary, but two acres per horse is a good rule of thumb. When horses have plenty of space, they tend to graze away from ?manure piles.
  • Harrow pastures in hot, dry weather to break up manure piles and expose eggs and larvae to the ?elements. Strongyle larvae can’t ?toler?ate heat and drying for long, ?though they can withstand freezing.
  • Don’t spread manure on fields where horses graze. Compost it. The heat in a good compost pile will kill worm eggs.
  • Rotate pastures by resting fields for three to four months or allowing other animals?sheep or cattle, say?to graze. This interrupts parasite life cycles. Small strongyle larvae may hatch, but if they find no horse to ?infest, they can’t mature and will die.
  • Do fecal egg counts and deworm new horses, if needed, before letting them share pastures and paddocks with others. Ideally, put them in a quarantine paddock for two weeks after treatment.
  • Remove bot eggs on your horse’s coat by clipping hair or washing with warm water. Bot flies lay the eggs in late summer and early fall; horses ?ingest the larvae as they hatch.

Applying these concepts will help protect your horse from parasites now. And by cutting back on the ?indiscriminate use of medication, you may delay the day when superworms show up.

Dianne Little, BVSc, PhD, MRCVS, DACVS, is currently a postdoctoral ?fellow in equine orthopedics at the North Carolina State University College of Veterinary Medicine. She has also done colic research as part of the ?Carolina Colic and Digestive Disease program at NCSU.

This article was reprinted from the May 2007 issue of Practical Horseman magazine.

For more information on deworming, check out Deworming Your Horse: How to Find the Best Deworming Schedule for You and Your Horse, a free guide from MyHorse Daily.Save

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