War on Worms

News from the front: Your deworming program may not be as good as you think.

You faithfully give your horses a daily dewormer or a paste every six to eight weeks. And you sleep well at night, sure that you’ve protected them from deadly parasites. Maybe you have—but then again, maybe not. Your dewormer may not be as effective as you think, or you may be overtreating and wasting money. Worse, you may be paving the way for a horse-health nightmare: drug-resistant worms that fight off the effects of the chemicals you rely on to control them.

In the war on worms, blindly following a one-size-fits-all treatment schedule “concedes any intellectual advantage to the worms,” says Cliff Monahan, DVM, assistant professor Veterinary Preventive Medicine at Ohio State University. The indiscriminate use of dewormers is helping parasites to become resistant, he warns. This happens when a few worms survive treatment and pass the traits that helped them survive to their offspring. After a few generations—not much time, where worms are concerned—you have superworms.

To win the war, you need a strategy that fights worms on multiple fronts and is tailored to your horses’ environment and lifestyle. The person best equipped to help plan your strategy is your veterinarian, who knows what parasites are common where you live and how they’re transmitted. Here, we’ll outline what the program should include.

Know Your Enemy

Some 150 different parasites can infect horses, but the most harmful are strongyles. Horses generally ingest larvae of these parasites while grazing. Forty years ago, large strongyles were responsible for many fatal colics, but good deworming programs have made them far less of a threat than they once were.

Small strongyles now top the worm-least-wanted list. Larvae of these worms burrow into the walls of the small intestine, where they spend about six weeks encysted. When they emerge—often en masse—they can cause colic and inflammation of the intestine. Adults of both types live in the large intestine. Females shed their eggs there, and the eggs leave the horse in manure.

An effective deworming program will take dead aim at strongyles and go after other parasites as needed, Monahan says. For example, ascarids are mainly a problem for young horses; by age 2, most horses have some natural immunity to these parasites. Tapeworm risk varies—your horse is much more likely to pick up this parasite in the East or Midwest, for example, than on the West Coast. And bots are a seasonal problem in most parts of the United States. They’re the larvae of botflies, which lay their eggs on the horse in late summer. The eggs hatch, and the larvae enter the horse’s mouth and attach themselves to the stomach lining over winter. In spring, they’re passed in manure and pupate into adult flies.

Choose Your Weapons

Visit a feed store or page through a catalog, and you’ll find dozens of different dewormers. Most fall into one of three broad classes: avermectins and milbemycins (such as ivermectin and moxidectin); benzimidazoles; or pyrantel salts. Veterinarians used to recommend rotating between the classes, switching yearly or every six to eight weeks. If worms survived one drug, the idea was, they’d be knocked out by the next one.

But rotation is no longer effective, Monahan says. There’s already a high level of small-strongyle resistance to benzimidazole and pyrantel, and drugs in the avermectin/milbemycin class are more effective against more types and stages of parasites than the other classes. They form the basis for most current deworming programs, at least for adult horses.

Ivermectin controls large and small strongyles, bots, and most other worms except tapeworms. If tapeworms are a concern, you can use an ivermectin product that includes praziquantel, such as Equimax or Zimecterin Gold.

Moxidectin (Quest) has a range of action similar to that of ivermectin and can kill at least some small strongyle larvae while they’re encysted, something ivermectin does not do. Moxidectin also stays active in the horse’s system longer than ivermectin, which seems like a plus—Fort Dodge, which makes Quest, says you can go 90 days between treatments.

Monahan sees a danger in moxidectin’s long life, however. Plasma levels (the amount circulating in blood) drop over time, and the low levels remaining after a couple of months may not be enough to kill all the small strongyle larvae that the horse picks up. That raises the risk of resistance—the strongest worms survive and multiply. This hasn’t happened in horses yet, but resistant parasites have appeared in sheep and cattle. Since parasites that survive moxidectin can also survive ivermectin, resistance would be bad news for the horse industry.

Pyrantel salts and benzimidazoles can still play a role against parasites that aren’t resistant to them. A double dose of a pyrantel pamoate paste, such as Strongid P, will kill tapeworms. Monahan also suggests deworming foals and yearlings every four to six weeks with pyrantel pamoate, year-round, to keep ascarids at bay. Fenbendazole (Panacur) can also be used for this.

Daily dewormers in the pyrantel group (such as Strongid C and Continuex) are designed to continuously kill parasites in the horse’s gut, before larvae do damage or mature into egg-shedding adults. Horses on these treatments still need a paste at least twice a year, to kill bots and tapeworms.

There’s growing concern that daily dewormers foster resistance, because some worms will be strong enough to shrug off the low daily dose. But Monahan says daily dewormers can be an option when your horses go off your property, to a multi-day show or similar event, say, as a way to keep them from bringing home foreign worms.

Gather Intelligence

How often you deworm depends on many factors. Are your horses stalled or on pasture, eating grass that may be contaminated with worm eggs and larvae? How many horses share the pasture? Do the horses go off the property for trail rides and shows, or do new horses often come onto the property? Do you have foals and yearlings, or only adult horses? Does cold weather halt parasite activity in winter where you live?

Your veterinarian can help you sort out these factors. But the best way to figure out how often to deworm—and whether the drugs you’re using are effective—is by monitoring your horses with fecal exams. These tests look for eggs of strongyles, ascarids, and some other parasites in manure samples. (They won’t find bots and several other parasites.)

You don’t need to test every horse. On a big farm, Monahan says, you can use a few horses as sentinels, letting their egg counts reflect the health of the herd. Choose young horses (under 6, if possible), as they tend to have the highest counts. You should also test any horse that has unexplained weight loss or poor condition, both signs of worm infestation. Collect a ball of manure from each test horse with a plastic bag, and then invert the bag over it.

Testing your sentinels before and soon after deworming will tell you whether the treatment was successful. Look for the egg count to drop by at least 90 percent; if it doesn’t, the horse may not have swallowed all the dewormer, or you could have resistant worms.

Follow-up tests will tell you when to treat again. As a general rule, when the sentinels’ counts reach 200 eggs per gram, it’s time to treat the herd—but you don’t need to keep testing until you hit that number on the nose. If you test ten weeks after treatment and find that your sentinels’ counts are creeping up but still below 200, you can decide to wait a couple more weeks and then deworm.

The cost of testing varies depending on the vet’s fee. (You can also submit samples directly to Horsemen’s Laboratory,, an Illinois company that charges $9 to $12 per sample.) If testing allows you to eliminate unnecessary treatments, it’ll save you money.

Wage Strategic War

Chemical dewormers give you a huge edge in reducing the health threats, including fatal colic, that parasites can cause. But these drugs are only one part of the plan. Environmental controls—steps that break the life cycles of the worms—are equally important. Here are six practices that a good program will include:

1) Remove manure daily from stalls and at least weekly (twice weekly is better) from paddocks. In some tests, this step alone was as effective as chemical deworming in controlling parasites.

2) Compost manure (the heat buildup in compost will kill worm eggs) or spread it on ungrazed areas. Except in extremely hot, dry conditions, worm eggs will survive in manure spread directly on pastures, reinfecting your horses.

3) Give pastures a break by rotating horses to different grazing areas, if you can. Make the break a long one—in cool, damp weather, it may take six months for worm larvae to die out.

4) Avoid ground feeding, and keep water buckets and troughs clean.

5) Remove bot eggs by clipping hair or sponging with warm water. The eggs look like little yellow seeds, stuck to the hair mainly on the legs or around the mouth.

6) Quarantine new arrivals until they’ve been dewormed.

Following these steps and monitoring your horses with fecal exams will allow you to design a deworming program that’s truly effective, saves money, and delays the day when resistant superworms show up on your farm.






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