Strategy Versus Rotation

Worms are an ever-present concern for barn owners. But does the traditional rotation model of deworming still work? Some say it's overkill.

Are you controlling your horses’ internal parasites? In the 21st century, parasites continue to thrive, partly because of how drugs are administered. What’s “in” is resistance, and what’s “out” is rotation.

The target species (worms in adult, larval, and egg stages) remain the same, and so do the chemical classes of anthelmintics (dewormers). What’s changed is the use of a new approach, which is called a strategic deworming.

Lydia Gray, DVM, MA, explains, “It’s so easy to put on the calendar which dewormer each month, and put it on the board bill. But that’s not how these wily worms are working. They develop a resistance to many of our favorite products.”

Dr. Gray, staff veterinarian and medical director for SmartPak, suggests that instead of rotating through drugs six times a year, you focus deworming on those horses that have high infestation. By concentrating on those selected horses, you can reduce treatment of the rest of the herd and reduce chemical contamination of your soil. Strategic deworming is a way to improve horses’ health and also save time and money.

Counting Parasite Load

Adult parasites in the horse are what we aim to destroy. A horse may look healthy, but how horses shed parasite eggs indicates which animals need treatment. “You need to know the eggs per gram (EPG) shed from each individual horse,” says Gray.

Wendy Vaala, VMD with Intervet/Schering-Plough, says, “Different horses are more or less susceptible to parasites.”

When a veterinarian conducts a test for fecal egg count, the EPG measurement categorizes a horse as a low, moderate, or high shedder. One with 500 EPG would be a high shedder. Vaala says, “You’d test that horse more times to see how many eggs he throws on your pasture.” A horse shedding eggs at 1–50 EPG is a low shedder, who won’t need deworming as often—maybe only twice a year.

Gray notes, “In horses, you will never have zero [eggs]. So we do these quantitative fecals and get the EPG.” She advises barn owners not to worry—“You won’t have a whole barn of high shedders!”

Realize that the test may not show adult parasites encysted, such as small strongyles. Vaala explains, “In the fecal egg test, what we’re most worried about are small strongyles and tapeworms. Tapeworms don’t shed eggs routinely, so they can be missed.” She adds that small strongyles hibernate in the bowel wall. “Kill them going into winter after the first good cold frost, or just when you come out of winter when the parasites break out of cysts.”

Testing for Effectiveness

Overall, veterinarians have noted an increase in small strongyles. Vaala says that adult horses tend to be more resistant to roundworms than are foals. In addition, some horses are genetically more resistant than others.

She adds, “Use different products for different horses. There’s no one product out there that does it all.”

Resistance develops when drugs affect weak parasites, and stronger ones survive and reproduce. “We do have resistant parasites,” says Vaala. “It tends to be the worst where we have well-cared-for horses. We see overuse and indiscriminate use of drugs.”

You can easily investigate to see how well your deworming works—“to see what is a particular horse’s innate ability to shed parasites,” says Vaala. “Before you deworm, collect fecals. Then, two to four weeks later, you do another test to see whether that drug reduced the egg count. Expect a 98 to 100 percent reduction by ivermectin or moxidectin, 90 percent reduction by fenbendazole.”

Available Drugs

“There are no new dewormers coming down the pipeline, so make sure the ones we have now continue to work,” says Gray.

Gray explains, “Deworming used to be very strict, to deworm all horses at the same time. The rotational way has been around 40 years—but it doesn’t do any good if the horses on your farm are resistant to that particular class.

“You had to rotate because we didn’t have the comprehensive dewormers that would get all worms. So now, why would we rotate?”

She adds that treating tapeworms is different. “For tapeworms, only one drug gets them. So it’s not a rotation as much as one or two times a year, make sure you include praziquantel.”

And when it comes to daily deworming, Vaala says, “Daily dewormer may be helpful, but we’re finding resistance to that. It’s not a reason to think you’re safe, and it’s not the only drug you should be using.”

Strategy for Timing and Dosage

Your location affects the lifecycle of parasites. For most horses, you’ll schedule your deworming when horses shed parasite eggs.

Climate affects your parasite transmission season. “Deworm before you send horses out to graze in the spring,” recommends Gray.

In strategic deworming, usually you won’t need to deworm in the hot summer months. However, “A high shedder may need to be dewormed every six weeks, and throughout the summer,” says Gray.

Realize that parasites hibernate in the host (horse) or in the soil. Vaala advises, “The snow and cold up north don’t kill the larvae—they hibernate.”

“Ascarid eggs can live in the environment for years—strongyles not so long, but they can live over winter,” says Gray. And if you live in a temperate climate, your parasite transmission season could be year-round.

Pasture management and regular paddock cleaning can reduce the eggs or larvae that horses pick up. “Pick up manure twice weekly, before eggs hatch and infect other horses,” says Vaala.

According to Gray, “It takes two days for a strongyle larva to develop.” She notes that dragging pastures to spread manure can spread parasites further. “When you break up a pile of manure, the strongyle larvae can crawl 20 inches—you’ve extended their range.”

When you’ve identified your high shedders, you might group them in a specific pasture. “They can help maintain a lower pasture burden of eggs, depending on who you turn out there,” says Vaala.

Horses pick up parasites on many other surfaces. While the pasture is the main source, also consider paddocks, stall floors and walls and trailer walls. “Roundworms can stick to anything,” says Vaala. “Pinworms are also acquired indoors or out. And tapeworm eggs can be picked up with hay.”

To develop your own strategy, talk with your veterinarian. “It’s not a cookbook any more,” says Vaala. “Know when a horse is most at risk for picking up parasites in the pasture or stall, which parasites are the biggest problem, and what drugs are really working.”

Treating Your Pasture for Parasites

To enrich your pasture, Rhizoboost is a product that stimulates forage growth and reduces parasite eggs.“It digests the chitin egg shell on the nematode eggs, so it cuts down the reproduction of the nematodes,” says Ron Whitehurst of Rincon Vitova. “In the pasture you get improved soil structure, improved plant growth, improved vigor of plant, and reduction of nematode load.”

Rhizoboost is a microbial soil inoculant that enhances biodiversity and discourages growth of undesirable microorganisms (parasites). Treating the soil while reducing the amount of drugs in manure helps your soil. Whitehurst says, “In the organic community, there’s a lot of concern about the effects of chemical dewormers on the life in the soil, and the effect on pasture and bird populations.”

For More Information

See these websites for product details:

1) Intervet/Schering-Plough Animal Health (Safe-Guard, Panacur)

2) Merial (Zimecterin, Zimecterin Gold)

3) Pfizer Animal Health (Anthelcide EQ, Strongid C and Strongid C 2X)

4) Rhizoboost

5) SmartPak Equine

For more details about strategic deworming:

• Presentation from Virginia Tech, “Emerging Issues in Equine Parasite Management”

Article, “The New Equine Parasitic Threat”






"*" indicates required fields

The latest from Stable Management, the #1 resource for horse farm and stable owners, managers and riding instructors, delivered straight to your inbox.

Additional Offers

Additional Offers
This field is for validation purposes and should be left unchanged.