Virtually every farm manager agrees that having an effective deworming program in place is a good idea. With the regular use of modern deworming medications and a few other management techniques, it’s possible to ensure that the herd and the property in your care is practically free of damaging internal parasites such as strongyles, pinworms, roundworms and bots. But though we’ve been tackling worms for decades and now have an array of effective anthelmintics at our disposal, there’s still very little agreement on what constitutes an effective deworming program. Here’s how five farms across the continent handle it.

The Tried and True

Holli Fontenot of Circle 8 Paints in Richmond, Texas, stands the black tobiano stallion QTS Blue Eyes (by QT Poco Streke and out of Bunny Skip A Run) and has a selection of weanlings, yearlings and broodmares for sale. Her deworming program for her adult horses is based on two drugs—ivermectin and pyrantel (Strongid)—administered alternately every two months in paste form. “My babies get dewormed every month until they are a year old,” she says. “I give them only Strongid paste until they are three months old, then start alternating with ivermectin.”

A couple of thousand miles north, at Franz and Elke Hollenbach’s Long Lane Farms in Puslinch, Ont., the herd of Trakehners is dewormed four times a year. Resident trainer Bruce Mandeville explains: “We administer the first dose after the thaw in the spring and then do a second dose one month later, using ivermectin. Then we deworm again in the fall after the first good freeze and again a month later. We usually use ivermectin three out of four times, and another type of dewormer, like Strongid, once.”

With 30 to 60 head to deworm, Mandeville says ivermectin, in its liquid form, offers an advantage. “It’s easy to shoot liquid down the throats of the horses with a syringe.” Long Lane’s program takes advantage of Canada’s climate to do some of the worm-killing: The parasites do not survive the long, cold winters.

Making Deworming a Science

Iron Spring Farm, one of North America’s leading warmblood breeding farms, in Coatesville, Pa., has deworming down to a science. “Having a veterinarian involved in designing your deworming program is key in the arsenal,” says Robert Croteau, Iron Springs farm manager who cares for about 130 horses on 1,500 acres. “He or she will be up-to-date on all the cutting-edge information on battling parasites.” Unionvillle Veterinary Services’s Dr. John Lee has been instrumental in creating a program for Iron Springs, which sees foals treated every 30 days according to their weight (monitored on a weekly basis). “We’re very pro-active where the youngsters are concerned,” says Croteau. In addition, broodmares, yearlings and older competing horses are each set up with their own, scheduled and monitored deworming programs.

Iron Spring finds it most effective to use a different product in a different drug class with each deworming. “Some of our horses on the road are on the daily dewormer, Strongid C, as extra insurance,” Croteau says, “and they get ivermectin a couple of times a year as well.” Foals are double-dosed with some products. “We have foals of different ages and their dams may be shedding cysts, which the foals can pick up in the field, so you don’t want to under-dose them.” Croteau notes that ‘outside’ mares coming in to be bred are required to undergo fecal checks when they arrive, and are never housed in the same fields with the general population. Broodmares are kept on a two-month program and are dewormed the day after foaling.

Monitoring each horse’s overall condition is important as well. “You have to be aware of the whole picture,” says Croteau. “Look at the condition of the hair coat, the attitude of the horse, the way he sheds in the spring—these small indications can tell you whether your deworming program is working. If a horse looks ‘off,’ we may run a fecal check and do bloodwork. The eosinophil count can help tell you if larvicidal work needs to be done.” He notes, however, that deworming won’t replace the basics—“groceries or dental care.”

Though paste dewormers form the core of the Iron Spring program, Croteau also believes in administering anthelmintics by nasogastric tube once a year. “When you’re treating as many horses as we are, some of that paste is bound to be spit out. Tubing ensures each gets a good dose.”

Natural Approaches

Another ‘old-time’ solution is employed with success at Lauren Sellars’s 25-acre farm in Cawston,

B.C., where she raises and trains Norwegian fjords. Though Sellars relies on liquid ivermectin as her main anthelmintic of choice, she supplements her program by feeding garlic to her 25-strong herd. “They get a taste for it and soon love it,” she says, “and I do believe it helps in the deworming process.” She also uses her horses in harness to drag their own pastures—a process which breaks up manure, exposes worm eggs to the sun and kills them.

At Dream Catcher Paints in New Prague, Minn., Theresa Marsyla and Carrie Welch stand two stallions and have a total of nine mares, all of which are treated on a rotation with pyrantel paste and ivermectin liquid every two to three months. “We have 20 acres for the seven mares who aren’t showing, so they have a lot of room and worms don’t build up that badly,” says Marsyla. “But we scrape out and clean the outdoor shelters every week, stripping and scrubbing down the 21-stall barn from top to bottom every spring and fall.”

“We rarely breed outside mares and are careful about what comes in,” she adds. “When we purchase a horse, we have a health examination and look at the previous deworming and vaccination schedule—and we never turn a new horse out with the others until we’re sure it’s healthy.”

Marsyla says she has based her deworming program on talking to other breeders and finding what has worked for them. “We’re very happy with the results of our routine.”

Though no two of these farms has an identical deworming program, all seem to be happy with their results. Barns need to tailor deworming programs based on such considerations as climate, pasture, the ages of the horses and the maount of travel on and off the property they are doing. Small operations with isolated populations and lots of room can afford to be more casual about deworming than large commercial setups with breeding stock or show horses.

The best recommendation seems to be to keep up-to-date on the latest suggestions for deworming and don’t get stuck in a rut. Use your eyes and fingers, as well as the latest diagnostic techniques to help assess the effectiveness of your program.

The Veterinary Viewpoint

According to Dr. Andrew Peregrine, associate professor of pathobiology at Ontario Veterinary College in Guelph, Ont., the horse industry has been very lucky when it comes to deworming drugs. So far, he says, problems with parasitic immunity to drugs have been relatively minor. But to assume that condition will continue indefinitely “is being naïve. It’s coming. Parasites are very successful at adapting.”

In other species, most notably cattle and sheep, problems with resistance to deworming drugs, including ivermectin, are rampant. In some parts of Australia, for example, resistance is so widespread that some farmers have had to stop raising sheep entirely. Sheep parasites have a much shorter lifecycle than equine ones (a couple of weeks vs. up to 10 months or so) and farmers tend to deworm sheep far more frequently, but the resistance among sheep should be a warning sign. And, with the drug industry coming up with relatively few new drugs to add to the arsenal (only one, moxidectin, has been developed in the past 20 years), we should be doing all we can to preserve the power of our current deworming medications.

“If you use different compounds each time you worm, you have the highest chance of resistance,” Peregrine says. “You expose each generation of parasites to a wide variety of different drugs.” A better solution, he suggests, is to rotate deworming products once a year. For example, use ivermectin for one year and then switch to pyrantel the next. When doing this, use the same drug on all ages and groups of horses on your farm.

It’s also important to resist the temptation to overuse dewormers. Young horses faced with their first exposure to internal parasites may need to be wormed every two months, but adult horses who’ve developed a bit of healthy immunity of their own should only be dewormed (according to the manufacturer’s directions), for the first four months they are on new pasture after a winter kill (or a summer kill, if you live in the deep South where drought and heat kill worms). This ‘strategic deworming’ takes into account the natural lifecycle of the worms and attacks them when they are most vulnerable, Peregrine says.

Fecal checks can become a valuable part of parasite control program. “I recommend taking fecals from as many horses as possible, every year or two, in the summer months; on the day of treatment (with a dewormer) and again 14 days later. You should see a 90 percent reduction in egg counts (eggs per gram of feces).”

“Targeted” deworming, in which only horses with high egg counts get treated with dewormers, may be the best strategy for warding off drug resistance. “But it has to be done right,” Peregrine says. “It takes frequent fecal checks, and most people would rather just buy a tube of dewormer and hope for the best.”

Having a low level of internal parasites may not necessarily be the worst thing for a horse. Though owners may be horrified at the thought, carrying a load of worms is natural for wild equines—and when exposed to low levels of these parasites, most horses build up a degree of immunity and suffer few ill effects. It’s only when we crowd domestic horses on small pastures and multiply the effects of parasites that extreme health problems develop. —KB