Clipper and Doc

Clipper and Doc header image
Researchers in OSU's veterinary college say EPM is Oregon's worst horse disease.

Clipper and Doc don't know each other, but they have a lot in common. Both are slick-looking Thoroughbreds. Both are dark brown. Both stand 17 hands tall. And both have suffered from the worst ailment ever to hit the horse industry.

The disease is equine protozoal myeloencephalitis, or EPM. A terrible neurological disease, it causes even the best of horses to stumble, fall, have seizures, go blind or even die. EPM is considered the number one neurological disease of horses in the Western Hemisphere. Oregon is one of the hot spots, probably because of the moist climate the EPM parasite thrives in. Before 1986 there were no EPM cases in Oregon. Now at least one new case comes in every other week to Oregon State University's Veterinary Clinic.

OSU veterinary researchers have made control of the disease their top priority in horse research. Their primary thrust is to find ways to strengthen horses' immune systems to prevent them from getting the disease. Officials at the OSU Agricultural Experiment Station have put extra funding into the control effort.

EPM's cause is a single-celled parasite called Sarcocystis neurona. This protozoan infects horses only when they eat pasture grass, hay, grain or water contaminated by infected opossum feces. Both birds and opossums can spread the disease, making the spread nearly impossible to control.

Woman holding bridle of horse. The horse is in a body sling.

Robin Neilson of Oregon City with her treasured Thorughbred, Raja Jet, at OSU's veterinary college. The horse was diagnosed with equine protozoal myeloencephalitis, or EPM, and had to be euthanized. Photo: Lynn Ketchum

On top of that, EPM treatment is expensive and doesn't always work. And when you're through treating, you may not get back the horse you started out with. In pregnant mares, the medication of choice, a combination of different sulfa antibiotics, may cause abortion or birth defects. Newborn foals from these mares may be irreparably damaged and have to be put to sleep. In stallions or geldings, like Doc and Clipper, the horse may get better with therapy, or it may not.

Doc is a lot better now, although his owner worries about a relapse-again. Clipper has also improved, although his athletic days seem over-even after eight months of treatment that cost more than $3,500.

Here are their stories.

* * *

Clipper was beautiful-cocoa brown, almost black. Jill Bartlett fell in love with him at first sight. Clipper had such potential. His previous owner had paid $50,000 for him when he was a year and a half old. But the Thoroughbred had health problems and never made it to the winner's circle.

Jill got Clipper "for free" when he was 5 years old and still suffering the after-effects of pneumonia. Clipper couldn't race, but Jill figured with a little tender loving care and a lot of training he could be a hunter/jumper. She imagined him bounding over rails, sloshing through water barriers and jumping over hedges. That dream soon faded.

An opossum walking on a branch.

Horses get EPM when they eat pasture grass, hay or grain, or drink water contaminated with opossum feces containing a parasite. Birds and opossums can spread the disease. Photo: Tom and Pat Leeson

"I brought Clipper to Oregon in June 1996 and he was diagnosed with EPM by mid-October," Jill recalled. "We started treating him in October and weren't able to take him off medication until mid-July of the next year."

The treatment was twice as long as the usual given to horses, and Jill was crestfallen. "I couldn't ride him. He was ataxic-uncoordinated. He'd stumble and fall down. He got a little better under medication, but you can't ride a horse when he's medicated, because then he might get stressed, which could cause a relapse."

A month after the treatment ended, Jill's attitude was more positive. Clipper was doing well, "on the verge of being normal."

Still, today Clipper is not the jumper or racer his genes would have him be. "He's mostly a pet," said Jill, as she watched him romp along on her place three miles east of Corvallis, where she has two other horses, two dogs, three cats, five birds and a sheep. Clipper stands the tallest in that menagerie.

For Jill Bartlett, her association with Clipper has been bittersweet. She loves the horse, but she's watched a lot of money slip through her fingers as she's tried to make him better. The medication cost $230 a month for eight months. Diagnosis was $500. Then she had to keep bringing him back to the veterinarians for a spinal tap-three times at a cost of $150 each.

It isn't the money that bothers Jill most, though. She can really empathize with her horse because she sees so many similar cases in her work. You see, Jill's day job in the College of Veterinary Medicine at OSU includes doing electromyography (EMG) tests on horses that come into the OSU clinic. In other words, she uses needle electrodes placed in muscle to detect the spontaneous voltage that indicates disease. "The EMG can detect abnormal muscles whose nerves are damaged," she said, "and I could tell Clipper had a lot of damaged nerves."

The protozoa-the one-celled animal that causes EPM-can affect the brain, the spinal cord, or both. It is these parts of the central nervous system that give rise to the peripheral nerves that make muscles contract and relay sensory information to the brain.

Woman and horse next to a electromyography machine.

Dr. Linda Blythe, a neurologist and the associate dean of OSU's College of Veterinary Medicine, gives Clipper, a horse with EPM, an electromyography. The test checks the nerve supply in muscles. Here, she's examining nerves in a muscle in Clipper's head. Photo: Lynn Ketchum

"A horse can get better if its nerve cells aren't irreversibly damaged," said Jill. "After treatment, we may have to give a horse a year to recover. By the end of a year, that's probably as good as it's going to get. Clipper has had a harsh life. He's really nice, a really sweet horse. But I haven't had a saddle on him for more than a year. It's a real bummer."

* * *

Doc's story is a happier one, although it wasn't always that way. Dr. Katie Avison, a specialist in human internal medicine in Portland, got him when he was 5 years old. Horse trainer Mary Nash had seen Doc at a race track and recommended him to Avison.

"Doc is probably the kindest, quietest Thoroughbred I ever saw," Avison said. "I put my 3-year-old on him, and she can feed him apples and carrots out of her hand; he never bites."

Foaled in 1987, Doc is one of the best jumpers and show horses in the state. At age 10 he won the training level at the Freeman Farm against 85 competitors. Then he won the adult amateur division at the Pine Desert Horse Show in Bend. It was there that Nash first noticed Doc had a slight limp.

"We took Doc to veterinarian Tim Phillips," Avison recalled. "He told us, 'I don't think it's lameness; it could be EPM.'"

Doc's next stop was the OSU veterinary clinic, which offers clinical service to the public and referring veterinarians. Dr. Linda Blythe, associate dean in the College of Veterinary Medicine and an expert neurologist, did a neurological workup including a spinal tap and EMG tests. Sure enough, Doc had EPM.

Woman dropping liquid into a test tube.

Searching for clues to how a horse with EPM is getting along, Dr. Sue Tornquist, an OSU veterinary professor, examines a sample of fluid taken in a cerbral spinal tap. Photo: Lynn Ketchum

And there lies a mystery. Doc had never been turned out to pasture where he might have been exposed to opossum feces or some other source of the EPM parasite. In fact, he had been housed with other horses who seemingly got the same feed he did, and none of them got EPM. It is that kind of mystery that has bothered veterinarians and horse owners from coast to coast. In Oregon, Blythe had detected antibodies for EPM in the blood of 45 percent of the clinically normal horses she sampled statewide. The number rose to 60 percent in horses west of the Cascade Mountains.

This indicated that many horses in Oregon have been exposed and have successfully warded off the disease. Research is focused on what goes wrong with the immune system of some horses that allows the protozoa to enter and damage their nervous system.

Doc's treatment for the disease was complicated. In July 1995, Avison got up each day at 5 a.m. to drive from Portland to Newberg to give Doc his medicine.

"He had to have the medication on an empty stomach, and he was fed at 7 a.m.," Avison said. As it turned out, Doc's medication, a combination of sulfa medication and a drug called Daraprim (pyrimethamine), was causing anemia. His hemoglobin had dropped from 12 to 8. The normal range for a horse is 12 to 16. "Doc's bone marrow wasn't making redblood cells. The medication was interfering," Avison said. "So we stopped treatment and watched him. No problems."

Man using a shot to draw liquid from a horse.

Dr. Terry Gerros, another veterinary professor, draws spinal fluid for EPM testing. Horses get a local anesthetic before the 7-inch needle goes in. Photo: Lynn Ketchum

Then in February 1997, Avison and Doc's trainer, Mary Nash, noticed Doc "hitching his leg," snapping it up at the hip joint with every stride. A spinal tap showed the EPM antibody was present-Doc had had a relapse. This time he was treated with liquid Daraprim and sulfadiazine, specially formulated for the horse by pharmacists at Mortar and Pestle, an Iowa-based company working with OSU scientists to develop better treatment methods.

Doc's treatment went better. For one thing, it was easier. "He just needed a squirt a day at 6 a.m., and this time we had him at a barn closer to home," Avison said.

Treatment stopped in May 1997. Doc has had no problems since. In fact, in June he placed in six of eight jumping classes at Spruce Meadows against 64 competitors. Today, Avison figures Doc is worth at least $35,000.

"I think the main reason Doc is better is that Mary Nash spotted a problem early, and we treated him early," Avison said. "She taught me to watch my horse every day."

* * *

The OSU effort against EPM is a three-way thrust. First, researchers, led by Linda Blythe, did a statewide survey to establish the disease's presence in Oregon.

Second, the OSU veterinary clinic offers a daily clinical service to the public and referring veterinarians. The service includes physical and neurological examinations, diagnostic tests like EMG and spinal taps, and suggestions for treatment. Analysis of the spinal tap is the key to making the diagnosis.

"Often we take a sample of the fluid around the brain and this is sent to a lab in Kentucky for specific analysis and diagnosis," Blythe said. "The tap can be a tricky process and the horse has to stand still for it. A lot of veterinarians prefer not to do it." Once a horse is diagnosed with EPM, horse owners and their veterinarians get detailed counseling on therapy and case management.

The third thrust of OSU's research effort is to find out how the horse's immune system works. Horses can't spread EPM to each other. But how come one horse, like Doc, will get the disease when 50 other horses in the same barn, that have eaten the same hay, don't? A team of OSU researchers is looking at levels of substances called antioxidants in horses, particularly vitamin E and selenium, which are components of their immune system.

Meanwhile, Dr. Terry Gerros, another OSU veterinary professor, has been analyzing EPM medications in cooperation with the Mortar and Pestle pharmaceutical company. "Even with the best drugs we have available, there is at least a 10 percent relapse rate," Dr. Gerros said.

Woman with horse in stable.

Doc with owner Katie Avison at a Portland-area stable. Despite relapses, he's better. OSU is studying horse care that helps prevent EPM. Photo: Lynn Ketchum

While pharmaceutical companies keep trying to develop better drug treatments, some OSU researchers have taken a different tact. "We are looking at ways to prevent-to ward off-the disease," Blythe said. "We want to make horses more resistant by enhancing their immune systems."

Good horse care is one way to do that. "Provide the horse with good nutrition, protect it from bad weather, keep it parasite free and give it plenty of vitamin E to strengthen the immune system," Blythe said.

She said doubling the recommended amount of vitamin E in rations of young horses has markedly reduced another horse disease, equine degenerative myeloencephalopathy (EDM), a disease responsible for 25 percent of spinal cord clinical cases seen in Oregon 10 years ago. Today, thanks to the extra vitamin E, EDM is pretty much gone from the Oregon scene.

With EDM out of the way, OSU veterinarians have been working aggressively to educate the state's horse owners and veterinarians about EPM. Besides monitoring disease spread, they participate in workshops, distribute literature and do public presentations.

OSU veterinary science professor Sue Tornquist, a clinical pathologist, has received an Agricultural Experiment Station grant to identify immune factors associated with disease. Tornquist performs immune assays on samples from EPM cases. She is looking at differences in certain types of lymphocytes, which she says may be critical to a horse's immune responses.

"When a horse is exposed to EPM, it makes antibody-a defense against the organism," she explained. "But the antibody alone does not protect the horse against disease. That's where the immune cells, the lymphocytes, come in. They give the horse cell-mediated immunity. In other words, the lymphocytes may react to the Sarcocystis neurona, directly attacking it, recruiting other cell types to attack, or producing cell substances that attack the parasite.

"Now we know more about horse immunity than we did before our project started. However, we need to know even more before we can optimally treat the horse or prevent the disease."

The OSU researchers are optimistic about their chances for success.

Woman riding a horse.

Jill Bartlett of Corvallis exercises Clipper. After he got EPM, Jill couldn't ride the Thoroughbred. "He'd stumble and fall down," she remembers. As the photo suggests, Clipper's better, thanks to lengthy and expensive treatment. But he isn't the jumper or racer his genes would have him be. Photo: Lynn Ketchum

"Drug treatments keep getting better; we keep learning more about the horse's immune systems," Blythe said. "Meanwhile, most horse owners are well aware of the disease and are watching feed supplies to avoid contamination with opossum feces and bird droppings, as best they can. They are also keeping careful watch on their horses, looking for any signs of EPM. And they are making sure the horses have good rations with plenty of vitamin E.

"Hopefully," Blythe added, "it's just a matter of time before we get EPM under control."


OSU's College of Veterinary Medicine leads the way in research and extension efforts to deal with EPM.

For openers, researchers in the college affiliated with the OSU Agricultural Experiment Station sensed an impending problem and monitored cases extensively. They did a survey of serological tests to find out how serious EPM was in Oregon. They told veterinarians that taking blood samples alone was not enough to verify the disease, and that the horses needed a spinal tap and analysis of their cerebral spinal fluid to check for antibodies to the parasite.

OSU scientists and specialists took part in a continuing education effort to share what they had learned with veterinarians nationwide. They took every opportunity to teach veterinarians and horse owners how to watch for EPM and how to treat it.

OSU veterinary researchers also participated in an EPM treatment study in collaboration with Mortar and Pestle, a national pharmaceutical company that formulates EPM medication, especially for horses. OSU research documented the efficacy of the treatment.

Today, OSU research concentrates on the horse's immune system, learning why some horses get EPM and others don't.

Finally, OSU veterinarians operate a referral service. If veterinarians suspect a case, they can refer it to the OSU Veterinary Teaching Hospital where clinicians and specially trained technicians can do electromyography and complete neurological examinations. It is one of the best-equipped veterinary clinics in Oregon to perform these multiple tests on horses.


You can't eliminate all chances of your horse getting EPM. But you can reduce the odds of that happening with good food, good water, good shelter, vaccinations, and monitoring. Here's the to-do list from OSU veterinarians:

  • Follow a regular worming and vaccination schedule.
  • Follow a regular feeding schedule. Use high-quality feed, especially hay. Avoid feeding horses old hay.
  • Keep hay covered and away from opossum feces and bird droppings.
  • Give the horse vitamin supplements, especially 2,000 IU (international units) of vitamin E daily. Supplement separately with selenium if you are feeding hay from a selenium-deficient area.
  • Look at the horse-often. Watch how it moves. Look for atrophy of muscles, abnormal behavior or lameness.
  • Avoid high-stress situations. Provide shade when it's hot, cover when it's wet. Avoid excessive workouts, especially if the horse is in poor condition.
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