How to recognize and treat Equine Protozoal Myeloencephalitis, a protozoal disease that causes inflammation of the brain and/or spinal cord leading to muscular incoordination in horses.

Hot Rodn My Vette, “Ida B,” a 6-year-old American Paint Horse/Thoroughbred mare, was not settling into a new training routine in August 2017. Chancie Bryan of Whitesboro, Texas, bred, raised and started the mare before sending her off to the McLeod family to finish her preparation or fall futurity season. The first week of training went well. So did the second. Then Katelyn McLeod called Bryan to report the filly was dropping her lead and throwing a bit of a fit.

“I wasn’t worried because it was the first time she had been away from home and the first time somebody new was riding her,” Bryan said.

The mare also dropped weight. Again, Bryan wasn’t concerned. She chalked it up to midsummer Texas heat. Plus, her exercise routine had increased from four days of riding a week to seven. Weight loss during the adjustment period was to be expected.

However, when Michele McLeod returned from a road trip, she took one look at the horse and said that the mare had EPM, Bryan recalled.

“She wasn’t showing any neurologic signs so I just didn’t think that’s what it could be. She had only been with the McLeods for two weeks and had to have been exposed before ever going to their barn,” Bryan said. “I picked her up on a Saturday, had the chiropractor and veterinarian look at her the same day we got home, but we decided to have her tested.”

The titer results came back at 360, which meant it was 87 percent positive. With those results, the veterinarian said she should have barely been able to walk. But her only signs were weight loss and dropping a lead.

Bryan’s veterinarian recommended treating the mare with MARQUIS® (15 percent w/w ponazuril) Antiprotozoal Oral Paste, a treatment given over 21 days.

“With that high of a titer, I didn’t have 21 days. And you can’t always be sure an oral medication is absorbed,” she said. “I opted to use a five-day treatment prescribed by Dr. Brunks.”

Nebraska-based veterinarian Doug Brunks, DVM, owns Equine Veterinary Associates. He recommended a proprietary formulation given intravenously twice a day and once a day intramuscularly.

“This treatment is something I’ve been using for 40 years, before there was even a test for EPM,” he said. “There isn’t science behind it, but clinically it works.”

Bryan also increased the filly’s ration, added Vitamin E and Diatomaceous earth. Within 60 days, Ida B had gained 200 pounds and was swimming in an exercise pool. Ida B looked as if she had recovered well. But in December 2017 she appeared to have trouble breathing. Bryan scoped her and discovered her throat flapper was partially paralyzed. Bryan treated the mare again, turned her out and gave her time to recuperate.

“We tested her again in August 2018, a year later and her titer was normal and when she was re-scoped, her flapper was back to 100 percent,” Bryan said. Bryan was fortunate Ida B’s symptoms didn’t progress. Other horses can quickly progress from being “off ” to exhibiting full-fledged neurologic symptoms.

University of Pennsylvania School of Veterinary Medicine (Penn Vet) veterinarian, Michelle Abraham, BSc BVMS DACVIM (LAIM), has seen a horse’s situation change in 24 hours. One dressage horse she treated arrived with mild incoordination and within a day, started staggering and tilting his head to one side.

When horses arrive at Penn Vet’s New Bolton Center they are graded on a scale of zero to five on the equine neurologic grading scale. Ataxia is the severity of one’s ability to control his/her movements and balance. Zero is considered normal and five indicates a horse is unable to stand. A grade 3 is defined as a horse that exhibits neurologic deficits including ataxia that is prominent when walking and turning, when loin pressure is applied or in a neck extension.

“The first symptoms are a change in a horse’s attitude and just not working like he used to,” Brunks said. “But it’s important to remember soreness can cause an attitude change, too. Poor tail tone, weakness in the hind end and progressive muscle loss are other indicators.”

The infective form of EPM develops in opossums.
The infective form of Equine Protozoal Myeloencephalitis develops in the definitive host for EPM, which is the opossum. Photo courtesy of iStock.

What is EPM?

The acronym EPM stands for Equine Protozoal Myeloencephalitis, which is a protozoal disease that causes inflammation of the brain and/or spinal cord leading to asymmetrical ataxia, commonly known as muscular incoordination, in horses.

EPM was discovered in the 1960s, but it wasn’t until the 1970s that the organism was named Sarcocystis neurona. S. neurona remains the primary parasite responsible for EPM, but Neospora hughesi is emerging as another. Both rely on host animals to survive and to be transmitted to the horse.

The life cycle of EPM is complicated and involves multiple host species including an intermediate host, a definitive host and an aberrant host. The intermediate host(s) include raccoons, armadillos, cats, sea otters, harbor seals and skunks. The parasite gets into an intermediate host such as a cat, raccoon, armadillo or skunk, with cats being the most likely host in barns.

The intermediate host ingests sporocysts from the environment and the sporocysts travel to muscle tissue where they form sarcocysts. However this is not the infective form of the organism.

The infective form develops in the definitive host for EPM, which is the opossum. The opossum ingests sarcocysts by ingesting dead intermediate host muscle tissue, from a cat or skunk, and the sarcocysts reproduce in the gut forming sporocysts. The sporocysts are passed in the opossum’s feces and are the infective form of the organism.

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