The difficulty in diagnosing equine protozoal myeloencephalitis, or EPM, is a source of investigation and consternation for veterinarian Stephen M. Reed. But the challenge really hits home when he encounters a particularly vexing case like the one he treated on familiar turf.
A horse being ridden at Dr. Reed’s home clinic, Rood and Riddle Equine Hospital in Lexington, Ky., appeared to be dragging a toe, even as it was able to clear the fences. The owner brought the horse in two days later for an exam.
“His gait was a bit off, so we did a lameness exam, and there were some subtle changes but nothing so dramatic to think there was a neurological problem,” said Reed, DVM, Dipl. ACVIM.
A blood test for EPM revealed barely any antibodies, “So it didn’t look like a recent big exposure to a causative organism,” Dr. Reed said. “Because we couldn’t make a diagnosis, we recommended that we see the horse back in a week or two.”
Six days later, the horse was down and unable to rise. The owner struggled to get him into a trailer and to the hospital, where a serum test was a strong positive and was backed up by a cerebrospinal fluid test.
“Don’t I wish I had tapped the horse (originally) and put him on (EPM) treatment,” Reed said. “Now I think the antibody count was so low because it was a very recent exposure and there wasn’t time to build antibodies. But the train was on the track, and it was moving.”
Reed and his colleagues treated the horse “very aggressively” over the next 18 months, “but we finally lost that horse.” A necropsy confirmed EPM.
“A case like that burdens you,” Reed said.
Plenty of other equine practitioners have had to face similar burdens associated with EPM, which has been called “the master of disguise” because its symptoms can mirror those of other neurological disorders.
There is no vaccine for this central nervous system infection caused by a single-cell protozoa most often transmitted through the feces of the ubiquitous opossum, which means prevention is the first of many challenges.
That these days, navigating diagnosis and treatment is at least a little clearer and features a few more positives than previously was the case.
“The biggest improvement is in diagnosis,” said Martin O. Furr, DVM, Ph.D., Dipl. ACVIM. Dr. Furr is a professor and Adelaide C. Riggs Chair in Equine Medicine at Virginia Tech’s Marion duPont Scott Equine Medical Center.
Although a number of EPM tests are available, research and his own experience tell Furr that the most reliable is the Surface AntiGens (SAG) 2 ELISA test for sarcocystis neurona, which is by far the most common of the two protozoan causes of EPM.
“It is the most accurate (test) when it is applied to blood and spinal fluid and a specific antibody index is calculated,” Furr said.
The test of both serum and cerebrospinal fluid, coupled with mathematical corrections, is not yet widely used, Furr noted.
“But I hope it is becoming more widespread, because it’s the best that we can do,” he said.
Still, there is no perfect test, noted Sharon Witonsky, DVM, Ph.D., Dipl. ACVIM. Because EPM remains a challenge to diagnose, Dr. Witonsky works closely with clients, “And there are times, working in the field and based on clinical signs, we have started treatment even though we don’t have a true positive test.”
In all suspected EPM cases, it’s critical to do a full neurological exam, said Witonsky, associate professor in Equine Field Service, Department of Large Animal Clinical Sciences at Virginia Tech. And not just for the first exam but for follow-ups as well.
“One thing I’ve noticed is that a lot of horses have a change in personality and behavior,” she said.
Asymmetrical muscle atrophy is another common clinical sign, said Reed. He noted that it’s important to make neurological exams a component of routine checkups, so any subsequent abnormalities become more apparent.
Once EPM is diagnosed, Reed’s treatment plan features one of two oral medications to kill off the protozoa: Protazil (diclazuril) or Marquis (ponazuril). Protazil just became available in 2011, and Reed said he likes that it comes in pelleted form.
“It also works at very low dose–1 milligram per kilogram,” he said. “At that dose, you get high concentrations in blood and CSF.”
Length of treatment is important, said Furr. With ponazuril, the 90-day recurrence rate is less than 10 percent after treatment for a month, he noted.
“We believe we can minimize recurrence by treating longer” – as long as two months, the doctor added.
Daniel K. Howe, Ph.D., and his colleagues at the University of Kentucky’s Gluck Equine Research Center are working to identify parasite proteins and, it is hoped, to elicit a strong immune response as a protection against EPM.
In addition, through a genome-sequencing project, they hope to unearth the genetic makeup of the parasite, taking a step toward identifying molecules that might be used for an EPM vaccine.
“There have been a lot of efforts into developing vaccines for protozoans, but not a lot of success,” Howe said. “We still don’t have a vaccine for malaria, for instance. But that doesn’t mean it isn’t possible.”
This article originally appeared in Veterinary Practice News magazine's special Equine Practice section.
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