Although somewhat rare, veterinarians do get the calls: “My horse tripped twice. I think it might have EPM.” “The horse isn’t working well. Could it be EPM?”
EPM Diagnostic Challenges
Equine protozoal myeloencephalitis (EPM) is called the Great Pretender for good reason: It can present in a variety of ways, confusing horse owners and making a definitive diagnosis a potential challenge. Amy Polkes, DVM, DACVIM, owner of Equine Internal Medicine and Diagnostic Services (IMED), says a thorough neurologic examination is a critical first step in accurately diagnosing the disease.
“Part of the challenge with EPM is that it can present in so many different ways,” she said. “And because it shows up in so many ways, people say it could be EPM and simply start treating for it only to find out it wasn’t EPM. For a horse to have EPM and not something else, the horse has to have neurologic signs.”
Components of a Neurologic Exam
To uncover these signs, Polkes is adamant in her recommendation of a thorough neurologic examination, generally conducted by an internal medicine specialist or neurologist.
For that examination to be completely thorough it should include:
- The horse’s medical history.
- A head and cranial nerve examination.
- Neck and trunk examination.
- Evaluation of gait and posture (walking, circling, backing, and the tail pull test).
- Reflex testing.
- Tail and anus examination.
“EPM is a neurological disease, so if a horse is lame but not neurologic, it’s not EPM,” says Polkes. “That’s why an internal medicine specialist or neurologist can help with this examination, using their training to discern whether it’s something musculoskeletal versus neurologic.”
Next-Step Diagnostic Procedures
If clinical signs of a neurologic disorder are found, Polkes says the next step is a blood test followed by a spinal tap. The blood test can detect the presence of antibodies to Sarcocystis neurona (or Neospora hughesi). However, the presence of the antibodies only confirms the horse has been exposed to the disease.
To determine whether there is an active infection, a Cerebrospinal Fluid (CSF) test is conducted, which will show the presence of antibodies in the spinal fluid. If the antibodies are present there, it is a strong indication there is an infection in the central nervous system.
Finally, ratio testing comparing the antibody levels in both the blood and CSF can be done to determine if the infection exists solely in the central system.
EPM Treatment and Prognosis
EPM is treatable with the antiprotozoal medication MARQUIS (15% w/w Ponazuril), and Polkes has had success with many horses, including a recent case of a show horse that recovered well enough to go back to a full competition schedule. However, statistically, only about a third of horses will make a complete recovery. Of the remaining horses, another third will recover, and a third will not respond well to treatment.
For that reason, it’s important for the veterinary team to manage client expectations.
“A horse with EPM, or any neurologic issue, isn’t going to be headed back to the show ring anytime soon,” says Sarah Reuss, VMD, DACVIM, technical manager at Boehringer Ingelheim. “This can be a very hard conversation to have, especially when we are talking about a range of recovery from being well enough to be a pasture pet or well enough to compete at a high level.”
For more information about diagnosing EPM, go to the Boehringer Ingelheim online diagnosis tool, located here.
Related Reading
- Tips for Diagnosing EPM in Horses
- Improved Understanding of the Genome of Sarcocystis neurona, the Primary Cause of EPM
- Is That Horse Ataxic or Just Lame?
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