The words equine protozoal myeloencephalitis first appeared in this publication more than five years ago, and while the scientific community has made great advances in unraveling certain aspects of this disease, other areas still beg for answers. In the past, increased awareness–and fear–of EPM led to it being “diagnosed” as the cause of everything from lameness to a change in behavior. Now, horse owners and their veterinarians are accepting that EPM is not the cause of every stumble, limp, or failure to perform up to expectations. They instead are relying on good medical diagnoses to eliminate other causes of problems, and are putting horses on available treatments earlier in the disease process if EPM is determined to be the cause of a physical problem.

“We’re seeing it just as much as before,” said Doug Byars, DVM, Dipl. ACVIM, ACVECC, head of the medicine facility at Hagyard-Davidson-McGee veterinary practice in Lexington, Ky. “Now, we’re taking a more rational approach.”

That rational approach, however, is tempered by problems with diagnostic testing and unproven medical treatment. While five years of field use of certain drugs and combinations of drugs has given veterinarians means by which to address the problem of EPM, practitioners don’t see one as better than any other.

“There are no studies that show one product to be better than another,” said Bill Bernard, DVM, Dipl. ACVIM, of Rood and Riddle Equine Hospital in Lexington, Ky. “All seem to give us the same percentage of improvement and response to therapy. But economics have to be considered.”

Each of these experienced veterinarians has seen hundreds of cases of EPM in the past half-decade. Their advice to horse owners is the same: Get an appropriate diagnosis to start with, then treat based on what your veterinarian discovers.

Other neurologic problems or diseases need to be ruled out before assuming symptoms are due to EPM. Wobbler syndrome, especially in Thoroughbreds, Thoroughbred crosses, and other fast-growing breeds, is a common cause of neurologic signs in horses. Wobbler occurs when vertebrae in the neck press on the spinal cord, and can be caused by bony compression in the spinal canal, laxity of the intervertebral ligaments, and abnormal bone development, such as osteochondritis dissecans (OCD).

While there aren’t many diagnostic tests available for horses with neurologic problems, radiographs of the neck (including mylograms, which involve an injected dye before radiographs to help veterinarians visualize problems with vertebrae) can be diagnostic. Veterinarians actually can see where the vertebrae are pressing against the spinal cord, and in some cases can surgically correct the abnormality.

There are several diseases or problems that can cause neurologic signs in horses, including encephalitis, West Nile virus, botulism, lower motor neuron disease, and rabies. There are diagnostic and serologic tests that can be done to rule out some of these diseases. There also are tests that can be done on horse blood serum and cerebrospinal fluid to detect antibodies for EPM. While used extensively in the early years of the disease’s prevalence, they are used less today, and used more wisely. Veterinarians also stress that EPM can be present at the same time as any of these other diseases.

“Owners must realize that diagnostic tests (for EPM) need to be used in the right way,” emphasized Bernard. “The positive CSF tap is not diagnostic. It can be good to do the tap because if it’s negative, then it’s diagnostic (that the horse doesn’t have EPM).”

A positive spinal tap could be the result of blood conatamination.

Byars added that a spinal tap is not the only tool used for diagnosis. A negative serum or blood sample means that the horse is less likely to have the disease. Byars said that this test is statistically the most correct, however, it can also be misleading. “There is no perfect test,” he said.

Management Is Key

Owners need to understand that diagnosis is difficult, if not impossible, with EPM. A horse can be exposed to the single-celled protozoal parasite that causes the disease, but not have the disease. Most horses are exposed, develop antibodies, and their immune systems fight off the parasite or prevent the disease from causing symptoms. Therefore, management to prevent exposure to the disease is critical.

Management factors you can modify are:

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  Keep feed sealed to prevent contamination.

  
  
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  Remove opossums from the farm and barn or discourage their presence.

  
  
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  If feeding horses outside, do not feed on the ground to avoid contamination.

  
  
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  Reduce or eliminate birds from barns, especially around feed and water

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