EPM Diagnosis Summary  

A diagnosis of EPM is composed of three parts: ruling out other diseases, an exam and laboratory tests.  EPM is difficult to diagnose because the symptoms can occur anywhere in the body, and can mimic the neurological symptoms of several other diseases.  The tests can be performed on blood (serum) collected at the horse’s location by your vet, or on cerebrospinal fluid (CSF) collected at an equine hospital.  All three parts are necessary for a diagnosis of an active infection of EPM.  

Photograph horse's eye  'I'm so blue'
   © EPMhorse.org   I'm so blue
A firm EPM diagnosis is difficult to achieve and may require multiple laboratory tests, farm calls, and perseverance by the owner.  If EPM is suspected early in the disease progression, the symptoms can be very subtle.  Some veterinarians will suggest that the horse be taken to a teaching hospital for a CSF tap.  See the Tests page for detailed information.

The only 100% certain test for EPM is a post-mortem (after death) autopsy.  However, treating a horse for EPM without a firm diagnosis may leave the horse still suffering from a different, but undiagnosed disease.  It will also waste a good deal of your money.

Ruling Out Other Diseases

There are several equine diseases that produce neurological symptoms that mimic EPM.  If the horse has a fever, it is generally not EPM.  The vet will want to know the history of the past few weeks and the symptoms that you have seen.  Depending on the situation, history, location, season, and observations during the exam, some or all of these diseases may be ruled out.

Wobblers is cervical spinal cord compression that affects horses younger than three years old.  It can produce various neurological symptoms that mimic EPM.  A myelogram and the age of the horse can help rule out Wobblers Syndrome.

Equine Herpes Virus 1 (EHV1) is a viral disease that can have neurological symptoms.  The horse generally will show a fever and cough before the neurological symptoms, and the onset is rapid.  A blood test will rule this out. 

Lyme Disease from the bite of an infected deer tick can cause neurological symptoms.  An antibody blood test can (sometimes) confirm or rule out this disease.

West Nile Virus can produce neurological symptoms.  It is carried by mosquitoes, and has a shorter incubation period than EPM.  A January vet call or blood test could rule this out.

Cushings Disease can also have some of the same symptoms of EPM such as lethargy and muscle loss.  The vet can administer one of several drugs, and later collect blood for a test. 

Selenium deficiency can create neurological symptoms.  If your soils are known to be Selenium deficient, your forage and local hay are probably deficient.  A deficiency in the diet can cause neurological symptoms.  Perform a blood test specific to Selenium to determine deficiency, and before supplementing with Selenium.  It is used in minute quantities by the horse, and an overdose of Selenium can also cause symptoms and be toxic. 

Orthopedic lameness issues can be very hard to separate from EPM.  One test often performed is to administer Bute for one week.  If there is no change in the symptoms, it may not be an orthopedic lameness.

EPM Neurological Examination

The neurological exam is performed by your veterinarian to look for, and grade physical symptoms.  The vet will ask you to lead the horse through several different slow maneuvers, designed to show neurological deficits in the limbs.  They will also look for muscle atrophy, abnormal facial movements, sweating, eyesight, and more.  Any deficits are scored on the Mayhew Scale, a way to rate the neurological problems the vet is observing.  Make sure that a complete neurological exam is given, and print out a copy of the Mayhew Scale to follow along during the exam.  This is the time to ask questions, and relay to the veterinarian any changes to normal patterns you have seen, even if they don’t seem related.

Please see the pages on Symptoms and Tests necessary for diagnosis.

December 2011


AAEP Proceedings, Vol 54, 2008


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