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Introduction to testing for EPM

Equine Protozoal Myeloencephalitis has many presentations.  Horses can be ataxic, lame, or stumble.  Some horses have behavior changes.  And some horses may have trouble eating (dysphagia), show abnormal airway function, or even have seizures. Vital signs are usually normal and can have muscle atrophy.  A horse with suspect EPM may or may not have active protozoal infection.  It is likely the clinical signs are due to inflammation, a part of the disease syndrome.  Some horses may look like they get repeat infections, although this can be due to chronic inflammation.

The first “test” is done by a veterinarian, it is the neurologic examination.  Signs can include gait deficits, we call the exam a GAIT ASSESSMENT SCORE, GAS, the horse has a number value between 0 to 5.  Clinical signs can include behavior issues or cranial nerve deficits without an abnormal gait.  These horses receive a GAS of 1.

The next tests analyze for antibodies in serum and/or CSF. ELISA Submission Form EPM Chart

There are several antibody tests, pathogen specific tests are the SAG 1, 5, 6 ELISA.  These proteins define a serotype of the S. neurona that infected the horse. Horses can have multiple exposures.  A positive test is suggestive that the signs are due to S. neurona. The tests can be used to test antibodies present cerebrospinal fluid (CSF). We produced a video for a standing procedure for CSF collection, an experienced veterinarian can collect the fluid in a matter of minutes.

An important part of the analysis of serum is the level of C-reactive protein.  This is a measure of the inflammation, An elevated CRP that doesn’t decrease after treatment indicates a chronic condition.

A response to treatment is often used by a field veterinarian to help diagnose EPM.  If a horse has not been treated in the last 90 days it is considered untreated.  If there are protozoa present an antiprotozoal treatment is indicated. If there is no response to treatment in 14 days the treatment is considered a failure. A partial response to treatment may mean the horse has post-treatment EPM syndrome.

There are additional tests to run if a horse fails on treatment.  A serum test for S. fayeri is appropriate, especially if there is muscle wasting or loss.  A significantly elevated CRP value (>39) may indicate polyneuritis.  The MP2/MPP test can determine if there are antibodies against myelin protein.  This is an important distinction because treatment is different than those used for horses with EPM. If there is a partial response to treatment a change in CRP is a good indicator of treatment effect.

Lyme disease is also considered when horses are exposed to ticks that carry Borrelia.  Our screening test is used to determine if Lyme should be considered in horses entering the field trial. If the screen test is 40, positive, confirmatory testing or treatment is indicated.

A horse with a diagnosis of chronic EPM, also known as relapsing/remitting EPM, should be tested for MP2/MPP and CRP.  The CRP value is an indicator of sub-clinical disease and can be used in evaluating the end of treatment in autoimmune polyneuritis.

Here is a link to our testing options.  Pathogenes Testing Options