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A New Look at EPM


By Peggy Gaboury with Cynthia Weis, DVM DABVP
Equine Protozoal Myeloencephalitis (EPM) is a neurological disease in horses.  It is caused by protozoa, Sarcocystis neurona, that can invade the brain and the spinal cord, affecting nerves that control the horse’s movements. 

The parasite causing the disease is commonly spread by opossums; other possible intermediate hosts include cats, raccoons, armadillos, cowbirds, and skunks.  Horses may pick up the protozoa while eating in places contaminated by the host animal’s feces. The protozoa do not occur in horse manure, so EPM is not spread between horses.

Many horses are exposed, but only a few actually develop the disease.  In most cases, the protozoa will not get past the horse’s natural defenses to penetrate the brain and spinal cord.  For a horse that does develop EPM, symptoms may include stumbling with loss of coordination, a weak or wobbly gait, multi-limb lameness, and an asymmetric gait. If the disease penetrates the brain, which is less common, symptoms may include depression, behavioral changes, and facial nerve paralysis, including inability to chew and head tilting. Severe symptoms may include paralysis or seizure.

This is the EPM with which horse owners are fairly familiar.  It is an EPM with quite severe symptoms, which can lead to paralysis and death in some cases.  It is not a disease with subtle and contradictory signs, and it is not one that can be overlooked.

So, I was very surprised when a horse I had in training was positively diagnosed with EPM.  This horse was an actively competing event horse, who routinely jumped 3’6” courses with no problem, and who galloped across country with great enthusiasm.  She did have an asymmetric gait, with uneven muscle development behind, and rather considerable “sidedness.”  She also suffered from narcolepsy and was fairly depressed; she would buckle at the knees and start to go down, sometimes while standing on the cross ties, and even under saddle if we paused during a lesson to talk with the rider.

I asked my veterinarian, Dr. Cindy Weiss from Mid-South Equine, for a general lameness exam.  After watching the horse go and doing flexion test, she surprised me by asking if she could do a test for EPM. 

A blood sample was sent to Dr. Siobhan Ellison at the University of Florida.  Dr. Ellison has specialized in EPM since completing her doctorate in 1999.  She is looking at EPM as a disease syndrome that includes both the initial infection and inflammatory encephalitis that is consequent to the infection.  She has developed a more specific blood test, which returns fewer false positives than did the older tests, and is working on a new treatment protocol.  In her research program, she is using pre-and post-treatment monitoring of blood antibody levels and inflammation levels, as indicated by presence of C-reactive protein. 

Through that monitoring she has learned that “horses can be re-infected, and that re-infection is consistent with a rise in the anti-body titer before the clinical signs appear.  If a horse had, and is treated for the infection, a second exposure induces a quick antibody response.  An inflammatory response can accompany the immune system response, and the inflammation causes clinical signs.”  In other words, a horse that has had EPM should be monitored by a veterinarian on a regular basis for neurological symptoms and have blood antibody levels checked twice a year.  If anti-body and/or inflammation levels rise, the horse requires palliative treatment to control the inflammation.

In practical terms, the trainer, rider and barn manager who deal with a horse who has had EPM all need to be aware that that horse can be re-infected.  They need to know that any stress to the immune system of a post-EPM horse, including routine vaccinations, can cause a spinal inflammatory response.  A horse who has recovered from EPM who starts to show any clinical signs – for examples, drag the hind toes, falter in the hind-end on a downward transition, fall out more than usual on a circle,  jump a little crooked – may need to be treated quickly for the spinal inflammation before more severe clinical symptoms begin.  Treatment for the inflammation is specific to the inflammation; it is not the same as the initial treatment to eliminate the EPM parasite.

The good news! The horse in my care is now clinically normal.  It has been several months since she has fallen asleep on the cross-ties.  She spooks and bounces around and interacts with her environment as I would expect for a normally healthy and fit horse.  Rarely does she miss a step behind, although we are still doing a lot of cavalleti work to retrain the motor response patterns in the hind end.

She received an initial ten-day treatment to kill the EPM pathogen, and multiple weeks of follow up treatment to control the inflammatory response.

Read more about EPM on the American Association of Equine Practitioners (AAEP) website:
Heather Smith Thomas. “EPM Update,” The Equine Chronicle Online, November 29, 2010.
Cindy Weiss, DVM DABVP
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