EPM Symptoms Summary  

EPM can cause lesions anywhere in the brain or central nervous system (CNS). That means symptoms or deficits can show up anywhere in the head or body. The lesions can cause temporary inflammation and permanent nerve damage. Symptoms that are a result of inflammation may get worse during treatment, and then disappear faster after the protozoa are killed. Symptoms that are a result of permanent nerve damage may never go away. The horse may learn to compensate for the loss of feeling, or may generate new nerve pathways around the damaged cells.

Photograph walking horse
   © EPMhorse.org   Drifter
Not all researchers agree on the incubation period; however, clinical trials and horse shipments to continents without EPM, have suggested it takes from  weeks to two years for symptoms to appear after infection by S. neurona. A very rapid onset is occasionally possible, with severe symptoms occurring quickly.  Rapid onset is thought to occur with ingesting large amounts of protozoa.  The longer the disease is left untreated, the more numerous symptoms become, and the worse they will be. The earlier these symptoms are identified, a diagnosis obtained, and treatment begun, the better the chance of a substantial recovery. 

The long list of possible symptoms can make EPM very hard to diagnose. You should not suspect EPM from a single symptom like lameness, rather from a pattern of symptoms that are not explained by other diagnoses. Many times the owner will recall possible symptoms that occurred months before a diagnosis, but did not recognize them as a pattern.

Important note:  A published study indicates that subtle cranial symptoms occured first IN ALL CASES of the experimentally infected horses.  Dropped feed, facial paralysis, drooping ear, or a lack of tongue tone occurred in each horse studied, before ataxia was apparent.

Detecting Subtle EPM Symptoms

The best way to detect subtle symptoms early in the disease is to know your horse well. Learn the way that the healthy horse eats, walks out to the pasture, behaves, poops, and moves under saddle and free. Changes to the norm should be noted and watched. If you board your horse and only ride occasionally, the barn staff may know your horse’s off-time behavior better than you do. If they mention that something doesn’t seem right, pay attention: the earliest symptoms may not be the ones felt under saddle. There are many possible symptoms; you are looking for changes to the norm. A fever accompanying the symptoms generally means it is not EPM.

Common EPM Symptoms of the Body

  • A very common symptom reported for EPM is uncoordinated movement of the rear feet, worse on one side (asymmetrical ataxia). This symptom is often worse moving uphill or downhill, and while stopping or raising the head.

  • Lameness issues that come and go, often switching sides

  • Changes to any gait, lethargy

  • Hind end weakness - worse on ground that slopes left to right or front to back

  • Problems balancing when a hoof is lifted

  • Circling, slipping, or falling while walking

  • Muscle atrophy, often over the rump or shoulders

  • Leaning on a stall wall for balance

  • Standing with a hoof cocked out or in, not standing square

  • Dragging a hoof, especially while turning

  • A sore back, changes in the fit of the usual saddle

  • Unusual sweating patterns or times

  • Carrying the tail to one side, or away from the body

  • Lack of sensation or heightened sensitivity in skin or hooves

  • Falling in the herd dominance order

Common EPM Symptoms of the Head

  • A drooping lip or repeated facial twitch

  • Change in vision

  • Dropped feed or trouble swallowing

  • Head tilt

  • Drooping ear

  • Changes in behavior, including throwing the rider

Less Common EPM Symptoms

  • Geldings that drop other than when urinating

  • Extreme lethargy

  • Changes to manure (Sn toxin may cause diarrhea)


"Early Signs of Equine Protozoal Myeloencephalitis", Ellison, S. et. al.


January 2012


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