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Equine Health Resource

Equine Protozoal Myeloencephalitis, EPM

Synonyms: Sarcosporidiosis

Equine Protozoal Myeloencephalitis, Myelitis is the most common infectious cause of neurologic disease in horses. It is caused by one of two protozoal parasites (Sarcocystis neurona or Neospora hughesi), that attack the brain and/or spinal cord and damage the nerve tissue.

Sarcocystis neurona has been implicated in the disease for years. Neospora as a causative agent was discovered more recently. Sarcocystis has a complex life cycle involving the opossum as definitive host, so it is more prevalent in areas where opossums live.

Usually, a horse contracts Sarcocystis by ingesting contaminated opossum feces that have been deposited in water, feed, hay and pasture areas.

Signs of this disease are similar to those associated with other spinal cord disease disease and severity can range from mild to severe. Classic signs are wobbliness, weakness and areas of muscle loss. Often, signs appear asymmetrically, and are worse on one side.

A great deal is written about EPM and much can be found on the Internet. However, there is also a great deal not known about the disease. The hardest part of dealing with this disease is establishing a definitive diagnosis. There is a great deal of misdiagnosis surrounding EPM.

There are many limitations and uncertainties associated with this diagnosis. One of the greatest difficulties in diagnosing this disease is separating horses that have been exposed to the disease from those that truly have the disease. In addition, many cases are complicated by unclear or confusing laboratory results.

There currently are several different tests being used. We use the UC Davis IFAT test, which returns a likelihood of infection.

Your vet may lean toward this diagnosis after taking a careful history and performing a careful physical and neurologic exam. Usually the signs are worse in a localized area or on the left or right side. Antibody testing is performed on a blood sample, and cerebrospinal fluid may be taken and analyzed. Radiographs may be taken to rule out other possible diseases.

I Might ObserveRelated Observations

Noticeably Wobbly or Weak
Hind-End Leans or Falls to One Side, One Hind Limb Seems Weak
Stumbling, Seems Uncoordinated Under Saddle
Crab Walking or Uneven Tracking
Leads, Trouble Maintaining or Changing
Obvious Stumbling or Tripping, Even when Not Under Saddle
Disjointed Feeling Under Saddle
Localized Muscle Loss in an Area
Lameness, Generally
Hindquarters Seem to Fall Away or Collapse while Ridden
Sweating, Anywhere on Body in Local Areas or Patches
Knuckling Over or Rolling Over on a Fetlock
Stands with Hind Limbs Crossed
Lying Down & Paddling
Hind Hoof Swings Outward at Walk or Trot
Local Muscle Twitching
Seizures or Convulsions (in Adult)
Dog-Sitting, Sitting on Hindquarters, Forequarters Raised
Struggles to Rise, Gets up with Difficulty
Head Tilted or Cocked to Side
Hind Limb Bows Outward at Hock When Walking
Cannot Swallow, Difficulty Swallowing
Choking or Gagging, Fluid (Clear, Frothy, or Green) Coming Out of Nostrils &/or Mouth
Hind Hooves, Toes Worn Off or Toe Dragging
Abnormal Foot or Limb Placement, at Rest
Urination, Straining or Difficulty
Tail is Limp or Does Not Move
Hypersensitive to Touch on Flanks & Belly
Loss of Shoulder Muscle on Right or Left
Hoof-Limb Contact, Foot Interfering or Overreaching
Stands with Front Limbs Crossed
Change in Personality, Strange Behavior
Incontinence, Urine Dribbling or Dripping
Tail Raised or Held Off to One Side
Straining to Pass Manure (in Adult)
Groove, Dent or Divot in Neck or Shoulder
Hind Limb Swings Inward, Viewed from Behind
Drooping Lip, Face or Muzzle
Front Limbs Spread, Wide Stance
Penis Dropped, Will Not Retract, or Persistent Erection
Leaning Against Stall Wall or Fence
Sweating Excessively
Pregnancy Loss, Mid-Term (6 Weeks to 6 Months)
Eye is Making Abnormal Rapid & Jerky Movements
Irritability, Moodiness, or Aggression Toward People
Pupil, One Larger than Other


  • How severe is the disease process?
  • How long will the course of treatment take and what is the cost?
  • What are the chances of relapse?

    There is no vaccine for EPM, so other preventative measures are very important. Keep opposums, cats, skunks and other animals out of your horse facility, and especially away from your horse feed storage areas.

    Good facilities management to decrease the population of rodents is recommended, including the use of traps and bait. Remove brush piles where rodents like to live. Keep your tack room clean and keep all grain and treats in sturdy trash cans or plastic bins with lids.

    Keep your horse in good health to maintain strong general immunity.

    Author: Doug Thal DVM Dipl. ABVP


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