Stacy Bethke, DVM

Equine protozoal myeloencephalitis (EPM) is a disease of horses that affects the brain and spinal cord.  Protozoal refers to the type of organism that causes the disease.  “Myelo” refers to the spinal cord and “encephalitis” refers to an infection/inflammation of the brain. So, EPM is a disease of the brain and/or spinal cord of the horse, which is caused by a protozoan organism. While great strides have been made throughout the last decade in an effort to understand EPM, many questions remain unanswered concerning its cause, progression, occurrence, treatment, and diagnosis.

 What is a Protozoa?

 Protozoa are the smallest and most simplistic members of the animal kingdom.  They are a single-celled organism.  The name of the protozoan organism that causes EPM is Sarcocystis neurona.  The disease was first described in the 1960s.  But it wasn't until the early-1990s that the organism Sarcocystis neurona was isolated from the spinal cord of a horse with signs of EPM. 

 Parasite Life Cycle

 Although much has been learned in recent years about the life cycle of S. neurona, there is still much we do not know.  Sarcocystis neurona has a complex life cycle which includes both definitive and intermediate hosts.  A definitive host is an animal which sheds an infective form of the parasite, S. neurona.  The intermediate host is an animal that does not shed an infective form but provides the parasite a place to live and is required to complete the parasites' life cycle. 

 The opossum is the definitive host of S. neurona.  There may be other definitive hosts for this organism yet to be identified.  The opossum sheds the infective form of the parasite in the feces; this infective form is called a "sporocyst."  The sporocyst is then ingested by the horse.  Once inside the horse, the parasite goes through a maturation process and eventually produces the form of the parasite called "merozoites."  Eventually the merozoites reach the brain and spinal cord of the horse thus causing damage to this vital area and creating the condition called EPM.

 There are several intermediate hosts identified and probably several more yet to be discovered.  The current list includes: skunks, armadillos, raccoons, cats and sea otters.  Since we do not believe the horse is a "normal" host for this parasite, the horse is referred to as an aberrant (abnormal), intermediate host.  This means that the disease cannot be passed from horse-to-horse.

  What does EPM look like?

Since the parasite that causes EPM can attack the brain or spinal cord at multiple locations, the signs and severity of this disease can vary dramatically.  While both the brain and spinal cord can be affected, it is more common for the signs of disease to be associated with damage to the spinal cord. 

 The signs include 3 "A"s: Asymmetry, Ataxia and Atrophy.

 Asymmetry is a term used to describe a symptom that is worse on one side of the body than the other side. 

 Ataxia is a term used to describe incoordination or the inability of the horse to move its legs in a normal balanced manner.  These horses are commonly described by owners as acting "drunk." 

 Atrophy describes a condition where the muscles decrease from their normal size.  With EPM, this results from damage to the nerves that normally control these muscles.  Muscle atrophy is not seen in all cases of EPM.

 So, with this disease we will see horses with incoordination that is worse on one side of the body as compared to the other side.  These horses may or may not develop muscle atrophy.  It is also common to see muscle weakness in these horses.  Some horses may display abnormal gaits, lameness, and loss of sensation along the face, neck or body.  Paralysis of the muscles of the eyes, face or mouth may occur and this will be evident by drooping eyes, ears or lips.  Horses may also demonstrate a head tilt, poor balance and difficulty in swallowing or vocalizing.  On rare occasion even seizures and collapse may occur.  This disease may progress rapidly or very slowly.  Horses generally deteriorate over time but some animals may "level off" or plateau for a period of time only to worsen days, weeks or even months later.  Most horses afflicted with this debilitating disease continue to have a bright, alert attitude.

 How do we diagnose EPM?

 As you can see from the preceding discussion, clinical signs of this disease can vary significantly, and no two horses with EPM will look exactly the same.  For this reason it is difficult to make an accurate diagnosis of this disease.  There are several diseases of the brain and spinal cord of the horse that can look very similar to EPM, especially in the early stages of the disease.  These other diseases include: West Nile Virus, Eastern and Western Encephalitis, and Equine Herpes Virus.  Finally, there is no single "test" that can be done which is 100 percent accurate.  There are multiple tests that can be utilized by your veterinarian, and these are used to support the diagnosis of EPM as well as rule out other diseases.  It is important to remember that we can never be absolutely sure a particular horse has EPM.  So, it is important that you have your horse examined by your veterinarian at the first suspicion of clinical signs because early diagnosis and treatment are essential.

 How Do We Treat EPM?

 Treatment of EPM should only be done under the direct supervision of your veterinarian.  It is important to recognize, diagnose and treat early for the best possible outcome.  Currently, there are three FDA-approved treatments for EPM available.  These are all prescription drugs and can be used only by or on the order of a licensed veterinarian.  The products available include: Marquis (Bayer), Navigator (IDEXX) and Rebalance (IVX).  These products come in a paste or liquid formulation and are given once daily for a minimum of 28 consecutive days.  Veterinarians will also use other supportive drugs while treating EPM.  These treatments can include anti-inflammatory products such as corticosteroids, Bute, Banamine, DMSO and others.  It is also recommended that horses be removed from stressful environments and activities while receiving treatment for EPM.  It is important to remember that successful treatment may eliminate the parasite, but the effects from damage to the central nervous system by the parasite can be permanent.

 What Can I do to Help Prevent or Minimize the Risk of EPM?

 Horse owners can take advantage of some basic husbandry practices and good common sense to help reduce the risk factors that can contribute to this disease.  One thing that all of us can do is to keep our feed storage areas neat and clean.  Untidy areas with spilled feed invites unwanted guest such as opossums.

 - Keep your feed in sealed or closed containers.

- Cover your hay storage area if possible.

- Keep rodents under control on your property.

- Don't leave cat or other pet food out.

- Check with local authorities with respect to trapping and relocating opossums or eliminating them.

- Properly dispose of any animal carcasses that you may see on or near your property.

- Clean your equine water sources on a regular basis.

- Do not feed on the ground.

- When transporting horses, make them as comfortable as possible.  Transportation stress can be a contributing factor for EPM.

 Currently there are no USDA approved vaccines available for use in horses to aid in the prevention of this disease.  Without a vaccine, horse owners must rely on keeping their horses' water and feed from becoming contaminated with opossum feces to prevent this devastating disease. 

 The information for this article was taken from the American Association of Equine Practitioners (AAEP) publications.  For more information on this and other horse health concerns you can visit the AAEP's website www.aaep.org.