EPM in horses

EPM- Part II: Symptoms, Diagnosis and Treatment

  1. EPM – Part I: What is EPM and How Did My Horse Get It?
  2. EPM- Part II: Symptoms, Diagnosis and Treatment

How do I know if my horse has EPM?

Equine protozoal myeloencephalitis, known as EPM, attacks the horse’s central nervous system and causes inflammation and damage to the brain and/or spinal cord. EPM is passed on to the horse when they consume feed contaminated by opossum feces. (See: EPM – Part I: What is EPM and how did my horse get it?) EPM is defined as a progressive, degenerative disease, which means as time passes, the inflammation can become widespread and the damage can increase in severity. Once affected, the function of the tissues in the central nervous system may continue to deteriorate. EPM in horses can be fatal. Unfortunately, the symptoms of EPM can be difficult to distinguish from other diseases, such as wobbler syndrome, Herpesvirus 1, West Nile virus, rabies, or even equine encephalitis, so this makes it hard to diagnose.

Symptoms of EPM in horses

If your horse has been unlucky enough to ingest the sporulated oocysts that carry the immature EPM protozoa which will infect your horse, the protozoa will hitch a ride on the horse’s white blood cells and migrate through the blood-brain barrier where they will target tissues in either the brain or the spinal cord or both. Once in the tissue, these protozoa cause inflammation that damages the cells, causing a loss of function.

A key feature in EPM is that the symptoms are asymmetric – which means that they can be worse on one side of the body. Signs of illness may come on suddenly or progress slowly. Some signs are almost imperceptible at first but progressively get worse. It is more common to see symptoms related to spinal cord involvement than brain damage. Symptoms can range from very mild to very severe.

Indication of spinal cord involvement:

  • Asymmetric or symmetric weakness, and poor coordination or unsteadiness of one, or all limbs.
  • Areas of spontaneous sweating
  • Loss of reflexes
  • Loss of sensation in the skin
  • Obvious muscle atrophy

If the spinal cord is affected farther back by the tail, signs of cauda equina syndrome can develop, such as:

  • Neurological symptoms in the lower body
  • Altered sensation in the saddle area
  • Incontinence (bladder and/or bowel)
  • Lower back pain or sharp pains in the legs

 Indications of brain involvement:

  • Depression
  • Head tilting
  • Facial paralysis

If the cranial nerve nucleus is involved, seizures, visual deficits and behavioral changes are often noted.

Things to be on the lookout for at the walk or during a neurological exam:

  • Asymmetric stride length
  • Beating movements of the eyes
  • Circumduction of the hindleg (swinging it very wide)
  • Floating or marching of the front limbs
  • Head tilting or facial paralysis
  • Pelvic sway
  • Toe dragging

Things to be on the lookout for when riding:

  • Difficulty staying on a specific lead or trouble changing leads
  • Difficulty maintaining balance in a turn
  • Excessively high head carriage
  • Frequent bucking
  • Head tossing

Are some horses more likely to contract EPM than others?

  • Horses ages 1 to 5 years old are at highest risk of contracting EPM.
  • Horses on farms where previous cases of EPM have been diagnosed are more likely to develop the disease.
  • EPM is seen more often in the spring, summer and fall.
  • Horses that are stressed or those with compromised immune systems are more likely to contract the disease.

Testing for EPM in horses:

If you suspect your horse has EPM your veterinarian will conduct a full neurological exam and perform certain tests. Testing both the cerebrospinal fluid (CSF) along with a blood serum sample is best. However, spinal taps can be risky and expensive so in many cases a positive serum IgG test combined with neurological signs and a history consistent with exposure to EPM will serve as a positive diagnosis.

My horse has EPM; now what do I do?

There are several treatment options for your horse. Your veterinarian will work with you to pick the best one for your horse’s situation. The earlier you can start treatment, the better. Severely damaged nerve tissues may not recover.

Marquis® (ponazuril)

Available in a paste form and administered for 28 days. Dosage depends on the horse’s body weight.

  • Sometimes it will be combined with dimethyl sulfoxide (DMSO) to increase bioavailability
  • Although not 100% effective, it has a stated success rate of 70% improvement or resolution

Protazil® (diclazuril)

  • Available in an alfalfa-based pellet and administered for 28 days. Dosage depends on the horse’s body weight.
  • Studies show it is as effective as ponazuril.

ReBalance®  (sulfadiazine and pyrimethamine)

  • Available in an oral suspension. Administered once daily for a minimum of 3 to 6 months. Dosage depends on the horse’s body weight.
  • Effectiveness is thought to be limited, providing only temporary relief. When withdrawn the disease often recurs.
  • Requires supplemental folate, especially in pregnant mares.

Recommended supportive therapies:

Along with the drug of choice, your veterinarian will also recommend other supplements and drugs that will keep your horse comfortable and help him or her recover faster.

Elevate® W.S.* (natural vitamin E)

  • Passes through the blood-brain barrier to impact CSF
  • Dosage is 10,000 IU per day
  • Can be fed at high levels for long periods of time with no adverse effects
  • Builds immune response
  • Supports healing of damaged neurological tissues

* Elevate W.S is the only vitamin E research-proven to cross the blood-brain barrier.


  • To control pain and inflammation as needed.
  • Dosages and length of treatment vary with each individual.


  • To support a robust immune response that helps medications do their job.
  • Dosages and length of treatment vary with each individual.

Will my horse recover?

How well your horse recovers depends a lot on how much neurological damage he has to start with, and how quickly he was treated. Horses that have mild symptoms and are treated early have the best prognosis. In fact, 80% to 90% recover completely. Over all 10% to 20% of horses will relapse. Horses that have mild cases tend to have a lower rate of relapse. If your horse has a severe case of EPM, the prognosis is not as good, but this can vary from case to case.

An ounce of prevention is worth a pound of cure.

At this time there is no vaccine or known preventative available for EPM in horses; however, you can take some precautions to limit your horse’s exposure to infected opossum feces.

  • Don’t inadvertently attract opossums on to your farm.
    • Clean up any grain that is spilled around outside ground feeders.
    • Don’t leave cat or dog food out where an opossum has access to it.
    • Clean up around your bird feeders and don’t leave birdseed where it is accessible.
    • Secure your trash in a critter-proof container.
  • Prevent opossums from gaining access to the areas where you store your hay and concentrates.
  • Restrict your horses from using natural water sources such as ponds or creeks. These are a favorite habitat for opossums and a source of contaminated water.

If you suspect your horse has EPM contact your veterinarian as soon as possible. Together you and your vet can decide on the best course of treatment for your horse.



  • Char Dasta

    Does a horse with EPM become mean like biting people?

    • Karen

      Aggression is not a typical symptom of EPM, but it’s not ruled out. EPM can impact both the brain and spinal cord, so symptoms can vary from case to case. Pain caused by EPM lesions may also cause a horse to become grouchy.

      Change in personality is more closely associated with Lyme disease, and Lyme can present with similar symptoms as EPM.

      In mares, ovarian dysfunction and bladder infections can cause aggressive behavior.

      The best thing to do is to contact your veterinarian and discuss your horse’s personality change with him or her. A full physical exam and lab work will allow your vet to rule out different possibilities, and then make a diagnosis and suggest a treatment.

      Karen Isberg

      • Janna Lindig

        I bought a 21 year old mare in June. The intention was that she would be a grandchild horse. She was ridden in the sale ring and appeared to be well-ridden. She exhibited some strange behaviors, but I assumed it was the result of a change in location, etc. I recently tried to saddle her and ride her, and it was impossible. Within a week, the ferrier who was shoeing her for the 4th time contacted me and described her behavior which resulted in him not being able to complete the job. He warned me against even getting into the stall with her and suggested that she be tested for EPM. The result was 4000. She has not exhibited any muscular issues, but Iam concerned given her changes in behavior and some head tilting that there is an issue with her brain. Are any of the treatment protocols considered to have successful results when the infestation is in the brain?

        • Karen

          The best course of action is to work with your veterinarian to determine the most appropriate treatment for your mare. In cases presenting with neurological signs, we recommend supplementation with the water-soluble natural vitamin E provided by Elevate W.S. liquid (https://www.kppvet.com/elevate-w-s/). It is research-proven to cross the blood-brain barrier into the cerebrospinal fluid, offering antioxidant support directly to the tissues of the spine and brain.

          Antioxidants reduce the level of oxidative stress that can damage tissues and they also support healing in compromised tissues.

          The recommended level of Elevate W.S. (https://www.kppvet.com/elevate-w-s/) oral supplementation for a horse diagnosed with EPM is 5,000 IU (10 cc) to 10,000 IU (20 cc) per day. Work with your veterinarian to determine the correct level of support for your horse.

    • Kim

      Three ours became agresive. Would try bite,buck and kick. It all stoped after treated for EPM

  • Gina Barbeau

    What criteria or indicators did you use to differentiate between mild and severe cases?

    • Karen

      That would be a question for the treating veterinarian. He/She will assign a severity level based on the physical exam and how a horse responds to treatment. Symptoms can vary from case to case depending on where the lesions are located in the horse. Symptoms can also worsen over time. Typically the worse the neurological symptoms, the more severe the case.

  • Julie B

    Hello, I have a horse that I am treating for EPM, I am getting conflicting information on where to continue working my horse after the first 2 weeks of treatment. What is the best protocol for success?

    • Karen

      In general, it is thought that exercise and rehabilitation are good for horses recovering from EPM. It can strengthen muscles and support nerve regeneration, but it needs to be a slow and steady process. Overtaxing a horse can cause further damage.

      Rehabilitation can begin after a horse starts to respond to drug therapy in a positive manner. What kind and how much exercise a horse can do depends on the type and extent of nerve damage that has occurred. It is best to work with your veterinarian to determine what kind of exercise is appropriate for your horse. Horses with severe neurological deficits can be unsafe to ride and work around.

      Here is a link to an excellent article published in The Horse magazine that gives a good overview of how to rehabilitate a horse with EPM. https://thehorse.com/14471/rehabilitating-the-epm-horse/

  • jane harding

    I f a horses 8000 result for EPM does that mean it is a new case or does it still hold true that a horse could have had it for years?
    Is there a way you can get a time table as to when EPM was contracted?

    • Karen

      According to guidance on EPM from the Association of Equine Practitioners (AAEP), horses can be exposed to EPM at any time in their lives. The incubation period is unknown, so determining a timetable would be extremely hard. Do clinical signs always appear directly after exposure? Can the microorganisms lie dormant until an opportune moment arises? These are the questions researchers are asking.

      A positive serum test reveals that the horse has been exposed but does not confirm central nervous system (CNS) disease, regardless of how high the titer is. A positive cerebral spinal fluid (CSF) test is more likely to correlate with an EPM diagnosis. It is recommended that after all other possible causes are ruled out, testing—along with a physical exam to confirm the presence of neurological signs consistent with spinal cord and brain dysfunction—should be used to accurately diagnose EPM.

      The following is the latest information on EPM from the AAEP:*

      Incubation Period: The time from sporocyst ingestion to clinical disease is unknown; the pathogenesis of S. neurona in horses is unclear due to the lack of an experimental model that reliably induces disease.

      Risk Factors: All horses are considered susceptible to EPM, but most horses exposed to the causative organisms never show signs of disease. A horse’s individual immune response and variation in protozoal inoculum (protozoal strain, dose, frequency) likely determine whether infection is controlled or progresses to neurologic disease. Young age (1-5 years old), old age (>13 years), breed (Thoroughbred, Standardbred, and Quarter Horse), and season (spring, summer, fall) have all been identified as risk factors in some studies. Stressful events such as heavy exercise, transport, injury, surgery, and parturition are also thought to increase risk.


  • Greg

    Can epm be passed in the fecal of an intermediate host such as a raccoon? They tend to sneak into the hay mound and soil some of the hay. Im am very careful not to feed that out to the horses however I’m always afraid I might miss one.

    • Karen

      No, racoons don’t spread EPM directly to horses, but they can play a role in the life cycle of the disease.

      Research has revealed that skunks and raccoons are considered intermediate hosts. An intermediate host can harbor Sarcocystis neurona in its tissues. After it dies, if it’s consumed by an opossum it will pass the S. neurona on to the possum, which then infects the horse.

      Regardless, it is a good idea to toss out any hay that is soiled with feces.

  • Dana Wilson

    Is there a way to test hay, water, or other factors in the environment to determine if EPM is present?
    i.e. if you are running a barn, can you periodically test your hay storage to be sure that it is not showing up in the food you are feeding the horses?

    Thank you.

    • Karen

      To the best of my knowledge there is no testing available to determine if hay, grain, or water is contaminated. The most important thing to do is to make your farm as possum-unfriendly as possible so the animals do not nest in your barns and spend a lot of time roaming around your farm. The good news is, less than 1% of the horses exposed to these disease-carrying protozoa develop clinical EPM.

  • Sally Summerall

    My horse tested 500 on EPM blood test in Jan 2019. We tested him in Jan 2020 and he tested 2000. He has been through all of the meds and protocols you list for over a year. He was improving but now fails most EPM tests. No one will tell me what to look for to know its time to let him cross the bridge. Can you give me any advice?

    • Karen

      Dear Sally,

      I am so sorry to hear that your horse is not recovering even after treatment. It is heartbreaking to watch a loved one decline. One of the hardest decisions we have to make as horse owners is when to put our dear friends down. It isn’t always a black and white decision, which makes it even harder. Some horses live long lives with minor disabilities while others continue to decline. In the end, only you can make the final call.

      I have always looked at this as a question of quality of life. Is my horse in chronic pain? Does he have more bad days than good days? Can he participate in normal activities such as grazing in the pasture, interacting with other horses, moving safely around in relative comfort? Is he able to eat and maintain a healthy weight? Does he seem happy or is he depressed? Listen to your horse. Have a frank conversation with your veterinarian about his prognosis and future needs. Your vet can’t tell you what to do, but they can give you information that will help you make the decision.

      I am posting some links to articles that might be helpful.


      • pam

        i am having the same questions with my epm horse

  • Marcia

    My friends horse may have EPM as he tested positive on several of the markers for the protozoa that causes EPM. Are there other diseases that mask as EPM as it just seems harder to catch on the West coast?

    • Karen

      Dear Marcia,

      In general, equine protozoal myeloencephalitis (EPM) is difficult to diagnose because a positive test tells us that a horse has been exposed to the disease, but exposure does not guarantee that a horse will develop the disease and exhibit symptoms. Many horses mount a significant immune response to the protozoa and keep the disease at bay. EPM is considered a disease of opportunity; it waits in the body until conditions allow it to flourish. Often horses that have been stressed or horses that are older and have a compromised immune response develop the disease years after the original exposure.

      Typically, veterinarians consider both test results and physical symptoms when diagnosing EPM. Other diseases do not “mask” EPM but rather EPM mimics other neurological diseases, which makes a definitive diagnosis more problematic. Overall, the incidence of EPM on the west coast is lower because of the smaller possum population, so in some cases what appears to be EPM is in fact some other neurological problem.

      Karen Isberg

  • Elizabeth Harreschou

    If my horse had epm and was treated, but has been left with a lack of downhill skills, is there anything I can do to help it improve?

    • Karen

      Hi Elizabeth,

      EMP impacts each horse differently, depending on the severity and location of the lesions. Work with your vet to evaluate your horse and set up an appropriate rehabilitation program. Some horses recover completely from EPM and others do not.


  • Pauline Aguilera Longano

    Thank you ! Our horse was diagnosed with a very slight positive in October however we treated him aggressively for three months with Marquis. He was given a clean bill of health in January and we geared and back up slowly and is now in prime fit health and looks like a supermodel. However yesterday he stumbled a little bit and his knees buckled and he fell to his chest and got right back up and we didn’t immediate physical test in which he passed that comes tomorrow for full analysis I’m worried about the fact that this might be a relapse although he has shown absolutely no other signs whatsoever 😌

    • Becky

      Hi Pauline,

      We hope your horse continues to have a clean bill of health.


  • scottwayne wootton

    If a horse with EPM is pastured with a horse without EPM is it ok?
    Is EPM contagious to other horses are animals?

    • Becky

      Thanks for asking this question. No, EPM is not contagious. A horse with EPM is considered a “dead-end” host. They do not pass EPM on to other horses or other animals.

    • Marci

      That is interesting because above you say horses on farms that have previous cases of epm are more likely to develop other horses with epm

      • Karen

        Dear Marci,

        EPM is not contagious between horses and a horse cannot pass the disease on to other animals, but it has been noted that some farms have ecosystems or management practices that increase the risk of horses being exposed to the sporulated oocysts passed out by the possum. On those farms, multiple cases are often seen, not because the horses are passing it between themselves, but because the horses have greater exposure to the same risk factors.

        The previous question asked if EMP is contagious between horses; it is not. If the farm in question has a history of EMP, particularly in certain fields, then further investigation may be warranted. Thanks for bringing up this topic. We appreciate the opportunity to provide further clarification.

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