Equine Protozoal Myeloencephalitis – The Master of Disguise

By Garrett Metcalf, DVM

Equine Protozoal Myeloencephalitis, or EPM as it is commonly known, is a devastating disease, and not all horse owners are fully aware of its debilitating and sometimes deadly capabilities to the horse. This disease has been known for some time, but it hasn’t been an easy road to figure out this disease. EPM has a wide range of symptoms, clinical signs, severity and manifestations that make it difficult to diagnose. This article will discuss the history of EPM and how it can affect your horse. Also discussed are ways to prevent and treat horses with this disease.

What was described as EPM today was discovered in 1968 and was found to be caused by a protozoa parasite in 1970s, but it wasn’t until 1990s that the causative protozoa was identified. The leading causative protozoa agents of EPM are Sarcocystis neurona and Neospora hughesi, which is found to be in the similar phylum as malaria. These parasites cannot be passed from horse to horse but has to be passed through other small wildlife animals that are commonly found around the farm and horses. The opossum is the most incriminated because it is the definitive host in the life cycle of the parasites, but there are other animals involved in the lifecycle, which are called intermediate hosts. These intermediate hosts are cats, raccoons, armadillos, skunks, and even from harbor seals and sea otters.  The horse is known as the dead end host, hence why they cannot pass these parasites to another horse or other animals.  The parasite is passed in the feces of opossums, which can then contaminate water and food supply of horses leading to the possibility of horses getting EPM disease.  Horses ingest the sporocyts of the parasite, which then enters the blood stream via the gastrointestinal tract. The sporocyts are able to cross the blood brain barrier into the central nervous system where they do their dirty work, which can cause damage very quickly or slowly. The longer the parasite is present, the more long lasting neurologic destruction it causes.

Symptoms of EPM

Unfortunately there are numerous symptoms and manifestations of EPM, which makes it the master of disguise.  A list of symptoms taken from the AAEP (American Associate of Equine Practitioners) website on EPM is listed below.

  • Ataxia (incoordination), spasticity (stiff, stilted movements), abnormal gait or lameness;
  • Incoordination and weakness, which worsens when going up or down slopes or when head is elevated;
  • Muscle atrophy, most noticeable along the topline or in the large muscles of the hindquarters, but can sometimes involve the muscles of the face or front limbs;
  • Paralysis of muscles of the eyes, face or mouth, evident by drooping eyes, ears or lips;
  • Difficulty swallowing;
  • Seizures or collapse;
  • Abnormal sweating;
  • Loss of sensation along the face, neck or body;
  • Head tilt with poor balance; horse may assume a splay-footed stance or lean against stall walls for support.

The symptoms can range from very severe to very subtle, which makes it difficult to diagnose or be noticed by owners until the disease is more advanced.  Factors that are thought to play a role in the progression of the disease include the duration of infection until treatment, stressful events the horse may experience, comorbidities with immunocompromising diseases such as Cushing’s disease, amount of parasite ingested and the area of the central nervous system that the parasite infests.

As mentioned before, EPM plays by its own rules and it is a very difficult to differentiate from other diseases or symptoms. In performance horses that experience EPM, they may have lameness issues, decreased performance and subtle weight loss with muscle loss from this disease. Whenever there are performance issues in a horse, the list of problems can be substantially lengthy, so it is sometimes overlooked that EPM can be the underlying cause of the decreased performance. Delay in finding the cause of the performance issues can be a financial and time consuming burden that owners may face if symptoms of EPM are overlooked or mimic other conditions.

Diagnosis and treatment

A definitive diagnose of EPM is difficult, to say the least. The first step is to have your veterinarian examine the horse for any symptoms of EPM. This includes a physical exam, examination for muscle asymmetry, neurologic exam by a tail pull test and making the horse move in tight circles plus even limb placement tests. If symptoms of EPM are noted further diagnostics are needed such as blood testing for titers to the parasite. Blood tests are difficult to interrupt because blood tests are looking for antibodies to the causative agent, but antibodies in the blood can be caused by exposure and not necessarily mean the horse has EPM. The higher the titer or antibodies in the blood the higher the likelihood the horse does have EPM, of course, coupled with clinical signs or symptoms. The parasite lives in the central nervous system so another method of testing is to collect cerebrospinal fluid (CSF) that bathes the spinal cord and brain. This fluid can help increase the accuracy of testing, but there are more risks when collecting CSF fluid and the risks need to be weighed with the data gained.

As you can see, making a diagnosis of EPM is challenging and very difficult at times but with a skilled veterinarian with experience to interpret the findings of these testing methods and clinical signs, a diagnosis can be achieved. There are other diseases that need to be considered in these cases and also ruled out on a case by case basis because symptoms of EPM can mimic other types of diseases.  It is also important to achieve a diagnosis as quickly as possible to not delay treatment before the disease worsens and injury to the central nervous system is irreversible.

Treatment options are available with good success in many cases, but the sooner the treatment is implemented generally the better the outcome will be. Most widely used treatments of EPM are fairly lengthy courses of antiparasitic or coccidiostat drugs that can cross the blood brain barrier where the parasite lives and destroy it. Some of these drugs have narrow safety margins and need to be administered or prescribed on the order of a veterinarian. Most of these drugs used are off label and are not approved for EPM treatment but are found to be effective in eliminating the disease. There are only two currently available FDA approved treatments for EPM. One is called Marquis paste and the other is Protazil pellets, but off label used drugs seem to be as effective at treating EPM.  One of the first drugs used to treat EMP is still used today and that is a sulfa antibiotic that has antiprotozoal action, but side effects can occur with long term use of this drug. It is important that a veterinarian be involved in the treatment plan and monitor of progress of the treatment to ensure that safe and proper treatment is being administered for the treatment of EPM.


Prevention is a very difficult task to achieve but here is a list of things recommended by AAEP to potentially decrease the risk of EPM:

  • Keep feed rooms and containers closed and sealed.
  • Use feeders, which minimize spillage and are difficult for wild animals to access.
  • Clean up any dropped grain immediately to discourage scavengers.
  • Feed heat-treated cereal grains and extruded feeds since these processes seem to kill the infective sporocysts.
  • Keep water tanks clean and filled with clean, fresh water.
  • Maximize your horse’s health and fitness through proper nutrition, regular exercise and routine deworming and vaccinations.
  • Schedule regular appointments with your equine veterinarian.

Unfortunately there is no end in sight for EPM and it’s going to be a challenge long into the future. This disease is destructive and debilitating to horses with little to no discrimination of breed, age or sex. Also horses that are not diagnosed with EPM and made aware to the owner can be a risk to themselves and any potential handlers or riders. These horses can become very weak and unsafe to be handled yet alone ridden, but don’t be discouraged because many horses are treated successfully and hopefully this will help any horse owners to detect early clinical signs in case their horse becomes ill by EPM. Remember to reach out to your veterinarian if this is suspected in your horses so early diagnosis can be made and treatment implemented as soon as possible.

Read more in the July issue of Oklahoma Farm & Ranch.