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Equine Protozoal Myeloencephalitis (EPM) in the Horse

EPM or Equine Protozoal Myeloencephalitis has plagued horse owners for decades with an estimated clinical prevalence of less than 5% of horses being affected.  Despite this being a low number, relatively speaking, impacting less than 5 out of 100 horses, the impact when it does happen can be devastating to the average owner.  The costs can be huge in regards to diagnostic expenses, therapy costs and overall loss of use and performance issues.  The incidence of EPM appears to be increasing at times, which is not uncommon as overall health is impaired on a higher level.  This can be frustrating, but if we take a step back and look at ‘why’ this may be happening, it can open doors for prevention and newer therapy options for the horse.

Equine Protozoal Myeloencephalitis (EPM) in the Horse
Equine Protozoal Myeloencephalitis (EPM) in the Horse

When I was in veterinary college, EPM was first being clinically recognized and treated.  In those days, we saw the worst of the cases that were admitted to the teaching hospital, and many of these horses were recumbent and unable to stand or even turn themselves.  As students on the late night shift, we were often given the task of hooking those horses up to overhead hoists, lifting and turning them to reduce bed sores and muscle wasting.  It seemed futile in many cases, as many of those horses could not eat due to not being able to chew their food or swallow.  This contributed heavily to the overall poor recovery in those situations.

In those days, we didn’t have the fancy therapeutic options that are available today, but most of what we did was supportive care in combination with standard antibiotic/anti-protozoal therapies.  The recovery rate was not good and this was due not only to our therapies available but also the severity of disease progression in those horses.  The truth was that there was too much neurological damage done due to the infection and the therapies were not eliminating the parasite completely.  Despite having newer therapy options today, for many horses the parasite is still not completely eliminated and many deal with high recurrence rates.  In truth, our therapy options were only targeting the invader and doing very little to support the body, helping it to recover and repair on a cellular level.

Let’s look at EPM a little closer and see what we can discover.

EPM Causes and Prevalence in the Horse

EPM or equine protozoal myeloencephalitis in causes by a parasite called Sarcocystis neurona, which gains access to the body and then targets the peripheral or central nervous system in the horse.  This invasion by the parasite creates an inflammatory reaction locally, which then impairs nerve function and creates many of the clinical signs.  The parasite is thought to be transmitted by the opossum primarily, but barn cats and various birds have also been implicated, with the horse inevitably being exposed while grazing on pasture or through contaminated feed.  The parasite is taken in by mouth, enters the digestive tract and then gains access to the horse’s body and nervous system.  The horse is considered to be an aberrant host and once infected, they are not at risk of spreading the parasite to other horses.

In my practice, the exact prevalence of EPM in our patients was less than 5%, but may be higher in other regions of the country.  Blood testing of horses has indicated a positive antibody titer in up to 80% of horses, but the majority of these do not demonstrate clinical signs.  A positive serum Western Blot simply implies exposure to the parasite and does not define a clinical infection.  It seems as if the prevalence of EPM has declined in my area over the past few years, but this may be due to lack of clinical recognition and testing.

Clinical Signs of EPM in the Horse

When I was a veterinary student, we saw the worst of the worst when it came to EPM cases.  Upon graduation, we were misled into thinking that they always presented this way, being recumbent or highly unstable.  With more clinical experience under my belt, I learned that there were many clinical presentations for EPM in the horse, with the vast majority being more mild in regards to severity.

In many cases, EPM would present as an unexplained or ‘shifting’ lameness in the horse.  The lameness may be present in one limb one day and then appear to shift to another the next.  If I was presented a lameness that I could not isolate or pinpoint, I would screen for EPM.  The clinical examination for EPM not only involves a full physical examination, but a lameness and neurological evaluation as well.

Peripheral and cranial nerves are evaluated for deficits, which may help us to localize the disease.  Limbs may be flexed and evaluated for lameness, but they will also be evaluated for conscious proprioception (CP) by crossing the legs and for overall signs of muscle loss and weakness.  In most cases, when the limbs are crossed, the horse quickly restores them back to a normal position.  Those horses with neurological impairments may have a delayed or absent response.  Tail and anal tone are also evaluated, in addition to the ‘tail pull’ test to determine if there is muscle weakness present.  The parasitic infection generally targets nerves and with this, we can have many types of presentations with varying degrees of severity.  The condition is progressive in most cases, with time being of the essence in terms of best outcomes with treatment.

The most common types of presentations for EPM in the horse would involve one or more of the following:

  • Unexplained lameness
  • Muscle weakness or atrophy
  • Poor performance or intolerance
  • Ataxia (uncoordination)
  • Stumbling or tripping
  • Recumbency or inability to stand or rise
  • Difficult eating or swallowing (recurrent chokes)
  • Head tilt
  • Behavioral changes

Diagnosis of EPM in the Horse

The diagnosis of EPM can be tricky, but in most cases we base the diagnosis upon the combination of clinical signs in addition to other laboratory data.  In our patients, if there is a suspicion of EPM, we would submit a blood antibody titer (Western Blot) to rule in or out the disease.  A positive test is not definitive, but a negative test would lower our suspicion of EPM.  If the blood test was positive, which would equate to a history of exposure to the parasite, often the next step would be further serum testing or a spinal tap for confirmative testing.  In more recent years, technology has allowed for more refined antibody testing of serum in order to obtain specific titers, which may help to differentiate exposure and actual clinical disease in the horse with EPM.

Outside of EPM in the horse, there are other differential diagnoses which must be considered, especially if neurological involvement is noted.  These include:

  • Toxicity or toxin exposure
  • Herpes viral infection
  • Eastern/Western/Venezuelan encephalitis
  • West Nile Virus
  • Rabies
  • Equine Degenerative Myelopathy
  • Equine Lower Motor Neuron Disease
  • Cervical Vertebral Stenosis (Wobbler)
  • PSSM

Treatment Options for EPM

For most of my early years in clinical practice, we used a combination of antibiotics which demonstrated ‘anti-protozoal’ properties in the horse with EPM.  These two antibiotics were sulfatrimethoprim with rifampin.  This combination was fairly successful in the more mild cases, but required many months of therapy.  Over the years, we have seen some improvements in therapy, which includes medications such as diclazuril, toltrazuril, ponazuril and NTZ, which have all shown promise and have reduced the time for therapy.  Some of these medications have been removed from the market in more recent years for various reasons.

Results from therapy can be variable with noted improvement in less than 75% of cases and a full recovery in less than 25%.  In some cases, the horse would actually get worse during therapy for a brief period due to the parasite being killed off and eliciting an inflammatory type of reaction.  In many cases, the horse would recovery but continue to have neurological issues or deficits, due to nerve damage, which could result in loss of use, decreased performance, and in some, euthanasia.

One of the biggest problems when treating an EPM horse is relapses, which can be quite high.  In many situations, horses would appear to recover, only to demonstrate clinical signs again within the next 6-12 months.  This is not only frustrating but expensive for the owner as it often entailed another course of medication.

Other supportive therapies which are used include non-steroidal anti-inflammatories, corticosteroids, DMSO and antioxidants, with vitamin E being commonly used.

Thoughts on EPM Therapy and Prevention in the Horse

Many years back, an astute veterinarian raised the question as to how the parasite was gaining access to the systemic circulation in the horse.  After all, most bacteria and parasites taken in by mouth are killed off by high acid levels in the stomach or by other factors in the digestive tract.  In order for the parasite to gain access, there would have to be a breakdown in the gut barrier in some shape or form. This just seems to be logical.  In order for that infection to occur, the horse has to be ‘compromised’ to some degree, which makes him more susceptible.  This is confirmed when we look at the true infection rate, being less than 5 out of 100 horses.  Other horses demonstrate ‘exposure’ but are not clinically sick.  So, what’s the difference?  There in lies the key to prevention and therapy for EPM in the horse.

Upon further exploration of this question or theory, it was proposed years ago that many of these clinically impacted horses had gastric ulcers, which would be a breakdown of sorts in the lining and allow for penetration of the organism.  There are many theories out there, but this ‘ulcer theory’ opens the door to many questions and a line of thinking that I could strongly agree with overall.  If 8 out of 10 horses have been clinically exposed, but only 1-2 of them are clinically sick, what is the difference? What makes those 1 or 2 horses more susceptible?

When we look at the ‘ulcer theory’ it raises questions regarding the impact of stress and diet upon the horse, impacting not just the immune response but also digestive health.  The body creates many barriers to prevent invasion by bacteria, viruses and other parasites, but the immune response is ultimately responsible for detecting and eliminating invaders into the body.  So, aside from a barrier problem in the horse, one also has to question the health of their immune response.  A deficient immune response can be linked to genetics, diet, stress, and many other factors.  It has been noted that stress and immune compromise increase the susceptibility of a horse to contracting many other disease, such as herpes and influenza, so why not EPM?  The incidence of EPM is also higher in competitive horses, which may be associated with increased levels of stress.

The immune response, along with inflammation, appear to be a key area in the EPM horse.  In all of our cases of EPM, I have sent in bloodwork, which included a CBC or complete blood count.  In this lab test, what we are looking is the lymphocyte count, which are a type of white blood cell. In many cases of EPM, they have lowered or reduced levels of lymphocytes upon presentation.  Those horses with lowered lymphocyte counts also have a higher incidence of relapse.  It is not the most reliable means of assessing the immune system, but measuring a lymphocyte count can certainly gives us some insights as it is impacted by stress and cortisol.

In those horses with a lowered lymphocyte count, we would often add some supplement to enhance the immune response and by doing so, our relapse rate decreased.  Initially, we would use a dewormer called Levamisole to enhance the immune response, which worked in some but seemed rather incomplete.  Then, over the years, through reading research, we began to utilize mushrooms in the horse as a supplement.  This led to the Cur-OST EQ Immune Full Spectrum formula, which is what we used in our EPM patients along with standard therapies.  The horses did much better and recovered more completely with much fewer relapses.  In some horses, no other therapy was utilized in their recovery aside from the mushroom blend, mainly due to financial limitations by the owner, and the horses still did well.  How could that be?

As mentioned before, the immune response is key to helping the body of the horse to eliminate invaders, but that immune response is at the mercy of other factors including diet, digestive health and the process of inflammation.  Inflammation was obvious in most, as this cellular event was what was contributing heavily to the clinical signs in the horse.  It wasn’t pain, but inflammatory cellular damage at the nerve level which was evident.

Now, in most of these horses that developed EPM, there was a comoribidity or pre-existing condition. This may have been a lameness or other health problem, including ulcers, which opened up the door to increased susceptibility to EPM.  In those horses, if you reviewed the history, inflammation was always present at some level and in a high percentage, there were underlying digestive issues from ulcers to IBD or simply poor digestion.  Most EPM horses have a disturbance or imbalance in their digestive microbiome at the time of EPM infection, which likely was present before the condition was diagnosed.  This is what is noted in our laboratory after hundreds of fecal culturesIf this microbiome imbalance is present, it would explain the ongoing inflammatory events and immune related problems.

Given this involvement of inflammation and digestive health with the poor immune response, it makes sense to address these as they could be the main contributors to the increased susceptibility.  If they opened the door for the infection, then supporting and rebuilding those defenses could close the door as well.  If you just treat a horse with medications to impact the parasite, without correcting the underlying issues, the recovery may never happen or be incomplete.  In our equine patients, we now utilize an approach to both manage the inflammatory events and digestive health, along with the immune response.  When we do this, our recoveries are much quicker and more complete in the EPM horse.

At this time, we tend to put the horses into one of two categories for supportive care.  Either they are an easy-keeper or a harder-keeper type.  If they are an easy-keeper type, we will utilize the Cur-OST EQ Total Support along with the EQ Immune formula.  If they are a harder-keeper, we will utilize the Cur-OST EQ Plus along with the EQ Immune formula.  In most we will also monitor a fecal culture, keeping tabs on the digestive microbiome and balance.  All horses have their diets modified and in many this is enough along with the above supplements to balance out the digestive system, but others require additional support.

What we have seen in our EPM cases over the years has been confirmed by many horse owners across the United States.  When you address the inflammatory and digestive events, the recoveries from EPM are much more complete, even in cases of relapse.  The results are directly due to enhancement of health and host defenses, rebuilding the body so that it can repair and defend itself.  This is taking therapy for EPM to a new level and looking at the body as a whole, rather than just seeing the parasite.

Prevention of EPM from my point of view is rather straight forward.  We can make attempts to ‘clean up’ the environment and make it less attractive to the opossum, but it is next to impossible to eliminate the parasite completely.  We can, however, enhance overall health in these patients by improving their immune function, digestive health, and managing the inflammation.  Factors such as stress, lifestyle, diet, genetics and many others directly contribute to inflammation and digestive health, which then can impact immune function.  It becomes obvious in many EPM cases that inflammation was a pre-existing problem as many of these horses have other concurrent issues such as lameness, poor performance, laminitis, COPD and even insulin resistance.  We need to look at these horses as a whole to get a better understanding of how poor health influences the prevalence of not just EPM, but all diseases.

When we stop looking to pharmaceutical medications as being the final answer and start looking at the horse as a whole, including their lifestyle and diet, then our success rates in EPM cases will be much better.


Author:  Tom Schell, D.V.M., CVCH, CHN


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