EPM is a commonly diagnosed (commonly over-diagnosed, as well, but we'll get into that!) neurologic disease of horses in the US.  Let's break the name down a bit.  "Equine" obviously means it affects horses.  A microscopic parasite (a protozoa) is responsible for the disease, thus the "protozoal" part of the name.  And "myeloencephalitis" describes what happens: the spinal cord ("myelo-") and brain ("-encepha-") become inflamed ("-itis").  There is no breed or gender predilection, as all horses are susceptible.  There's also no specific time of year that transmission is more likely.  

"Could this be EPM?" This is a question we frequently get asked.  To understand the answer, the first thing is to know how the signs are caused.  

Once S. neurona gains access to the central nervous system it begins to travel through the spinal cord and brain.  Clinical signs develop due to inflammation and necrosis (cell death) caused by the tunneling of the organism.  Lesions can be macroscopic (visible to the naked eye) as shown in the spinal cord picture above, or microscopic. Most often the spinal cord and brain stem are affected. 

Given that information, it's easy to understand why the clinical signs are always related to a neurologic deficit.  However, since there' s no single path that the organism takes, the damage in the spinal cord can be quite random, leading to a variety of signs.  Most commonly we see muscle atrophy coinciding with unilateral or bilateral gait abnormalities.  This can sometimes be mistaken for lameness, but is actually a slowly progressive ataxia. 

Occasionally, more severe signs can be seen such as cranial nerve deficits (facial paralysis, dysphagia, or facial muscle atrophy.  These tend to correlate more to lesions in the brain. 

Things we do not see are signs associated with typical illness, such as fever, inappetance, or a dull attitude.   Generally horses with EPM are fairly bright, but may be anxious because they are not sure where they are placing their feet. 

Keep in mind, though, that just because neurologic deficits or signs may be present, does not immediately make a diagnosis of EPM.  There are many other neurologic diseases that we have to watch out for as well.   So that leads us to our next point...

It's easy to suspect EPM but much harder to verify.  The difficulty comes in interpreting titers, and making decisions to treat based on a serum titer, or perform more invasive testing to try to confirm a diagnosis.  We'll try to keep this relatively simple.

ELISA Testing

We can test for the disease in a number of different ways.  There's Western Blot analysis, ELISA and even IFAT!  It would take an entire newsletter to describe all these so let's just suffice it to say that each of these tests looks for specific proteins or concentrations of proteins to confirm exposure.  We currently send our samples to a lab which performs ELISA and looks for SAG2,3,4.  These are outer surface proteins of S. neurona. 

Okay, so, we do a blood test, and it's positive.  Now what?  Unfortunately, the serum just confirms exposure to the protozoa, and doesn't necessarily confirm clinical disease.  To understand why that is, remember that S. neurona is SUPPOSED to be found in the muscle or blood.  Only in horses, it sometimes goes to the spinal cord.  But (perhaps) not always.  So, if there are protozoa burrowed in the muscle, or for some other reason the horse cleared the infection on its own, we'll often still get a positive titer on serum.  Bummer.  It's reported that nearly 50% of horses in our area will test positive for EPM on serum, while only about 1% will actually have clinical disease. 

No, not that Spinal Tap!

So how do we make our diagnosis more sure?  Ideally, we  would do a paired sample of blood and  cerebrospinal fluid.  This means that a spinal tap would be necessary.  Spinal taps can be done under standing sedation in horses, but can be a bit tricky to get a good sample. That's not to say it can't be done, but your vet should go over the specifics with you prior to the procedure.   With a high titer for EPM on a CSF sample, we can be much more sure about our diagnosis. 

Our general recommendation is to run a serum test first - if the serum test is negative, we know that the clinical signs are not due to EPM.  If we have a positive serum test, we have the option of pursuing a spinal tap for a CSF test to confirm, or we can try a course of treatment and monitor for response. 

So, we have our positive serum and/or CSF test, and we're going to pursue treatment.  Given the makeup of  S. neurona, and the fact that treatment must be directed at the central nervous system, medication options are quite limited. 

Historically, the disease has been treated with a combination of antibiotics (sulfadiazine and pyremethamine).  While somewhat effective, treatment was of LONG duration (3 - 9 months) and had side effects including anemia and colitis.  Overall, the success rate with this combination is just over 60%.  Today the product is sold as Re-Balance. 

The gold standard for treatment at this point is ponazuril, the trade name of which is Marquis.  This product is extremely safe as it's mechanism of action is directed at a cellular organelle that is contained in protozoa but not in mammalian cells.  (Pretty cool, huh?)  Studies have been performed to confirm safety with this medication up to 7 times the dose (!) with no side effects seen.  The typical treatment regimen is 30 days but in some instances it is doubled to 60 days.  Marquis comes in a paste with seven doses (for a typically-size horse) per tube.  A similar drug which recently became available is diclazuril, which is marketed as Protazil - this is a related compound to Marquis, but is manufactured as a top-dress pellet which can be added to the feed.  The results with Protazil aren't quite as good as Marquis, but for horses which will absolutely not tolerate a paste medication, it's a good alternative.

Adjunct treatment with neuroprotectants (such as vitamin E) and anti-inflammatory drugs (such as Banamine) are also important, especially at the beginning of treatment with these EPM drugs.  When the medications are begun, there is a slight risk for worsening of symptoms as the drugs work to kill the protozoa within the central nervous system.  These adjunct treatments can help reduce the body's inflammatory reaction to the dying protozoa. 

Please keep in mind that there are several other compounds which are available from compounding pharmacies, which have made claims to be very effective at treating EPM.  Be skeptical.  Be especially skeptical if the same lab who is advertising a test for EPM is ALSO selling a unique treatment.  Remember that MANY horses test positive on serum without actually having clinical EPM, so many of the "positive results" are horses that just needed tincture of time.  Always check with your veterinarian before pursuing testing or treatment for EPM.