Equine Protozoal Myeloencephalitis— more commonly known as EPM— is a disease caused by a single-celled organism, found in opossum feces, that effects the brain and spinal cord of the horse. Horses are exposed to the organism when they accidentally ingest opossum feces. This can happen if an opossum comes into contact with a horse’s water and food sources. EPM is not transmissible from one horse to another. Fortunately for the horses whose owners are unable or unwilling to keep them in a hermetically-sealed, opossum-proof chamber, exposure does not equal infection. The organism causes damage to the central nervous system, resulting in a range of neurological symptoms that can progress in a matter of weeks. There are a few risk factors that make an EPM infection more likely to occur, but active infections of EPM are not all that common. Treating a horse for EPM will stop the progression of neurological damage, and many horses will recover pretty well— the extent to which they are able to recover depends on the extent of neurological damage that took place before treatment.
Exposure vs. Infection:
Exposure to EPM, as indicated by a blood test, simply means that your horse’s immune system has seen the EPM-causing organisms (Sarcocystis neurona and, rarely, Neospora hughesi). Luckily, horse’s immune systems are actually really good at fighting off infections of all kinds— including EPM. A horse that has merely been exposed (had an immune response) to the EPM organisms suffers no ill effects, because the organisms have not been allowed to make it to the brain and spinal cord. A blood test only indicates exposure, not an active infection, and a positive (non-zero) result does not necessarily mean that your horse has an active infection. This means that the blood test for EPM can only say “definitely no”, or “maybe yes” about the presence of an active infection.
An active infection is determined by a cerebrospinal fluid (CSF) test. This requires a trip to a referral hospital, because your horse will need to be sedated and likely put in stocks in order to perform the spinal tap. A blood sample will be drawn at the same time, because it’s inevitable for a small amount of blood to “contaminate” the CSF sample while the needle is passing through tissue on its way to the spinal cord. The ratio of antibody production in the blood sample vs the CSF sample will be calculated in order to determine whether or not there is an active infection taking place.
Symptoms
Because the organisms don’t take up residence in the brain and spinal cord in the exact same place every time, EPM can do all sorts of weird things. However, horses with EPM will typically exhibit some form of ataxia (incoordination of muscle movements) and atrophy (muscle wasting). These main symptoms are nearly always asymmetrical, because the organisms aren’t lining themselves up dead-center where the nervous system forms its signals. If the symptoms are occurring on both sides, one side will usually be way worse than the other. Any and all symptoms are best evaluated by your equine veterinarian. A great way to document and track your horse’s behavior and possible symptoms is with your cellphone camera, so you can record any possible decline for use by you and your equine vet. It can be difficult to notice incremental changes when you’re seeing your horse on a daily basis, so taking pictures and videos can be very helpful!
Another distinguishing characteristic of EPM can be the rapid rate of symptomatic progression. EPM horses can develop ataxia and atrophy that gets worse in a matter of days or weeks. While this doesn’t guarantee that it is EPM, it can suggest that EPM should be considered as a possible cause for your horse’s symptoms.
Occurrence and Risk Factors
EPM is a bit of a hot topic right now, but it’s much less common than one might think. In an “EPM Hotspot” area, like the southern-central US, upwards of 90% of horses will test positive for exposure to EPM via a blood test. Of that >90%, only 0.5-1% of those horses will experience an active infection of EPM, as confirmed by a CSF test. That tells us that about 99%-99.95% of exposed horses can fend off an infection on their own.
So, why does that small portion of exposed horses experience an active infection? Immune response— or lack thereof— is key. If a horse’s immune system is compromised, they become more susceptible. Various adverse health events or conditions can weaken the immune system, causing an EPM exposure to become an active infection, but stress has the most notable impact on immune response as it pertains to EPM infections. The two biggest risk factors are being a racing breed and being younger than 5 years old. Racing breeds— Thoroughbreds and Standardbreds, specifically— have a 5 times higher risk of being infected with EPM. It’s believed that those breeds tend to experience more daily stressors due to their lifestyles as racing horses: training regimens and frequent trailering to new locations. As it so happens, they also tend to be younger than 5 when they are being trained and physically conditioned to race. Remember, though, that’s 5 times the standard 0.5% to 1%, so it’s still not especially common, and between 95%-97.5% of those higher-risk horses can fend off an infection on their own.
Treatment
Treatment for EPM is often a 3-month course. Your veterinarian might prescribe one month of the strong dose of medication (typically Marquis or Protazil, which are the most common, but Ponazuril and ReBalance are another two) followed by two months of a weaker dose. Those drugs stop the progression of EPM by killing the organisms responsible, which makes rehabilitation (improvement of neurological grades and muscle wasting) possible in the future.
It should be noted that treatment for EPM— though a bit pricey (low four-figures)— has no adverse effect on horses that do not have an active infection of EPM. This means that you could decide to treat your horse without performing a CSF test to confirm an active infection with no consequences to the health of your horse. EPM medications, as named above, do have anabolic (mild anti-inflammatory/steroidal) effects, so your horse may appear to feel better after a few days of treatment. This doesn’t necessarily mean that they did or didn’t have EPM— it’s just a side effect of the treatment.
A drawback of treating a horse for EPM without confirmation of an active infection via CSF test is the possibility of spending time and money on treating a neurologic horse for a disease that it doesn’t actually have— leaving you, the horse owner, out a good chunk of money and time with no answers, and leaving your horse with another few months of progression of their disease or disorder with further diagnostics and treatment ahead. The CSF test isn’t all that much cheaper than the course of treatment itself, but it does have the ability to provide definitive answers and potentially expedite an accurate diagnosis. Ruling things out can be just as important as ruling things in, especially if they can be caught and treated with medication.
Outlook and Recovery
Horses who were suffering from EPM and have since completed treatment tend to have a pretty good outlook. Rehabilitation from the neurological damage incurred up to that point is dependent upon their neurological grade at the time treatment was started. Through proper conditioning, horses can typically improve by one—maybe two— neurological grades. This takes time and work. Nerves don’t grow back overnight. This process is somewhat akin to a stroke victim learning to walk and talk again; the brain has suffered damage and needs to rewire itself. Your vet will be able to guide you on the next steps with your horse and what they need during and after treatment.
EPM is not the death sentence it used to be. There is a huge amount of information and research newly available to us as horse owners that allows us to make informed decisions about diagnostic, medical, and rehabilitative care. That being said— your vet is your greatest resource when it comes to questions about neurological diseases and disorders, and the procedures to diagnose and treat them.
