There’s a crazy bug out there. It lives in critters that wander the pastures with your horses. In a very few horses this bug gets into the spinal cord, and wreaks all kinds of havoc. What am I talking about? Equine Protozoal Myeloenchepalitis, or EPM. Three letters that strike fear into the hearts of horse people everywhere. This week, let’s talk about why you shouldn’t be scared of a little old E, P, and M, and why, sometimes, you should be. But first here’s a spoiler: your horse most likely doesn’t have EPM. Cats love spoilers. We’re a little bit evil like that. 

Let’s talk Lifecycle

Where does this bug come from? That’s a complicated answer. You see, bugs like the one that causes EPM go through several different life stages inside several different animals on their way to adulthood. I’ve gotten ahead of myself. 

To begin with, there are two main bugs that cause EPM. They are Sarcocystis neurona, and Neospora hughesi. Being a cat, I’m going to start with the easier of the two: Neospora hughesi. Humans don’t know a thing about this one. That’s right. They don’t know what animals it comes from or how it goes through its life cycle. Moving on to Sarcocystis neurona. This guy hangs out in a wide variety of animals during its teenage years. Racoons, skunks, armadillos, sea otters, and, whoa, wait, what?!? Even cats(!!) can help these guys go from babies to teenagers. 

After Sarcocystis neurona reaches teenagehood, it moves onto opossums, and almost definitely some other critters, but humans aren’t sure which ones. From there the adults hang in the opossum GI tract, which sounds like an awful place to live, and spit out eggs which are then spread by opossums pooping. Did you notice I didn’t mention horses anywhere in there? Yeah, that’s because they aren’t supposed to get them. Horses don’t work as a good choice for completing the life cycle. The little guys get to be teenagers, but then can’t leave. I hear you humans sometimes have a similar problem.  The technical term for this is aberrant intermediate host. Kinda like Eastern Equine Encephalitis. Horses aren’t supposed to have this organism living in them. Good job, horses. 

Those troublesome teenagers

Alright, back to teenage EPM organisms. What do they do to cause trouble? When a horse gets infected, these guys head for the brain and spinal cord. When they do, they start to take up space where there already wasn’t much room. This is sounding more and more like human teenagers. That space was used by a nerve that was sending a signal to some body part, or doing something important if it’s inside the brain. This explains why EPM symptoms can vary so wildly! It all depends on where that tiny guy takes up residence. 

The most common symptom of EPM is an area of muscle wasting away before your very eyes. OK, maybe not that fast, but certainly over a few days. Now, there are a bunch of different things that can cause this. EPM does it by taking up the space where the signal is formed on the spinal cord, and squishing it. EPM usually causes those signals to be blocked in a very one-sided way. For example, if the muscle on the forehead is affected, it will be only the right or left one, not both. That little organism would have to line itself up very, very purposefully to hit the signal for both sides of the body. That’s probably tough when you are made up of only one single cell. These bugs can be anywhere in the spinal cord, or, less often, brain, which means any muscle can be affected. To make things even more confusing, they can be affected badly, or mildly. This can make diagnosing EPM a challenge. Whoa, that was an awesome segway into my next section.

Diagnosing EPM

I’m going to start discussing diagnosis with a few statistics. For the entire United States, current best estimates are that 0.14% of the horses get actual EPM. That’s not a lot of horses. The odds are it’s not your horse. For the State of Florida, that’s 539 horses out of the 385,000 horses that live here. Again, a very tiny number. I can hear you humans now…..but my neighbors, cousins, sisters friend had her horse tested and it was positive. More on that in a minute. 

So you’ve called my Docs because you checked with Dr. Google (you know you did) and Dr. Google said EPM. My Docs are going to start with a very thorough physical exam focusing on symmetry. Remember, I talked about how important that was earlier. Then they’re going to ask your horse to do some fancy footwork to make sure your horse knows where his feet are. They ask for backing, spinning in tight circles, and walking with the head up and the head down at the very minimum. Based on that exam, they’re going to decide if they should continue on with testing for EPM. 

There’s only one tried and true way to diagnose EPM, and that’s a spinal tap. This is where a needle is placed into the area around the spinal cord, and some fluid is removed. Yes, it sounds scary. It is a tiny bit scary. However, it is a low risk procedure that can even be done with your horse standing. This is the only test that will tell you if there are organisms in the spinal cord. That’s right. The one and only test. Yes, yes your friend of a friend of a friend had a blood test. There are several available. They all are good if they’re negative, but worthless if they’re positive. No, I am not being a contrary cat. Allow me to explain. 

A negative blood test for EPM means there is no evidence of exposure. This means your horse hasn’t seen an EPM organism in the past 12-18 months minimum, or that your horse was literally just exposed and hasn’t had time to acknowledge the presence of EPM yet. That second scenario is super unlikely. The body mounts an attack in as little as 3 days. The chances that you are seeing symptoms, have gotten an appointment with my Docs, and had a blood test done and three days haven’t passed are pretty miniscule. It’s close to the chance I could walk by a piece of tuna without gobbling it down. So that’s negative. 

A positive test only means your horse has had an EPM organism inside them. In some areas that can be as many as 90% of the horses. Back to that 0.14%. This means that of those 90%, 99.84% of them clear the organism by themselves.  The immune system is impressive like that. It’s how we stay alive. It’s also why a positive blood test doesn’t mean a whole lot. A positive spinal tap, on the other hand, is really, really significant! 

Treating the beast

Well, you’ve lost the horsey lottery, and gotten a positive spinal tap back. What now, you ask? The good news is we have some good drugs to treat these nasty bugs with. The biggest two we use are Marquis and Protazil. Both are a drug that specifically targets the EPM organism and knocks it down. Both are pretty pricey. This means typically my Docs start with one of these drugs then transition to a cheaper drug that does a great job killing these nasties once they’ve been weakened. The most common version they use is one called Rebalance. 

These drugs are usually used for about 3 months total. Then your horse is back to normal, right? Nope. Nerves take a long time to grow back. It’s typically 10-12 months before horses are fully healed, and it’s going to take a whole lot of rehab to get them back there. It’s also important to know that the worse off the horse is when treatment starts, the harder it is to get them back to normal. That’s a kick in the pants right there. This is why it’s important to call my Docs quickly when you think your horse isn’t quite right. EPM, like most things, does best when diagnosed and treated quickly. 

A little mythbusting

I can totally hear you muttering under your breath that you treated your horse for EPM and it got better from whatever it was it was doing. Let’s talk about why you may be right. The drugs we use to treat EPM have what my Docs call an anabolic effect. This is like the steroids those sports guys took, and they got in big huge trouble. They build muscle, reduce inflammation, all kinds of good stuff. Unlike the ones the sports guys took, these drugs only have a smidge of this effect. However, it’s enough to make your horse feel better. And that’s what really happened when you gave EPM medications and your horse felt better. Wouldn’t you rather know what was really bugging them, and treat that instead? A smart cat would. 

I get it. EPM is scary, and complicated, and Dr. Google, and all your friends say it’s everywhere. Rest easy tonight knowing it isn’t everywhere, and chances are good (0.14% good) your horse doesn’t have it. Instead, call my Docs, set up an appointment and talk with them about what’s going on. They’ll help you find the reason, and design a program that gets you back enjoying your horse in no time!

Until next week,
~Tony

P.S. If I haven’t given you enough information here, the humans did an entire podcast episode about EPM, and why your horse probably doesn’t have it. You can find it over on the Podcast Page.