EPM.

Three letters that get humans in such a tizzy. It seems these letters carry some special meaning. Since curiosity is in my nature, I launched an investigation. Moral of the story: your horse probably doesn’t have EPM, but it might. Read on to find out more from this intrepid cat.

It’s Everywhere

For large parts of the United States, the organisms that cause Equine Protozoal Myeloencephalitis (now you know why they abbreviate it EPM) are pretty common. There are two main bugs responsible: Sarcocystis neurona, and Neospora hughesi. To be honest, there are probably a couple more organisms, but they are super rare. S. neurona, and N. hughesi live a complicated lifestyle involving poop, possums, birds, and possibly some other animals. The short version is that if your horse eats hay, grain, or grass, they’re likely exposed to the organism on a regular basis.

But it’s OK!

I mildly apologize if that last paragraph sent you into a panic. Just because your horse has been exposed to EPM doesn’t mean they’re going to get it. First off, the overwhelming vast majority of horses, which is 99% or higher, can fend off EPM all by themselves. To say it another way, less than 1% of horses exposed to EPM actually contract it. Their immune system does a really great job with it. If you want to learn more about this part of equine immune systems and EPM, check out the EPM podcast that’s coming out this week on Straight from the Horse Doctor’s Mouth. It’s a lot easier to digest when you listen to Dr. Lacher explain it.

The second big reason your horse isn’t likely to get it, even when exposed all the time, is that the organism has to not only get into your horse, it has to get into the central nervous system before it can cause a problem. The central nervous system is rather like Fort Knox. There’s security, and then there’s Central Nervous System security. It’s a whole different level of tough to get past! The >1% of horses who actually get EPM have an immune system (the security system of the body) that doesn’t do a great job recognizing EPM as a bad guy. These immune systems have a blind spot for the organism. Fortunately the immune system is unbelievably complicated, so it’s an uncommon blind spot.

But the EPM test was positive

Testing for EPM is an excellent illustration of things that are even more complicated than human behavior, and I find that pretty complicated. There are a few different ways to run tests for EPM. My Docs commonly start with a screening type test. Blood is pulled, and then sent to a lab in Kentucky. There they check for some markers of the EPM organisms. If that test is negative, it’s a pretty good bet your horse doesn’t have EPM. There’s a tiny chance your horse could have EPM, but if so, it’s really, really early in the process and the immune system hasn’t gotten around to making the markers that are being checked.

Now that we know what negative means, let’s talk positive. A low positive likely means your horse has been exposed to the organism, but has fended off that attack and everything is going to be alright (this is the 99% of horses we talked about). A mid-range positive means the organism is in attack mode, and without further investigation we really don’t know how the battle is going. A high positive means we really need to move on to the next phase of testing to get a handle on the battle!

Yay! More tests!

…said no human ever. But it’s really important to follow up screening blood tests to find out if your horse actually has EPM. This is done by getting a sample of cerebrospinal fluid (CSF). CSF taps are a relatively simple procedure, but can have some serious complications. We find they are best done in stocks, or with the horse fully anesthetized. This fluid is then submitted to the same lab as the blood. They do the same testing as is done on the blood. These results usually give my Docs a clear answer about the presence or absence of EPM. Since there’s a horse involved, sometimes (but rarely) the answer isn’t so clear.

Important Things to Know

Every single horse suspected of having EPM should have a very thorough neurologic exam. During this exam, my Docs circle the horse tightly, pull on the tail, and do a wide variety of other weird things trying to determine if this horse knows where all its body parts are. The muscles are also VERY closely inspected. Muscle asymmetry is a common sign of EPM. Muscle asymmetry can also be a sign of a lameness, arthritis, and about fifty other things too, so don’t jump to conclusions!

Lots of people say, “But my horse got better on medication.” Yep. He sure did. That’s because every one of the medications used to treat EPM are also anti-inflammatory drugs, just like bute and Banamine. This means horses with some arthritis or low-level pain will get better on these medications from this “side-effect.” And they’ll get worse again when they go off these medications. That doesn’t mean they are having a relapse.

Speaking of relapse: They are pretty darn rare with EPM. Again, not saying they don’t happen, but it isn’t very common. Remember how I said the immune system is complicated? It’s also a very good learner. An infection with EPM is often enough to trigger it to fix that blind spot.

Whew. You can now cross off one thing to worry about!

Until next week ~

Tony

Want even more wisdom? Check out Straight From the Horse Doctor’s Mouth. It’s a podcast with Dr. Lacher about everything horse and veterinary. Sure, it’s missing my sarcasm, but it’s quality programming nonetheless.

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