The blog series I wrote about EPM early in 2020 remains one of the most-read series I have ever written. Clearly, this equine disease can be devastating to horses and their owners. It can also be difficult to understand exactly what is causing the visible clinical signs. In this post, I will describe some of the early signs of EPM. I would strongly encourage you to follow the link above and read the 4-part series I wrote in 2020. It will provide you with some background information to enable you to better understand what I will describe in this post.
Java, named for his color-changing coat, from black-bay to mocha!
The horse in the feature image of this blog used to belong to me. His name is Java, and for the past 7 years he has resided at the TREE House of Greater St. Louis. Their name has its origins in its mission statement:
Our Mission: Improve the lives of individuals with disabilities and their families through Therapy, Recreation, Education, and Exploration.
The TREE House has an incredible therapeutic riding program. I donated Java to that nonprofit organization in 2015 as I was going through the throes of a divorce. The main reason I found myself in the position of needing a new home for Java was that he was diagnosed with EPM.
Java had always been my “backup” horse. I nearly always rode Kadeen, and the occasional friend or family member rode Java. This was before Alan, when I didn’t have anyone to ride with unless I made it happen myself!
I knew that with all of the uncertainties ahead of me as I divorced and upended my life, I would not be in a position to rehabilitate Java. He would need a steadily increasing rehab routine, as well as a consistent and routine riding schedule. Additionally, he might never be sound enough to traverse the mountains. Yet, he recovered from the acute disease process (thanks to Pathogenes) and was in otherwise good health at the time that I donated him. That is, except for the hole he tore out of his ear a few weeks before he went to his new home!
I have kept in touch with TREE House, and I learned that a couple of years ago, Java developed an intermittent rear limb lameness. I don’t know if it is related to his EPM diagnosis. He is still in the program and he is still a character! His mild lameness means they only use him at a walk, but he’s one of their favorite horses.
Understanding Sarcocystosis vs EPM
Before you can fully understand what to look for in early clinical signs, you need to understand the difference between Sarcocystosis and actual Equine protozoal myeloencephalitis, or EPM. This post (part of the 4-part series) explains some of what I will reiiterate here.
In medical terminology, “itis” means inflammation, whereas “osis” means a degenerative or disease process. Therefore, “sarcocystosis” means a degenerative or disease process related to or caused by a parasite known as Sarcocystis. There are numerous species of Sarcocystis, but the one commonly associated with EPM is Sarcocystis neurona. Again, this blog post will explain this in more depth.
More than 80% of normal horses have antibodies to Sarcocsytis neurona. This means that the majority of horses are exposed to this parasite and develop immunity to it. Most horses get infected, develop immunity, throw off the parasite, and cruise happily through the rest of their lives. During the time they are developing immunity, they have sarcocystosis, meaning they have a disease process related to the organism Sarcocystis. They do not have EPM, they have Sarcocystosis.
EPM is very rare. EPM is defined as parasites in the central nervous system. The VAST majority of horses that come into contact with Sarcocystis neurona do not develop EPM. A horse with EPM will have a wobbly gait and most likely other central nervous system signs. Horses with Sarcocystosis present with different clinical signs. Read on…
Understanding this disease at a deeper level
The information I am sharing comes directly from the Pathogenes website. Siobhan P. Ellison, DVM PhD is the driving force behind the research and advancements made into and about this devastating disease. I reached out to her when Java was diagnosed in 2014. I bucked the trend of my veterinary peers by following her protocol instead of the standard line of thinking. I did so because Dr. Ellison’s belief that inflammation was intimately involved with the disease progression, and her research proving her beliefs, made a lot of sense to me.
It is not possible to predictably cause EPM in horses by orally dosing them with massive amounts of sporocysts obtained from opossums. (Please see this post about the lifecycle of the sporocysts.) That approach creates sarcocystosis in the horses, but not necessarily EPM, as differentiated a few paragraphs ago. Dr. Ellison’s research has established a method that has allowed them to reliably recreate the disease as well as study the difference in presentation between sarcocystosis and EPM. She wrote a blog about it and that is the source of the following information.
Early signs of Sarcocystosis
It is critical that you understand that signs of sarcocystosis appear before any evidence of EPM. Again, many horses will get sarcocystosis, but very few horses will progress to EPM. Sarcocystosis “in the field,” i.e. on your farm or property, starts with your horse ingesting oocysts from a skunk, opossum, or another intermediate host. Your horse mounts an immune response. The parasite is eliminated. However, the inflammatory response doesn’t necessarily stop immediately, and the inflammation is what causes the clinical signs.
Dr. Ellison’s blog post states:
If there is no new exposure, most horses exit this innate (inflammatory) response phase. However, some horses get stuck in a chronic inflammatory cycle. And other horses are chronically exposed from the environment. It is important to know the difference. Serotyping can tell you the difference.
In very rare cases, the parasite is transported to the brain or spinal cord causing encephalitis, this is EPM.
The first signs of sarcocystosis can often be detected about a week after infection. Weakness, often manifesting as stumbling, behavioral changes, laying down more than normal, and a change in tail carriage might be the first evidence of disease. Lip, facial, or eyelid paresis (partial paralysis) or dropping feed might also signal an early infection. Dr. Ellison notes:
These are signs that are seen before the parasite is taken into the brain or spinal cord, before EPM.
Dr. Ellison also notes that many veterinarians in the field do not see those clinical signs, because they are not called to examine the horse until the horse gets wobbly or noticeably lame.
Early signs of EPM
In Dr. Ellison’s research with experimentally infected horses, those that had persistent early signs had EPM by Day 30. Of the 10 horses in this study, five developed EPM, and five did not. The five horses that never developed EPM also never developed severe ataxia or a wobbly gait. She states:
Significantly, some horses with early signs resolved, and the disease in those horses was not as severe. Horses that continued to display early signs were the most severely afflicted by the end of our trials. It was the duration of the inflammatory cycle that predicted clinical outcome.
In the clinical trial, the five horses that developed EPM all showed severe ataxia (wobbly gait or stance) and depression. Muscle wasting and limb paresis were also noted in horses that went on to develop EPM. Let me repeat… the five horses that never developed EPM (the presence of parasites in the central nervous system) also never showed severe ataxia or depression. Let me share with you the summary paragraph from Dr. Ellison’s blog post:
In summary, horses with mild ataxia, toe dragging and any of the other multitudes of signs that prompt the horse owner and the veterinarian to suspect EPM should be tested for antibodies against S. neurona. We’d like you to suspect sarcocystosis first. If the horse is not depressed and not showing any vestibular (balance) signs along with the gait deficits, we are fairly sure you will have a good to great outcome with therapy.
Depression and vestibular or balance issues are evidence of disease in the central nervous system. This is EPM… parasites in the CNS. Sarcocystosis is usually a disease of the peripheral nerves. The latter condition has a much better prognosis than central nervous system disease.
One other point I would like to make here… All ten of these horses were infected with S. neurona in such a way that they were highly likely to develop full-blown EPM, and yet five of them were able to recover from sarcocystosis without developing EPM. Half of them recovered! In the field, the percentage of horses that develop EPM is much smaller than 50%. Remember that!
An article from Horse & Rider about EPM appeared in my Facebook feed
This article showed up in my Facebook feed recently. It was in response to a reader’s query as to whether or not a horse diagnosed with EPM could ever be safely ridden again. Notably, I started this blog telling you about my horse, Java, that has a wonderful life toting special needs riders around the ring.
However, what bothers me greatly about the article in Horse & Rider was the complete absence of any reference to the role that inflammation plays in the pathogenesis of this disease. When that article appeared in my Facebook feed, I made a comment on it. I talked about Pathogenes and the incredible research and knowledge they have contributed to the understanding of this disease, and I linked to my first EPM post referencing Pathogenes. The comment was removed within hours.
As I mentioned at the beginning of this post, my series on EPM is the most read and searched–for topic in my several years of blogging. I have personally experienced the decision-making nightmare of what to do with and for a horse diagnosed with EPM. I am a veterinarian by training who made educating my clients a huge priority. It is for these reasons and others that I am passionate about getting the word out about Pathogenes. Horse owners are not in a position to make an educated, intelligent decision about what to do for their neurologically unsound horse unless they have access to the latest and greatest information.
Don’t expect your field veterinarian to know the latest and greatest information. I have an absolutely awesome equine vet here in Colorado. He has listened to Dr. Ellison speak. He’s a very smart guy… but he had trouble absorbing all she had to offer. Nevertheless, he was motivated to learn the latest and greatest information, and he knows how to reach out to her if he has a case that requires clarification or testing. If you or someone you know has a horse suspected of having EPM, reach out to Pathogenes. What do you have to lose?
Not only that, but check out this link for a chance to win an Amazon gift card and potentially get your horse enrolled in a field study if you suspect he has EPM or Polyneuritis Equi. The blog link states that the program has ended, but Dr. Ellison tells me it has not… so check it out!