This is the fourth installment of a blog series discussing neurological disorders in horses. Part One discussed Sarcosystosis, which is an infection with the protozoa Sarcocystis neurona. The protozoa most often associated with Equine Protozoal Myeloencephalitis or EPM is S. neurona. In Part Two, I shared the expertise of Siobhan P. Ellison, DVM PhD. Dr. Ellison believes inflammation is the underlying cause of debilitating conditions in horses that are often misdiagnosed as EPM. Dr. Ellison states:
Autoimmune polyneuritis is a disease that causes weakness and ataxia (the loss of full control of bodily movements) in horses. It is related to horses diagnosed and treated for EPM. It is not a new disease but it hasn’t been in the EPM-discussion.
Part Three provided some critical bullet point takeaways about EPM.
Polyneuritis equi, or PE, is an inflammation of nerve roots and/or peripheral nerves. The inflammation results in the demyelination of nerves. Myelin is the protective cover around nerves. Demyelination means the myelin has been destroyed. Remyelination is the restoration of the protective sheath. Horses with PE show evidence of both processes. An example of a demyelinating disease in people is Multiple sclerosis.
Without their protective covering, nerves don’t work normally. PE is a description of a syndrome or collection of symptoms. To clarify, it is not a disease diagnosis in and of itself. There are likely multiple causes for this disease process. One probable cause is EPM.
What triggers the demyelination? In fact, scientists are not sure. However, in horses, it is believed that an overactive immune system is the cause. A horse infected with S. neurona (the causative agent of EPM) or Borrelia (the causative agent of Lyme disease) creates antibodies against the intruder. Now stick with me as I explain a couple of technical terms…
The Immune System
A cytokine is a general term for a substance produced by some of the cells in the immune system. Cytokines have varying effects on other cells in the body. There are many different types of cytokines. Researchers have identified IL6 as one cytokine involved in PE. A parasitic invasion stimulates the release of IL6. Usually, the destruction of the intruder signals the “turn-off” of the immune system. However, horses with PE seem to be “stuck” in a cycle of IL6 production. In turn, IL6 triggers the release of C-Reactive Protein (CRP) that was discussed in Part Two. These two agents of inflammation feed off of each other and escalate the problem.
Furthermore, myelin sheaths have IL6 receptors! This means that too much IL6 can create an inappropriate assault on normal body tissues, which in this case is the nerves. In other words, an overactive immune response can trigger an “autoimmune” disease. “Auto” means self, therefore “autoimmune” is an attack on one’s own tissues. What is the “takeaway” from this information?
Dr. Ellison’s Views
In her research, Dr. Ellison has shown that horses treated for abnormal IL6 reactions show significant improvement or completely recover from their symptoms. Treatment would involve a medication that temporarily shuts down the immune system. Untreated horses get worse. Sadly, a horse suffering from an out-of-control immune system for a long period of time sustains damage that can’t be reversed.
The immune system reacts to stress. Thus, a horse can have waxing and waning of symptoms as they experience varying degrees of stress. Veterinarians might interpret this improvement/decline cycle to be evidence of more protozoal infection. As a result, their response might be to treat the horse once again with an anti-protozoal drug. This would have no impact on the actual cause of the problem!
As previously mentioned, CRP levels change in response to the activity of the immune system. Therefore, an owner or veterinarian wanting to measure the effectiveness of an immunosuppressive treatment can monitor CRP levels.
I realize this has been a significant amount of technical information! Nevertheless, I believe it is important for horse owners to become more educated about what is happening inside their horse’s body. I will continue to write about this topic and do my best to explain the technicalities. In the meantime, if you are dealing with a horse diagnosed with EPM, I can’t encourage you enough to reach out to Dr. Ellison. Her lab and staff are extremely receptive to inquiries. Additionally, don’t hesitate to reach out to me if you need further clarification on this article.
In a recent phone conversation with Dr. Ellison, I learned that she is launching a research project on PE. She is looking for horses to enroll in a clinical trial. Do you have a horse that dribbles urine? A horse that seems to have lost the feeling or use of its tail? A horse with fecal incontinence? Your horse might be suffering from PE. Click here to fill out a questionnaire for possible inclusion in the clinical trial. Treatment would be free, although you would be responsible for a minimal amount of bloodwork.
More coming on neurological issues… stay tuned!