EPM education

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Equine protozoal myeloencephalitis (EPM) is a disease of horses that affects the central nervous system. Equine obviously refers to the horse, protozoal refers to the type of organism that causes the disease and myeloencephalitis refers to that portion of the animal, which is damaged. “Myelo” refers to the spinal cord and “encephalitis” refers to an infection/inflammation of the brain. So, EPM is a disease of the brain and/or spinal cord of the horse, which is caused by a protozoan organism.

Since EPM is a disease of the central nervous system (brain and spinal cord) and it can affect multiple locations within the brain and spinal cord, the signs and severity of this disease can vary dramatically. Though EPM can affect both the brain and spinal cord, it is more common for the signs of disease to be associated with damage to the spinal cord, but we can and do see insult to the brain. We typically refer to the 3 “A”s of this disease: Asymmetry, Ataxia and Atrophy. 

1. Asymmetry is a term we use to describe a symptom that is worse on one side of the body than on the opposite side. In other words, with EPM, the signs are generally worse on the left side than on the right or visa versa. 

2. Ataxia is a term we use to describe incoordination or the inability of the horse to know exactly where its legs are, resulting in inability to move its legs and trunk normally. 

3. Atrophy describes a condition where the muscles shrink from their normal size. With EPM, this results from damage to the nerves that normally control or “innervate” these muscles. Muscle atrophy is not seen in all cases of EPM, so it is not as consistent a sign of disease as is the asymmetrical ataxia. 

So, with this disease we will see horses that are incoordinated and this incoordination is usually worse on one side of the body as compared to the other side. These horses may or may not develop muscle atrophy. It is also common to see muscle weakness in these horses. Some horses may display abnormal gaits, lameness and loss of sensation along the face, neck or body. Paralysis of the muscles of the eyes, face or mouth may occur and this will be evident by drooping eyes, ears or lips. Horses may also demonstrate a head tilt, poor balance and difficulty in swallowing or vocalizing. On rare occasion even seizures and collapse may occur. This disease may progress rapidly or very slowly. Horses generally deteriorate over time but some animals may “level off” or plateau for a period of time only to worsen days, weeks or even months later. Most horses afflicted with this debilitating disease continue to have a bright, alert temperament.