Equine Protozoal Myeloencephalitis (EPM) in Horses

Symptoms can vary depending on whether the horse's brain or spinal cord is more heavily affected. But the following are common symptoms to watch for.

Symptoms

• Loss of coordination
• Muscle atrophy
• Difficulty swallowing
• Gait abnormalities
• Stumbling
• Behavioral changes
• Standing with feet splayed or leaning against a wall for support
• Weakness
• Drooping eyelid
• Head tilt

The symptoms of EPM can begin quite suddenly or slowly progress over time. As a general rule, the symptoms are asymmetrical, meaning they affect one side of the horse's body more than the other.2 So you might notice that your horse moves the legs on its left side in an uncoordinated fashion, but still moves them normally on the right. Commonly, the horse will seem to have poor balance and may stumble or move in an abnormal way. Its muscles might atrophy, leading to weakness and further inability to walk normally.

If the horse's spinal cord is most affected, which is the commonest presentation of EPM, you'll notice more difficulties with movement, balance, and coordination. If the horse's brain is more affected, symptoms can include paralysis in the facial muscles, making it difficult for the horse to swallow or move its tongue, as well as lethargy, behavioral changes, and drooping eyelid.1

Causes of Equine Protozoal Myeloencephalitis

Some protozoa, including Sarcocystis neurona, are capable of causing disease in animals. Typically, these types of protozoa require a host animal in which to carry out the reproductive part of their lifecycle. For S. neurona, that host is the opossum. An opossum ingests the protozoans, which reproduce in the animal's digestive system. The opossum doesn't become ill itself, but does, however, pass infectious sporocysts, or eggs, in its feces.

When a horse ingests the sporocysts from the grass, water, or ground, they enter its digestive system. For reasons that are not well understood, in a very small percentage of horses the sporocysts mature into a form called merozoites, which then migrate through the horse's bloodstream into its central nervous system. There, they infect the brain and/or spinal cord, causing the disease called equine protozoal myeloencephalitis.2

It is important to note that EPM is not contagious from horse to horse, nor is the horse contagious to other animals.