The 411 On Equine Protozoal Myeloencephalitis

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375 – July/August, 2026

By William Given

Equine Protozoal Myeloencephalitis (EPM) is a serious, debilitating disease that attacks the central nervous system of a horse. The infection causes an inflammation of the animal’s brain and spinal cord. The organism causing the infection is the single-celled protozoan Sarcocystis neurona.

The Clinical Symptoms
Clinical signs act as a map to where the parasite damages neurons, but the classic hallmark symptoms always include the “Three A’s”: Ataxia, Atrophy, and Asymmetry.
• The most common sign is ataxia. Horses may stumble, toe-drag, sway in the hindquarters, or appear “drunk”. This incoordination often worsens when going up or down slopes or if the head is elevated.
• Unexplained muscle atrophy, frequently most noticeable in the hindquarters or along the topline.
• Symptoms rarely appear on the same side of the body (asymmetry). For example, muscle loss may be worse on the left hip, or facial drooping may occur only on one side.

Clinical symptoms of EPM may also include depression, seizures, and paralysis. Many of the symptoms are similar to other equine diseases that affect the central nervous system, thus making EPM somewhat difficult to diagnose. In addition, the clinical signs of EPM can be subtle or severe. In some cases, progression of the disease is slow and symptoms may diminish, while in others, the symptoms may progress rapidly.

How Prevalent is EPM?

Click here to read the complete article
375 – July/August, 2026