Equine Protozoal Myeloencephalitis (EPM) is a neurological disease caused by Sarcocystis neurona, a protozoal parasite. EPM can be difficult to diagnose because of the wide range in severity and types of symptoms. It is transferred to horses by opossums, specifically, through their feces, which horses come into contact with while grazing or eating contaminated feed or drinking water.

Once ingested, the parasite targets the horse’s nervous system. The disease can progress at different rates and, if left untreated, can cause irreversible damage.

Clinical signs vary greatly and may include:

• Abnormal sweating
• Muscle atrophy
• Muscle paralysis, especially of the mouth, face, and eyes
• Loss of sensation
• Movement problems: incoordination and abnormal gait
• Weakness or trouble walking, especially when going up or down slopes and when the head is lifted up
• Collapsing
• Difficulty swallowing
• Seizures
• Poor balance and tilting of head, sometimes with a splay-footed stance or leaning against walls for support

Horses under stress seem to be especially susceptible to rapidly developing EPM. Other horses may host the parasite for months or years before developing symptoms. Many horses’ immune systems appear to be able to combat the infection without ever showing clinical signs.