Equine Protozoal Myeloencephalitis (EPM)

Equine Protozoal Myeloencephalitis, or EPM, is a serious disease that can be difficult to dignose because its signs and symptoms can mimic other health problems in horses and the signs can range in severity. More than 50 percent of all horses in the country may have been exposed to the causative organism so it is important to be aware of the signs and symptoms of this disease. 

The causative organism is a protozoa called Sarcocystis neurona, and is spread by opossums. Opossums are the definitive host which is an organism that supports the adult or reproductive form of a parasite. Opossums aquires the causative organism from cats, raccoons, skunks, and armadillos. The infective stage of the protozoa, the sporocysts) is passed in the opossum's feces and horses can come into contact with it while grazing or eating contaminated feed or drinking contaminated water. Once the sporocysts are ingested by the horse, the sporocysts will migrate from the intestinal tract into the bloodstream and will then cross the blood/brain barrier. Once this barrier is crossed, the horse's central nervous system begins to be attacked. Disease onset may be slow or sudden and if left undiagnosed and untreated, EPM can cause devastating and lasting neurological damage. 

The clinical signs and symptoms of EPM will vary and can be different in different horses. Clinical signs are usually asymmetrical, meaning not the same on both sides of the horse. Signs may include:

Ataxia (incoordination)

Spasticity (stiff movements)

Abnormal gait or lameness

Incoordination and weakness which worsens when going up or down slopes or when head is elevated

Muscle atrophy mostly along the topline or in large muscle groups like the hindquarters but can sometimes involved the muscles of the face or front limbs

Paralysis of muscles of the eyes, face, or mouth evident by drooping eyes, ears, or lips

Difficulty swallowing

Seizures or collapse

Abnormal sweating

Loss of sensation along the face, neck, or body

Head tilt with poor balance - horse may assume a splay-footed stance or lean against something for support

There are several factors that can influence the progression of the disease but these four things seem to play an important role:

The extent of the infection (number of organisms ingested)

How long the horse harbors the parasite prior to treatment

The point(s) in the brain or spinal cord where the organism localizes and damage occurs

Stressful events following infection or stressful events while infected

Almost every part of the country has reported cases of EPM, however there is a lower incidence of the disease in the western United States because of the smaller opossum population. About one percent of horses exposed to EPM will show clinical signs of the disease and due to the transport of horses and feedstuffs across the country, almost all horses are at risk to being exposed.

Diagnosing EPM can be difficult because there is no specific assay for this disease and it's clinical signs mimic other neurological diseases. A veterinarian will fist conduct a thorough physical exam to asses the horse's general health and identify any suspicious signs. One clue that can help lead to the correct diagnosis is that EMP tends to affect one side or part of the horse more than another. 

Early diagnosis and treatment are key to increase the horse's chance at a full recovery. Sixty to 70 percent of EPM cases that are treated aggressively show significant or complete reversal of symptoms with many horses being able to return to normal activity. Although there are drug combinations available to treat EPM, there are propriety anti-protozoal drugs specifically labeled to treat the disease. Anti-inflammatory drugs may be prescribed to alleviate symptoms. Vitamin E, an antioxidant, supplementation aids in healing the nervous system tissue. Treatment duration generally lasts one month when using the proprietary products, but can be longer (up to six months) depending on which medication is used. Treatments for this disease can be expensive and may affect stallion fertility and may pose health risks for unborn foals. Despite the high success rates, not all horses respond positively to treatment, with approximately 10 to 20 percent of horses experiencing relapse. During treatment taking blood samples may be needed to monitor potential side effects sushc as anemia, low platelet count, and low white blood cell count. Horses undergoing treatment should be closely monitored for sings of improvement or decline and any changes should be reported to your veterinarian immediately. 

The best way to prevent this disease is good horse keeping practices that discourage opossums and other rodents from getting into and contaminating hay, grain, bedding, and water sources. Here are a few suggestions for how to do that.

Keep feed rooms and containers closed and sealed

Use feeders that minimize spillage and are difficult for wild animals to access

Clean up any dropped grain immediately to discourage scavengers

Feed heat-treated cereal grains and pelleted feeds sind these processes seem to kill the infective sporosysts

Keep water tanks clean and filled with fresh, clean water

Maximize your horse's health and fitness through proper nutrition, regular exercise, and routine vaccinations and vet visits

If you have more questions on the topic, please contact your local livestock Extension agent and your veterinarian.