Equine protozoal myeloencephalitis, or EPM, is a neurological disease in horses caused by parasites that invade the brain and spinal cord. It’s one of the most commonly diagnosed neurological conditions in horses in the Americas, and it can range from mild coordination problems to severe disability. The parasite responsible is most often Sarcocystis neurona, though a second species called Neospora hughesi causes a smaller number of cases.
How Horses Get EPM
The opossum is the key player in EPM transmission. Opossums carry Sarcocystis neurona in their intestines and shed the parasite’s infectious form, called sporocysts, in their feces. When opossum droppings contaminate a horse’s feed, water, or pasture, the horse can accidentally ingest these sporocysts. Once inside the horse’s gut, the parasites enter the bloodstream and eventually cross into the central nervous system.
Horses are what’s known as a “dead-end host,” meaning they can’t pass the infection to other horses or back to opossums. Your horse can only pick up EPM from the environment. This is why the disease is concentrated in regions where opossums live, primarily North and South America. Exposure is actually quite common: a large percentage of horses in opossum-inhabited areas test positive for antibodies, indicating they’ve encountered the parasite at some point. But only a small fraction of exposed horses develop clinical disease. Stress, heavy exercise, transport, illness, and anything that suppresses the immune system can tip the balance from silent exposure to active infection.
Signs and Symptoms
EPM’s symptoms depend entirely on where the parasite settles within the central nervous system, which makes it one of the great mimics in equine medicine. No two cases look exactly alike.
When the spinal cord is affected, the most common presentation includes gait abnormalities, incoordination (ataxia), spasticity in all four limbs, and muscle atrophy. A hallmark feature is asymmetry: the signs are often worse on one side of the body than the other. You might notice your horse stumbling more on the right hind, for instance, while the left side looks relatively normal. Some horses stand with their feet splayed apart or lean against walls and fences for support.
When the brainstem is involved, the signs shift. Horses may become lethargic or show behavioral changes. Cranial nerve damage can cause facial paralysis, a drooping lip, tongue weakness, or difficulty swallowing. Cases caused by Neospora hughesi tend to start with hind limb weakness and ataxia that can progress to generalized weakness and, in severe cases, an inability to stand.
Symptoms can appear suddenly or develop gradually over weeks to months. The progression is unpredictable. Some horses plateau at mild incoordination, while others deteriorate quickly.
How EPM Is Diagnosed
Diagnosing EPM is notoriously tricky because its symptoms overlap with many other neurological conditions, including wobbler syndrome, equine herpesvirus, and trauma. The standard approach combines a thorough neurological exam with laboratory testing.
Blood tests can detect antibodies to the parasite, but a positive result on serum alone only proves the horse was exposed at some point, not that it has active disease. The more reliable method involves testing cerebrospinal fluid (CSF) collected via spinal tap, paired with a blood sample drawn at the same time. The newest and most accurate test measures antibodies against specific surface proteins of the parasite (known as SAG 2, 3, and 4). By comparing the ratio of antibody levels in the blood versus the spinal fluid, veterinarians can determine whether the horse is actively producing an immune response inside the nervous system. A serum-to-CSF ratio below 100 strongly correlates with active EPM, with a specificity of 97% and a sensitivity of 83%, according to data from Cornell University’s Animal Health Diagnostic Center.
In short, if your vet suspects EPM, expect a neurological workup followed by a spinal tap. Serum testing alone isn’t enough to confirm the diagnosis.
Treatment and Recovery
Three FDA-approved treatment options exist for EPM. Two of them, ponazuril and diclazuril, are antiparasitic medications given daily for 28 days. The third is a combination of two drugs (sulfadiazine and pyrimethamine) given daily for at least 90 days. All are oral medications. Your veterinarian will choose based on the individual case, cost considerations, and how the horse responds.
Treatment kills the parasite, but it can’t undo damage already done to nerve tissue. That’s why early detection matters so much. With FDA-approved treatment, roughly 57 to 62% of horses show improvement in their clinical signs. Some recover well enough to return to their previous level of work. Others are left with permanent neurological deficits, ranging from subtle gait changes to more significant coordination problems. About 10% of treated horses relapse within one to three years, so continued monitoring after treatment is important.
Anti-inflammatory medications are often used alongside antiparasitic drugs to reduce swelling in the central nervous system during treatment. Rehabilitation, including controlled exercise and gradually increasing workload, plays a significant role in how fully a horse recovers. Patience is essential: neurological improvement can continue for months after the parasite is cleared.
Reducing the Risk
Since there’s no vaccine for EPM, prevention comes down to keeping opossums away from your horse’s food and water. Store all horse feed and pet food in tightly sealed, sturdy containers. Remove fallen fruit from the ground and avoid placing bird feeders near feeding or watering areas, as bird seed attracts opossums.
Keep water sources clean and fresh. Standing water in troughs that haven’t been scrubbed is an easy route for contamination. Providing several clean water sources encourages horses to drink from uncontaminated options.
On the property management side, seal off entry points to barns, sheds, and garages where opossums might shelter. Remove brush piles promptly and dispose of any roadkill or animal carcasses quickly, since opossums scavenging on infected carcasses is part of how they pick up the parasite in the first place. Placing mothballs or open containers of ammonia around the property perimeter can deter opossums. If you’re dealing with a persistent opossum population, professional wildlife services can set humane traps and relocate them.
Reducing stress on your horse also matters. Since immune suppression appears to be a key factor in whether exposure leads to disease, managing the stressors you can control (travel schedules, herd dynamics, nutritional needs) is a practical layer of protection alongside environmental management.