EPM (equine protozoal myeloencephalitis) is not contagious. It cannot spread from one horse to another, and an infected horse poses no risk to other horses in the barn, pasture, or trailer. The parasite responsible for EPM requires a specific life cycle that horses simply cannot complete, making them a biological dead end for the disease.
Why EPM Cannot Spread Between Horses
The parasite behind most EPM cases, Sarcocystis neurona, has a two-host life cycle that depends entirely on the Virginia opossum as its primary host. Opossums eat infected prey animals like raccoons, skunks, armadillos, or cats. The parasite then reproduces sexually inside the opossum’s intestinal lining and produces millions of microscopic egg-like structures called sporocysts, which the opossum sheds in its feces for months.
Horses get infected by accidentally eating feed, grain, or drinking water contaminated with opossum droppings. But here’s the key point: once the parasite enters a horse, it never develops into the tissue-cyst stage needed to continue its life cycle. The parasite is essentially trapped. It can damage the horse’s nervous system, but it cannot produce the form that would allow another animal, whether horse or opossum, to become infected by contact with that horse. A consensus statement published in the Journal of Veterinary Internal Medicine put it plainly: S. neurona is not transmitted horizontally between horses, nor can it be transmitted to horses from other intermediate hosts.
The Role of Opossums
Opossums are the sole source of infection for horses. A single opossum can excrete millions of sporocysts over a period of months, contaminating pastures, hay, grain bins, and water troughs. The parasite cycle works like this: an opossum eats the carcass of an infected raccoon or skunk, picks up the parasite, and begins shedding sporocysts in its feces wherever it roams. If an opossum has access to your feed room or water source, that’s the risk.
Other animals like cats, raccoons, skunks, and armadillos serve as intermediate hosts. They carry dormant cysts in their muscles that opossums can pick up by scavenging their carcasses. But these intermediate hosts cannot directly infect horses. Only opossum feces contain the sporocyst stage that is infectious to horses.
A Second Parasite With a Different Story
A small percentage of EPM cases are caused by a different organism, Neospora hughesi. Its definitive host has never been identified, and its full life cycle remains poorly understood. Unlike S. neurona, there is evidence that Neospora species can be transmitted vertically from a mare to her foal during pregnancy. One Brazilian study found that about 35% of foals born to seropositive mares had antibodies before nursing, suggesting the parasite crossed the placenta. However, the role of this transmission route in causing actual disease or abortion in horses is still not fully understood.
What EPM Looks Like
EPM attacks the horse’s brain and spinal cord, and its hallmark is asymmetry. You might notice your horse stumbling or dragging a hind toe on one side but not the other, or muscle wasting that’s clearly worse over one hip. Signs can range from subtle gait changes to severe incoordination, difficulty swallowing, or head tilting. Because the parasite can lodge in different parts of the central nervous system, no two cases look exactly alike, which makes EPM one of the trickiest neurological diseases to pin down.
Many horses are exposed to the parasite without ever getting sick. Exposure rates are high in areas where opossums live, but only a fraction of exposed horses develop clinical disease. Stress, heavy exercise, transport, illness, or anything that suppresses the immune system can tip the balance from exposure to active infection.
How EPM Is Diagnosed
A blood test alone cannot confirm EPM because so many horses have antibodies from past exposure without ever developing disease. The more definitive approach involves comparing antibody levels in the blood to antibody levels in cerebrospinal fluid, collected through a spinal tap. If the ratio suggests that the horse’s nervous system is actively fighting the parasite rather than just showing old exposure, that supports a diagnosis. Your veterinarian will combine these lab results with a neurological exam to rule out other causes of incoordination, like cervical vertebral malformation (wobbler syndrome) or equine herpesvirus.
Treatment and Recovery
EPM is treated with antiprotozoal medications given orally, typically for 28 days or longer depending on the severity. Treatment kills or suppresses the parasite, but recovery of nerve function depends on how much damage occurred before treatment began. Horses caught early, when signs are mild, tend to recover more fully. Horses with severe, longstanding neurological deficits may improve but often retain some degree of coordination loss. Relapses can occur, particularly if the horse is re-exposed to contaminated environments or becomes immunocompromised.
Preventing Exposure on Your Property
Since the only route of infection is opossum feces, prevention centers on keeping opossums away from your horse’s feed and water. Store hay and grain in enclosed, opossum-proof rooms or containers. Cover water troughs or use automatic waterers that opossums can’t easily access. Clean up spilled grain promptly, especially overnight when opossums are most active.
One often overlooked strategy involves removing dead wildlife from your property. When opossums scavenge the carcasses of raccoons, skunks, armadillos, or feral cats, they pick up the parasite and begin shedding it. Disposing of roadkill and dead animals near your barn and pastures reduces the chance that local opossums become infected in the first place. Even with these precautions, it’s impossible to fully protect open grass pastures from contamination, since opossums roam widely at night. But reducing attractants like pet food, garbage, and accessible feed goes a long way toward keeping them off your property.