Preventing equine protozoal myeloencephalitis (EPM) comes down to two things: keeping opossums away from your horse’s feed and water, and reducing the physical stress that makes horses vulnerable to the disease. About half of all horses in the United States test positive for exposure to the parasite that causes EPM, yet only 0.5 to 1% actually develop clinical disease. That gap tells you something important: exposure is nearly unavoidable in many regions, but the horses that get sick are usually the ones dealing with other risk factors at the same time.
How Horses Get EPM
EPM is caused by a protozoan parasite called Sarcocystis neurona. The Virginia opossum is the only known definitive host. Opossums carry the parasite in their intestinal tract, where it reproduces and forms infectious packets called sporocysts. These are shed in opossum feces and can end up on pasture, in hay, in grain bins, or in water troughs.
Your horse gets infected by accidentally swallowing sporocysts while eating or drinking contaminated feed or water. The parasite then migrates to the central nervous system, where it can cause inflammation and damage to the brain and spinal cord. Sporocysts are hardy. They can travel through surface water runoff and even infiltrate groundwater, meaning contamination isn’t limited to the spot where an opossum defecated. If opossums are active anywhere on or near your property, the potential for contamination is real.
Keep Opossums Out of Feed and Water
This is the single most effective thing you can do. Opossums are opportunistic, nocturnal feeders that are drawn to easy food sources. Every prevention strategy starts with making your barn and feed areas unappealing to them.
- Store all grain in sealed, animal-proof containers. Metal bins with tight-fitting lids work best. Opossums can chew through plastic bags and squeeze through surprisingly small openings.
- Secure your hay. Research from Kentucky Equine Research found that farms where hay and grain were not secured from wildlife had higher contamination risk. Hay stored in open-sided shelters or left on the ground is especially vulnerable.
- Cover water troughs. An opossum walking through or defecating near an open trough can contaminate your horse’s water. Use covers when possible, and clean troughs regularly.
- Feed pets away from the barn. Cat and dog food left out overnight is one of the biggest opossum attractants. Feed pets in areas away from where horse feed is stored and horses eat.
- Reduce clutter and hiding spots. Opossums and rodents are drawn to cluttered barns with plenty of places to nest. Keeping your barn tidy and sealing openings in walls and rooflines makes it harder for wildlife to move in.
- Let barn cats and dogs patrol. Keeping cats or dogs loose in the barn can discourage opossums from making nighttime visits to your feed room.
None of these steps guarantees zero exposure, especially if your horses graze on pasture where opossums roam. But minimizing contamination of stored feed and water significantly lowers the dose of parasites your horse encounters.
Manage Stress to Lower Disease Risk
Since so many horses are exposed without ever getting sick, the question isn’t just whether your horse encounters the parasite. It’s whether your horse’s immune system can keep the parasite in check. Stress and immune suppression are the factors that seem to tip the balance toward clinical disease.
The American Association of Equine Practitioners identifies several specific stressors that increase EPM risk: heavy exercise, long-distance transport, injury, surgery, and foaling. Racehorses and show horses develop EPM at higher rates than breeding or pleasure horses, likely because of the combined effects of intense training schedules and frequent travel. Advanced age also plays a role, as older horses tend to have weaker immune responses.
In practical terms, this means being thoughtful about how you manage transitions and high-demand periods. If you’re hauling a horse across the country for a competition, that animal’s immune defenses are temporarily compromised. Stacking multiple stressors, like a long trailer ride followed by intense competition in an unfamiliar environment, compounds the risk. You can’t eliminate stress entirely, but you can space out demanding events, allow adequate recovery time after transport, and maintain good nutrition and turnout to support overall immune health.
Prophylactic Medication
There is some research supporting preventive use of antiprotozoal drugs in high-risk situations. A study published in the Journal of Parasitology tested ponazuril, one of the drugs commonly used to treat EPM, as a preventive. Horses given the higher dose before and after experimental infection had significantly better outcomes: only 40% developed neurologic signs compared to 100% of untreated horses. Seroconversion (the blood marker indicating active infection) also dropped from 100% to 40% in the treated group.
This suggests that short-term preventive dosing around known high-risk events, like a major move or stressful competition season, could offer some protection. However, prophylactic medication is not a standard blanket recommendation for all horses. It’s something to discuss with your veterinarian based on your horse’s individual risk profile, geographic location, and exposure level.
What About Vaccines?
A vaccine for EPM was conditionally licensed by the USDA in December 2000 and was approved for use in 43 states. However, the vaccine never achieved widespread adoption, and its efficacy remained a subject of debate within the veterinary community. It is not currently a routine part of most equine vaccination programs. No broadly recommended, highly effective EPM vaccine is available today, so prevention still relies on management practices rather than immunization.
Is Routine Screening Worth It?
You might wonder whether testing apparently healthy horses for EPM exposure makes sense, especially if you’re in a high-opossum area. The short answer: a positive blood test in a horse with no symptoms tells you very little. Because roughly half of horses in endemic areas carry antibodies, a positive result simply means your horse has been exposed at some point. It does not mean disease is developing or that treatment is needed. Screening blood tests in asymptomatic horses have minimal diagnostic value for this reason.
What matters more is recognizing early signs of clinical disease: subtle incoordination, asymmetric muscle wasting, stumbling, head tilt, or difficulty swallowing. If you notice any of these, particularly after a period of stress, that’s when diagnostic testing becomes meaningful. Early detection and treatment give horses the best chance of recovery, so knowing what to watch for is itself a form of prevention.
Regional Risk and Realistic Expectations
Your geographic location heavily influences EPM risk. The parasite’s distribution follows the opossum’s range, which covers most of the eastern United States and is expanding westward. Antibody prevalence varies by region, with some areas showing rates well above 50%. If you’re in the Southeast or Midwest, your horses are almost certainly encountering sporocysts at some point in their lives.
The realistic goal isn’t to eliminate all exposure. It’s to reduce the parasite load in your horse’s immediate environment and keep your horse’s immune system strong enough to handle whatever exposure does occur. Tight feed management, wildlife deterrence, and thoughtful stress management won’t make EPM impossible, but they meaningfully lower the odds that your horse will be one of the small percentage that develops this serious neurologic disease.