Treating equine protozoal myeloencephalitis (EPM) involves a course of antiprotozoal medication lasting 28 days or longer, combined with anti-inflammatory support and rehabilitation. About 60% of horses improve with treatment, though fewer than 25% recover completely. The earlier treatment begins after symptoms appear, the better the outcome tends to be.
Getting the Diagnosis Right First
Before starting treatment, confirming the diagnosis matters more than many horse owners realize. EPM is caused by a parasite, most commonly Sarcocystis neurona, that infects the central nervous system. Horses pick it up by accidentally ingesting opossum feces in contaminated feed or water. The tricky part: a positive blood test alone only tells you the horse has been exposed to the organism, not that it’s actively infected. In areas where opossums are common, a huge number of healthy horses will test positive on a blood draw.
The most accurate diagnosis requires testing both blood and cerebrospinal fluid (CSF), then comparing the two results. This comparison, called a serum-to-CSF titer ratio, pushes diagnostic accuracy to 93-97%. Testing blood alone sits at only 54-56% accuracy. Your veterinarian will also need to perform a neurologic exam and rule out other causes of the symptoms you’re seeing, such as wobbler syndrome or cervical injuries. Treating a horse for EPM when something else is causing the neurologic signs wastes time, money, and delays real treatment.
The Three FDA-Approved Medications
Three antiprotozoal drugs are currently approved to treat EPM in horses. All three work by attacking the parasite’s ability to survive and reproduce, but they differ in formulation, dosing convenience, and cost.
- Ponazuril (Marquis): An oral paste given once daily for 28 days. It’s the most widely recognized EPM treatment. A full course for a 1,200-pound horse runs roughly $1,400, since each tube covers about seven days of treatment and four tubes are needed.
- Diclazuril (Protazil): An oral pellet mixed into feed once daily. Many owners find this easier to administer than a paste, especially with horses that resist oral syringes.
- Sulfadiazine/pyrimethamine (ReBalance): This was the first targeted EPM therapy. It uses a different mechanism than the other two, blocking folate production that the parasite needs to survive. Treatment courses tend to be longer, sometimes extending beyond 28 days.
Your veterinarian will choose based on the severity of symptoms, the horse’s temperament, and practical considerations like ease of daily dosing. There is no strong evidence that one drug consistently outperforms the others in clinical outcomes.
Managing Inflammation During Treatment
When antiprotozoal drugs start killing parasites in the spinal cord and brain, the dying organisms can trigger a burst of inflammation. This “die-off” reaction sometimes causes neurologic signs to temporarily worsen during the first week of treatment. It’s alarming to watch, but it doesn’t mean the medication isn’t working.
For moderately to severely affected horses, veterinarians often prescribe anti-inflammatory medications like phenylbutazone or flunixin meglumine during that first week to blunt this reaction. Horses showing brain involvement or those in danger of falling may receive a short course of corticosteroids along with DMSO to bring the inflammatory response under control more aggressively. This supportive care runs alongside the antiprotozoal drug, not instead of it.
What Recovery Actually Looks Like
Recovery from EPM is rarely dramatic or quick. The antiprotozoal medication eliminates the parasite, but the neurologic damage it caused doesn’t always reverse. About 60% of treated horses show measurable improvement in coordination, strength, and gait. However, fewer than one in four return to full, pre-disease function. Horses that were mildly affected at the start of treatment have the best odds of a complete recovery. Those with severe ataxia, muscle wasting, or difficulty standing face a harder road.
Improvement typically becomes visible within the first few weeks of treatment, but the full picture can take months to develop. Nerve tissue heals slowly. Many veterinarians recommend controlled exercise and gradual reconditioning once the acute treatment phase is over, since rebuilding muscle symmetry and proprioception (the horse’s awareness of where its limbs are in space) takes active work. Turnout alone usually isn’t enough for horses that lost significant coordination.
Relapse Is a Real Possibility
Even after a successful treatment course, 10-20% of horses will relapse at least once. Some estimates place the figure higher, closer to 25% with certain treatment protocols. Relapses can appear up to two years after treatment ends, which means the horse may seem fully recovered for months before signs return.
Relapse can mean the original infection wasn’t completely cleared, or it can mean re-exposure to the parasite from the environment. Either way, the treatment approach is essentially the same: another full course of antiprotozoal medication. Some veterinarians will switch to a different drug for the second round, though there’s no firm consensus that switching improves outcomes. Monitoring your horse closely for subtle changes in gait, behavior, or coordination in the months following treatment helps catch a relapse early, when the odds of improvement are highest.
Preventing Re-Infection
Since horses contract EPM by ingesting the parasite shed in opossum droppings, prevention centers on keeping opossums away from feed and water sources. There is no vaccine currently available, so environmental management is your primary tool.
- Secure all feed storage. Grain bins, open feed bags, and garbage cans should be in closed, tight-fitting containers. Leaving feed out overnight is an open invitation.
- Remove wildlife attractants. Bird feeders and fallen fruit draw opossums to your property. Eliminating these reduces opossum traffic near barns and paddocks.
- Check outbuildings regularly. Storage sheds, hay barns, and rarely used structures make appealing dens for opossums. Simply entering these buildings regularly discourages them from settling in.
- Trap and relocate if needed. When other measures fail, opossums can be live-trapped and moved off the property.
These steps won’t eliminate risk entirely, since opossums are widespread across North America and nearly impossible to exclude completely. But reducing the parasite load in your horse’s environment meaningfully lowers the chance of initial infection or re-infection after treatment. For a horse that has already been through EPM once, this kind of management becomes especially important given the real possibility of relapse.