Not every opossum carries the protozoa. One study showed about 18 to 25% of opossums have this protozoa. In addition, not every horse exposed to the protozoa will become ill. In fact, out of all exposed horses, less than 1% will show up with clinical EPM. Illness requires the protozoa to cross the blood/brain barrier and many horses have immune systems that catch this invader long before that point. Still, S. neurona can, and does, reach the spinal cord and/or brain in some horses.
Diagnosing EPM simply by clinical signs is difficult. Horses can show a wide range of symptoms, depending on where the protozoa has attacked. These signs can mimic many neurologic diseases. In addition, most veterinarians don't see a lot of equine neurologic cases.
Many horses will show some muscle atrophy (though that takes time to show up); some will have a head tilt; some merely stumble or act uncoordinated. A few will show severe signs, including seizures. Unfortunately, many neurologic problems can have similar signs. One tip off may be asymmetric or unbalanced signs, i.e., a horse who only circles in one direction, muscle loss only in one hip, etc.
Laboratory diagnosis of EPM isn't much better. Blood samples for antibody titers can be drawn, but a titer merely indicates that a horse was exposed to S. neurona. Most exposed horses fight this protozoa off and never miss a beat. CSF (cerebrospinal fluid) tests are better, but even these tests may come back negative in a horse who is truly affected.
There are now some drugs FDA-approved for effective treatment of EPM, so you're best off relying on these as opposed to attempting unproven herbal medications. Success does seem to vary with the individual medication and the individual horse, so your vet may switch medications.
ReBalance combines the original standby treatment of sulfadiazine and pyrimethamine. This is sometimes used with trimethoprim. It doesn't appear to be commercially available at this time, but the combination can be drawn up by compounding pharmacies.
Today, many veterinarians use Marquis or ponazuril. The exact dosing schedule and length of treatment needed varies. Some clinicians go for 28 days, others prefer 56 days. Some like to dose at levels above the recommended amount initially, then drop back. This comes as a paste.
Diclazuril or Protazil is one of the newest treatments on the block and similar in many ways to ponazuril. Designed into an alfalfa-pellet mix, treatment time is 28 days.
The newest kid on the block is Oroquin-10, a paste. A combination of levamisole and deconquinate, it's currently being tested in field trials. Your veterinarian can apply for it, if it looks worth trying in a horse who has relapsed or is not responding.
These medications aren't cheap, but these approved drugs are your best options for restoring your horse to near normal.
It's important to remember that while treatment will (hopefully) kill the protozoa, it doesn't restore the damage already done to the nervous system. Some horses will continue to improve and get back to normal or nearly so, but others will plateau where they were at the beginning of therapy.
WHAT YOU CAN DO.
Supportive care is important for any horse with EPM. You may want to try massage, physical therapy including hand walking and specific exercises customized to the damage your horse is showing. Anti-inflammatories can be helpful as well. Some horses will show a slow improvement over months while others stagnate at a particular level of recovery. See PDF of Dr. Eldredge's horse after EPM treatment.
Can you prevent EPM? That's easier said than done. There is no effective vaccine for this disease. So your goal is to keep opossums out of your barn and away from your feed. If you have barn cats, get them used to meals, instead of leaving food out free choice. Keep any compost piles far away from the barn area. don't leave food out for wildlife, and don't blame your barn cat either.
If you spot an opossum, approach it with confidence. While some will run away, most truly do "play dead." Despite their fearsome set of teeth, opossums rarely bite. Still, you should probably wear thick leather gloves to pick one up. (Yes, you can literally pick them up, and they remain limp.) Then put the opossum into a dog or cat carrier and call your local conservation officer or wildlife rehabilitator.