My mare has EPM. For ease I will paste what I posted on another site below. Even though the new test is better there are no tests that are 100% accurate. The below statement was taken from
Blood Horse
"The newest test, the SAG-1 ELISA, has shown mixed results, depending on the study and population tested. The test looks for the presence of a particular surface protein of S. neurona, as its name implies, but not all strains express this particular protein. Thus, the test might be effective in detecting strains that express the SAG-1 protein, but it misses strains that do not. Therefore, false negative results (misdiagnosing horses that truly have EPM as being free of the disease) are common."
The trouble with the tests are there are false negatives AND false positives.
A lot of people that I have spoken with automatically start to blame the hay that was being fed. Hay can be one of the culprits as opossums and coons like to bed in them and defecate on the hay but there are other ways your horses/ponies are exposed to the parasite. We have a fence line that is the farmer's behind us (whom actually says it is ok for us to tear out) that has heavy brush, weeds, old hay bales and trees. A creek is only feet away from there. Places critters like to inhabit. If a coon or opossum climbs the fence and gets in your pasture and defecates your horses/ponies can graze in the effected area and pick up the parasite. If you are at a show and stalled where a opossum or coon who is infected with the parasite has defecated and not cleaned out your horse or pony can pick up the parasite. If you bed with straw or shavings that has been defecated on by an infected coon or opossum and your horse or pony eats the BM... well, I think you get the idea. Fact is life isn't cut and dry. Sometimes you need to step back and consider the whole picture.
On December 31st Baby started to act as though she had injured her right hip. I called the vet and he examined her. Nothing else was off about her. He even commented on how nice she looked. Since we had snow and ice on the ground it was assumed by all that she had hurt it. She had no fever or other signs that would make you go hmmm. We gave her Banamine for a couple of days and she was doing great. Then, after a while, Baby started being off on her gait. Called out the farrier. She did have a small bruise on her hoof but even her ambulation was perfect by the time he got here. No one else was seeing what I was. Baby would have a change and then she would be ok by the time someone else saw her. She had more good days than off days and sometimes she would be off for just a few minutes or an hour. It was just weird. At one point I may have even considered founder was a part of this but something kept telling me that this was something more. I kept thinking that every symptom came from a bigger picture. Vet came back out. Baby looked great and was playing out in the field. Strutting her stuff. These type of episodes kept happening since December 31st. Baby had her worse episode a couple of weeks ago. Her gait was way off, she was favoring her right hip and her hooves seemed like they grew at an astronomical rate in just a couple of weeks. Baby was also off with her grub. The vet was going to come out and my farrier was coming also. My farrier could not believe what he saw. In just a few weeks Baby had an inch of growth on her hooves. I asked about founder and he said no way. He showed me the rings in her hooves and said he thought something else was going on and even mentioned EPM. By the time the vet got here Baby was even holding her head in the corner and acting way off. At that time she was so classic EPM with her symptoms it was the first thing the vet said. She just breaks my heart! She had a hard time getting up this morning. Baby has started her medicine and we are all hopeful for her. The vets are giving her a good prognosis. She may just end up with a small drag on her right hoof. Please think good thoughts for Baby! I can't begin to explain what Baby means to me. I just love her!