Our local Vet, man of much experience, says he believes almost every horse in the State of MI, is carrying the protaza if they graze in fields. His own horses all tested positive, though non-symtomatic, when he volunteered them for a test group. They ended up unable to find ANY horse herds that didn't carry the EPM for that study.
It seems that when horse gets stressed, it allows the protoza to free themselves from the spinal cord and start causing problems. This is the tricky one, with all kinds of odd symtoms, nothing to point at for why horse is "off".
Sudden onset EPM, with the staggers, loss of body control, is usually so bad that horse needs to be put down pretty quick. Before the staggering, horse was FINE. We know some that hunted the day before, NO PROBLEMS in movement or working!! Next day they are in BAD shape, need to be put down.
Our own horse got EPM, the slow creepy kind. He was caught very early, treated and came back to his old self. Vet said he never would have suspected horse having an issue, passed all the physical tests he was given. We as a family, were VERY familiar with horse, and he just wasn't himself. Reading all the weird ways EPM presented itself here on COTH, I called for an EPM test. Vet used a blood test for checking, cheap, quick, showing our horse was carrying EPM protoza. Vet said to just treat with the Marquis, he would show improvement in a week if Marquis was working. The spinal tap method was the only other test method, and was extremely expensive. Same price for the Marquis treatment, and the Marquis would be doing something if it was going to work at all! So we treated horse, gave him time off any kind of work during treatment to prevent stressing him any more. He did show a LOT of mental improvement within the week, and at the end of treatment, was 95% back to his skills physically. He did heal himself fully within a couple months. Got back to 100%, used him hard in many things for another 10 years. Loved that horse.
Beyond the stress idea, there doesn't seem to be rhyme or reason why some animals come down with the EPM and others don't, if they are all carrying it. There are possums everywhere in the State, so horses are easily going to pick up the contamination if they graze a field. What happens then is up to that horse and his body.
I would agree that correct diagnosing of the EPM is now more common than in years past. Vets recognize it much faster now, so that would raise the numbers in reporting totals. There there might not actually be more horses sick with it than in years past, but now they know the disease to point at it. Horses just got put down with no testing, more often in the past, especially the inexpensive animals. No funds or unwilling to spend limited funds, to find out the "why he died" part.
Over 90% of the equine population in endemic areas have been *exposed* to the parasite (which will show up as a positive antibody titer from serum and CSF), but less than 1% of the exposed horses actually develop clinical disease.
Nobody knows why some horses get it and some don't. Stress events have been hypothesized, but not proven. Another (and better, in my opinion) hypothesis is that horses that develop clinical disease have/had some kind of previous neurological event (viral, bacterial etc) that caused minor damage to the blood-brain barrier, allowing access of S. neurona to cerebral parenchyma.
Well oh have two now being treated for it out of four horses. One was treated 2 years ago and found out it never got rid of it so doing another drug now and new horse was iffy on his levels but his crp came back high so we are treating him. I think more people and vets know about it more now then years ago and it's just becoming more recognized.
Horses aren't our whole life, but makes our life whole