Better safe than sorry. I've been feeding a high fat diet since I got him a 2. An acquaintance always poo poo'd the high fat diet and her draft came down with EPSM - they weren't able to save that horse but many horses can be saved even if they start showing symptoms and switch to a high fat diet.
I know there are draft owners out there that don't believe in the hype but it's better than not doing it as far as I'm concerned.
You can find more information here.
Rural Heritage Vet Clinic - EPSM in Draft Horses