I had an asb mare and two hackney ponies get chronic diarreah in about ten years ago. It started with one pony, then second pony, then asb mare. It seemed contagious, so I separated the three and basically quarantined them from my other horses. I sent the asb mare eto Ohio State, they ran every test they could and found nothing wrong. Mine were on reaform for two months or so which helped, but then when they went off it, the diarreah was back. This lasted from around July until January, when we had a really cold period, sub zero temps for several days in a row and then they all three were totally normal. I dont know what really cured them, but I always felt the cold did it???
Mine were tested for everything we could test for, salmonella, giarrdia, etc. the only thing that had changed prior to this is I had gotten a new load of hay from a different farmer, so I always wondered about that...my vets were stumped....the three who had this lived on with no other issues, all went back to showing. Two have since passed of old age the asb mare was sold. I can only tell you dont give up, I know I thought about putting mine down and they looked terrible and I was worried they were suffering and would die that way, but mine got better eventually. Good luck.