You can treat horse with thrush by washing the horses foot in bleach. Put bleach on the top and bottom and don’t get on their skin. Bleach kills the bacteria and dries it.
One of the people above said something about feed causing this, it may, so I would suggest a pelleted feed if you don’t already feed it. A pelleted feed with a high fat content. I feed Total Equine and it is a really good feed. Horses look really great on it and they are getting what they need from it.