We're in the epicentre of equine grass sickness here in Carnoustie. Needless to say, it wasn't mentioned in the particulars for the property

Local story is that it was brought back from the Crimea - we have a big and long established army camp next to us. Other stories is that it was because guano was used extensively on the fields here.
I don't worry about our boys - they're 18 and 14 respectively - but I do feed hay every day as this seems to help prevent it. TBH I worry more about lami and sweet itch.
It's a devastating disease and the prognosis is rarely good. You couldn't have foreseen this happening and I'm so sorry for you both and for the new owner, who must be feeling equally devastated.