I personally would do a cleaning of all dishes, and if you store your feed in a metal can check that, and smell the feed to make sure it smells right. I don't think they can get it from moldy hay, that would probably be something respiratory, it also could just be some moldy produce if you feed...
I was wondering, they could pick up a bit of moldy food anywhere, I would look around and make sure I don't see or smell any, you could do a quick search on here about botulism, it's often called limber neck.
Let's hope someone more experienced pops in here, I know they can first show symptoms when younger than a year, but I think they can show them at any point, it causes tumors that causes paralysis, usually in the legs first. Otherwise there are things like botulism that can cause neurological...