The only thing I can say is that I highly doubt that it has to do completely with the work that they are doing, as the symptoms don't present themselves until days later. Whatever it is, is probably related to something that they ate that harmed them, and that the work was just the catalyst to start their decline. Questions to ask are "do they go out to pasture, and is it possible that there is something in the pasture that is poisonous", like others mentioned, is it something in their feed that they are getting thats different from what the spanish horses and babies are getting. I would highly recommend tests, and necropsy of the horses who have died. I had a horse that was sick, and it wasn't until after we put him down and did a necropsy that we were able to find out what had been wrong with him, as his tests had all come back normal. I wish you the best of luck, and I hope you find out what it is soon.