The veterenarian called in was certain that , due to the pattern, the deaths were caused by intoxication by some chemical. His guess, before the autopsy and toxicology results , was that this was caused by an injection with some form of off spec shot. He was initially going through any medication data.
They said that the tox screening would take several days due to the number of animals involved.
My neighbor has a couple of ponies that he and his wife use for polo, theirs are really hyper animals and hes always keeping them separate from other horses that he rescues.