I think the veterinarian has suggested a toxic cause. If it were virus-induced, the incubation period and clinical course would be more variable. I would need further clinical information to assess the cause of sudden death. Differential considerations: cantharidin, monensin, OP or carbamate insecticides, cyanogenic plants, Oleander, Japanese Yew. More information on history, surroundings, and transportation methods are needed.
A similar incident happened someplace else in Florida just a few months ago. I think they determined that all the hay came from the same place, and there was some sort of toxic weed that had gotten into it.