Do you think just maybe it was bad hay or maybe rye or wheat grass bloat? It IS about that time of year.
I see some rancher looking for a payday.
My brother let six momma cows get into the range pellet feeder just free choice. They all died. Looked like pulmonary edema as well and a whole lot of bloat. Dug a big hole and buried them but we had a rain and their legs started to surface so he went out and cut them off with a chain saw. Really. I still laugh about that.
If it was in the lungs it was probably mold from something they were fed.
Cows and horses are no different than people when they breath in mold and grain dust.
Lungs fill with fluid.
One of the reasons for putting molasses on oats for horse feed, keeps the dust down so they don’t breath it in.