Your added comment about being in deep south Texas, near the Mexico border, is a crucial bit of info. Your second vet is probably right -- your cows probably are having problems from tick fever, AKA Texas fever and I think AKA "redwater". There are a number of "tickticides" that I would suppose any reputable large animal vet in deep south Texas will know about. Do you remember seeing old western movies, documentaries, etc. years ago where cattle are chased through a chute and then into a deep pool of medicated water --- they were treating for the ticks. The ticks were eradicated long ago from Texas but are still a problem in Mexico, and you are probably in the "buffer zone" area where outbreaks in Texas can happen.
Is it all mature animals that are dieing, and not the young calves? I recall that's another aspect of the disease --- exposure in adulthood can be fatal but not nearly so if exposure was in calfhood.
If you need more info I'd bet A & M has good info someplace on their site --- or do a google search for "tick fever in cattle" or "Texas fever" or "redwater fever".