As with any plage in a mostly isolated area, the disease soon runs itself out. Cattle ranching takes a hit for awhile, making many rich, influential members of society (read: pro-seccesionists) a fair deal poorer. Once over, say in another decade, cattle ranching picks back up again. In the meantime, alternative industries/ranching animals are tried: sheep are a big one, without cows to compete with.
Mexico, should it try and prompt secession, finds itself too poor, occupied, or weak to compete with the United States.