We usually treat one or two every year for diphtheria. Some we save, and some don't make the summer.
Diphtheria is the same bug that causes footrot, from what I have learned. From the treatment protocols we have been given in the past you can use basically any antibiotic that is effective for footrot. We have used, Resflor, Nuflor, Excenel and Oxytetracycline. And DEXAMETHASONE. Dex takes the swelling down which helps them to breathe.
Generally the idea is to treat for a minimum of 10 days. The one I had the most success with was to give Nuflor and Dex every 48 hours for 10 days. Then to switch to Oxytetracyline (LA) every 72 hours for another 10 days. Excenel seems to work about as well, but the downside is that you have to treat them every 24 hours.
We have used the above protocol and not only saved, but the animals had no discernible after effects and we kept them in the herd as replacements. OTOH, we just lost our only case from this spring the other day. Lost last year's only case as well. The earlier you catch it the better the chance of saving them is. And, some just don't seem to respond.