treatment

It is very dangerous to apply cat/dog treatment to the rat without a lot of adjustment.
I use Dexamethazone for a short period only. One of side effects it can have is making bones fragile if used for long (thinking about broken tail).
If Tetracycline did not bring relieve of symptoms, most likely the causative agent is not responding to it. In this case, another antibiotic may be necessary.
Also, as soon as you quit Tetracyclin, give your rattie yougurt with live cultures every day.