Thanks for all your replies. I had no idea that they were not a native species. Thank goodness I came and asked before I made an actual decision. We have decided to keep her until she is ready to be put down. I value her life, even though she is just a (mean) rat, just as much as any of my other pets. I wouldn't do anything that would harm her, and get me in trouble in the process. She served her duty and I believe she deserves the best.
Does anyone know of a way I can treat that ear infection?