I ALWAYS get nervous about the steroids in these situations, for a variety of reasons. Unfortunately, something like hemangiosarcoma also needs to be ruled out, and that's usually done with an ultrasound
As for the babesia, I do not believe that you can always see it. I would call this lab and ask, which is the one we use on the tick list, as they specialize in this stuff:
http://www.protatek.com/reflab.html
When did your dog last have a vaccination? Or any new meds before this all happened?