I'm putting this up in case someone else encounters dog anemia like this and searches for advice.
The short version: Our dog is on the mend. The cure seems to have been due to 1) LOTS OF PROTEIN AND LOTS OF LIVER, and 2) she was in good shape to start with. Oh and of
course 3) drugs from the vet (nothing expensive).
At first the pup didn't want to eat much of anything and we pretty much were trying anything she might take that has lots of protein. Mostly that meant chicken and hard-boiled egg and a little rice, although she'd take an occasional bite of this or that. She wouldn't eat straight beef, liver, or most of her regular treats, though. She was one sick puppy and we kept after her to eat. She was down about 2lbs+ at the lowest. Considering she weighed 12.5lbs to start, that's a big loss.
Once she started to get better, her appetite improved. (I suppose the prednisone helped that, too.) We kept giving her lots of chicken and added liver -- lots and lots and lots of liver. As in we were (and still are) buying it by the pound. We cook it so she won't meet with something her weakened system might have trouble with and she loves it now.
As she started getting back to something vaguely resembling her usual self, we added kibble and cut back on the meat, though not the liver and eggs. We took her to the vet to check on her progress and he was impressed. He described her recovery as "almost miraculous" and very rapid considering how sick she had been.
She's still not 100% but she's getting there. She's extremely pleased with the high liver diet. She has even learned the word "liver": shout "
LIVER!" happily with a piece in your hand and she'll jump in the air (like Odie). She gets lots of exercise usually and being in good shape to start with probably helped.
The vet is pretty sure that the anemia was a reaction to something but there's no way to know what. She tested negative for all the usual infectious agents. That doesn't mean there wasn't one -- it could have been something not usually encountered. But a reaction of some sort seems most likely.
As a result we're cutting out as many chemicals and treatments as possible. We especially suspect the topical anti-flea treatment. The vet didn't reject that possibility; he just said he has never seen that sort of reaction. I searched some veterinary databases and didn't find any research on that either. But it is the one correlation that seems to fit the timeline. Being that food grade diatomaceous earth is cheap, well, we aren't going to risk using flea treatments again.
Thanks for everyone's concern and suggestions! This was pretty shocking: a happy and very energetic pup just a bit over two years old laid low out of the blue.