Ava was back at the vet today, and, surprisingly, she had made a small gain. Hurrah! She also had a full bowel, so it's difficult to tell whether that was the cause!
The fullness of her bowel was a good thing, it shows there's not a lot wrong with her digestion, she's eating and it is clearly being processed as it should. So much to the good. It also meant that the vet couldn't feel her kidneys, and what with that and the fact that she's really feeling very bony now, I agreed to a full blood array of the kind Indigo had when she had her collapsing fit last year. And a urine test. I'm waiting on the results now, but I'm not really expecting any kind of anomaly... Ava's only symptom is skinniness, she's not appearing to be ill in any other way.