In the end, it was my fault. I mixed up the instructions from the vet and wasn't giving enough of the anti-diarrhea medicine. Now it's all straightened out and Susu is improving rapidly. She's more energetic and has started holding her tail straight up, which seems like a sign of good spirits. She watches all our goings-on keenly and likes to chase my fingers when I read and/or work on the computer.
We're not quite out of the woods yet, however, as she is still thinking outside the box occasionally when it comes to personal waste disposal. My sister is sending one of those amazing pet-smell neutralizers from the States, which will be much appreciated.
Chad was carrying Susu down the street in a cardboard box the other day when he ran into one Dani, one of the local motorcycle taxi drivers. After peeking into the box and seeing our bedraggled kitten, Dani said, "Are you throwing her away?"
He was joking -- I think.