See post in Cat Health.
I am too shocked to write more now.
Edit to add: The vet has just phoned. I asked them to do an autopsy of organs and see what they could find; I ask for them not to send out tissue samples for lab work.
Billy's liver was full of disease. It may or may not have been cancer. Some of you will know, if a cat stops eating, they can go into a liver disease called fatty liver disease.
But my vet told me, if that was what happened, because he hadn't eaten for 4 or 5 days, then what caused him to stop eating? There was nothing she could find wrong. No urninary blockage. No problem with heart, lungs, other organs. So she is thinking the liver disease became so extensive that it is what stopped him from eating.
Sigh. I still can't believe he is gone. I went to bed thinking I had to take him to the vet and periodically I get ready to phone them for an appt for him.
Billy was 6 years young.