You're speaking with Dr. Michael Salkin. Welcome to JustAnswer. I'm currently typing up my reply.
I'm sorry to hear of this Thea. "Swollen stomach" is the clue not to be ignored. Has the swelling been shown to be the result of an abdominal effusion - free fluid in the abdominal space? If so, the wet form of the FIP virus is the most important differential diagnosis. Fluid would need to be aspirated and sent to our pathogist for analysis of the cells within it and its physical qualities.
No, in fact, testing for FIP is problematic. There's no "FIP test". We use titers, the complete blood count, and the signalment (symptoms) of our patient to paint a picture. The wet form is easier to diagnose. We can use the fluid in the abdomen to test for feline coronavirus antigen by immunofluorescence within macrophages - the only manner in which to confirm FIP in the live patient.
Correct, only in the most extreme cases do we see horizontal spread of this virus (from cat to cat). In the great majority of cases, FIP arises as a mutation in the ordinarily well-tolerated coronavirus. The genetic predisposition to suffer this mutation can be passed to offspring, however.
I don't recommend testing. Titers against coronavirus tell us next to nothing. All cats in many populations of cats will test positive for this virus.
Thank you for your kind accept. I appreciate it. Please continue our conversation if you wish.
No, while prednisone might extend the life of some cats for a few weeks/months, this virus is invariably fatal. The vets need only google FIP virus and look for veterinary school publications rather than lay forums.