My 16 year old male cat (who was deemed perfectly healthy just 3 weeks earlier) suddenly got 0,5kg in liquid in his belly 1,5 week. Now his legs have swollen too and the vet here can't figure it out. Echo says his organs are normal, no signs of cancer in the fluid, but he is still dying. What could it be, and can he be saved? Pet's info: Cat | European Shorthair | Male | neutered | 16 years and 8 months old | 9.5 lbs

I'm so sorry to hear that you and Sam are going through this scary situation. With normal blood work and the normal ultrasound, I would be concerned about the disease called FIP (Feline Infectious Peritonitis) which is an uncommon response to a common virus in which the abdomen fills up with fluid and cats don't tend to recover from it. I would also continue to be concerned about cancer even though you're not seeing any signs in the fluid or on the ultrasound. A tumor has to be at least a centimeter in diameter before it can be seen an ultrasound, so it's not unusual for something to just be too small. Unfortunately, without a diagnosis, it's incredibly difficult to treat. And even with a diagnosis, at his age, it's mostly bad things that it could be. At this point, your vet is probably focusing on palliative care to keep Sam comfortable by removing the fluid, keeping him well hydrated, making sure he is comfortable. I would recommend talking to them about his prognosis and your goals for him.