Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry to hear about Scruffy's condition....I know this is very difficult because you do not know what is causing this condition.
The causes for this ascites fluid that Scruffy has and this depression are as follows:
Inflammation of the lining of the abdomen
Nephritis due to protein issues
The diagnostics so far have not told you what is causing this condition so it is important that you follow up with other diagnostics. The most common cause for this fluid , weight loss and lethargy would be Mesenteric Lymphoma which is difficult to diagnose without a CT and/or MRI. The fact that this responded to the prednisone which is an anti cancer medication may tell you that there is a cancer lesion that has not yet been detected.
If the liver enzymes are normal and the EKG and/or Echocardiogram shows that Scruffy does not have heart disease, then this may be the problem.
For your peace of mind, I would urge you to do the further diagnostics until the cause is detected.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, XXXXX XXXXX