Thank you for the additional information.
I am very sorry to hear that Rupert has been retaining fluid in his belly and is losing a lot of his muscle mass. The fluid in his belly is called ascites and it can be due to many different disorders. Without examining him giving you the exact diagnosis is going to be difficult though here are the common causes of ascites in dogs:
- Heart disease
or congestive heart failure. This is where the heart can not handle all of the fluid in the blood which causes the fluid to leak into empty chambers in the body such as the abdominal space.
- Liver disease in which the liver does not produce enough protein
- Low protein levels in his blood.which can be caused by liver disease or loss of the proteins because of a disease in the intestines or the kidneys.
- A mass or tumor in the abdomen which is secreting fluid into the free space in the abdomen
At this point, we need to get an exact diagnosis in order to determine the best treatment option for Rupert. All of these diseases can be treated with either medication or the worse case scenario would be a tumor in the abdomen which would require surgery. At 4 years of age though a tumor is very low on the list of possible causes.
I would recommend talking to your vet about performing some basic tests on him to help get a diagnosis. The first test would be a complete blood panel which can check his liver and kidney function and also his protein levels. A urine test can also be helpful to determine if he is losing any protein from his kidneys. If these tests are normal then the next best test would be an abdominal ultrasound. An ultrasound is a very thorough yet non-invasive way to look at all of his organs in his abdomen in addition to all of his intestines.
The Lasix is a diuretic or water pill medication that does help eliminate extra fluid in his body, but since the fluid is coming back we need to determine the underlying cause so that we know how to prevent the fluid from returning.
Most likely he is losing muscle in his body due to a possible low protein level in his blood. His body is breaking down the muscle to try to get more proteins that the body needs for the major organs.
Overall I would talk to your vet about these disorders and minimally a complete blood profile to look at his proteins, kidney and liver function tests.
Here is more information for you also:
Hope this helps to answer your question - if not or if you have any more questions please feel free to reply by clicking on the "reply to" button. Thank you for your question!