From what you have described, I am concerned your dog may have a liver problem.
The belly swelling could be due to ascites which is a build up of fluid in the abdominal cavity. There are a number of things that can cause this, from liver disease to cancer, to heart disease. Some of these things are treatable (such as liver infection) and some are not as treatable (such as cancer).
Here is more about ascites: http://www.petplace.com/dogs/ascites-in-dogs/page1.aspx
The (probable) ascites coupled with the swollen back leg makes me worried about hypo-proteinemia (low protein in the blood). This is something we see when there is low albumin. Albumin is a protein made by the liver. It is a very large molecule and helps to "hold" fluids in the blood vessels. Without it, fluid leaks out and we see edema of the lower limbs (sometimes just one if the dog was lying on it), and ascites.
Low albumin can be due to either not making it (as in liver disease) or losing it (as in kidney disease, or a protein losing enteropathy where it is lost from the intestines, with which you would see diarrhea).
You have not mentioned diarrhea.
So.... I am worried about liver disease and/or kidney disease. If your dog has either of these things, a diuretic would not help and could make him much worse. Treatment should be aimed at the underlying problem.
One liver problem is hepatitis (chronic active hepatitis, CAH). This can cause vomiting, loss of appetite, drinking more and urinating more, jaundice (very yellow urine, yellow in sclera and mucous membranes), and lethargy. Dogs with this may need hospitalization if it is severe. Intravenous fluids may be needed for rehydration. Dogs often need antibiotics, anti-inflammatories, and liver specific drugs to help it to function better.
Here is more about it:
So, as you can see, the symptoms you describe could be indicative or a number of different possibilities, though liver disease would be very high on my list of possibilities from what you have told me. The place to start would be a complete blood and urine analysis and x-rays. At my clinic that would cost about $400.
Based on the results of these tests, a veterinarian would be able to determine what is the most likely thing going on with your dog. I really feel there is SOMETHING going on with him, as what you are describing is certainly not normal. I do recommend that you take your dog to see a veterinarian as soon as possible to find out what is going on and get started on treatment. If your vet is not open, you may wish to consider taking your dog to see an emergency veterinarian.
If this is not an option for you, then I would certainly plan to BE on your vet’s doorstep when they next open so your boy can be examined, diagnosed and treated.
In terms of what you can do until you see a vet, I would suggest the following for a patient in my care:
1. Offer lots of clear fluids.
So, water is fine, but also you can give pedialyte, Gatorade, apple juice diluted 50:50 with water, or chicken or beef broth (no onions in it) diluted 50:50 with water. Give the fluids in small amounts frequently.
2. offer small amounts frequently of a bland diet.
For patients that I see, I usually suggest a mixture of 75% cooked white rice, and 25% low fat protein. For the protein you could use extra lean ground beef, boiled with the fat scooped off, or chicken breast boiled with fat scooped off or even scrambled egg cooked without fat in the microwave. Feed small frequent meals.
3. Keep your dog as quiet as possible - just out to relieve himself and back in.
If Pono is becoming more distended, if his gums are becoming paler, if he is unable to eat or drink, or is becoming more lethargic, or develops laboured breathing, then do contact an emergency veterinarian immediately!
Good luck with your boy, and please let me know how he does! I hope that with treatment he will be feeling better soon!
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.