A. I would add that protein losing enteropathy or nephropathy could contribute to weight loss. I also reccommend a good physical exam and blood work as mentioned by my colleagues
A. Your dog needs to see a veterinarian ASAP. There are various causes possible for this, one being food indiscretion or sensitivity, and also, since your dog is very old, a tumor. Sometimes Greyhounds suffer from hyperthyroidism, which should also be ruled out. Weight loss also occurs in renal and liver disease. Your vet will do blood tests and possibly radiographs and should be able to treat your dog according to his exam findings.
A. Sounds like your pet could use a good exam and bloodwork done to look for underlying disease. He is an older dog and can have a multitude or organ malfunction at this age including Diabetes, Lymphoma, IBD, etc. This should be looked into with a vet for a hands on evaluation to see if there are any appropriate and plausible treatments for his condition.
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