weight loss in dog

Question
Please help! I have a 5 year old purebread black lab. He has lost about 15 pounds in the past 6 weeks (87 to 72), with his rib cage starting to show. I took him to our vet yesterday. The vet had stool samples tested and blood work done, checking for parasites, etc. The results came back late today with everything being perfectly normal. I am taking him back to the same vet in the morning where he will do a complete xray on him. I am obviously worried as the vet said "he is deteriorating fast". My dog (Shadow) still has a good disposition, but he is not himself, actually a little on the lethargic side. His appetite is still good, and he is eating normal as he has since he was a puppy. I did find a dead possum in my back yard about 6 weeks ago, and when I disposed of it I did check for any bite marks on other abrasions that I thought may have come from my dog. However, after checking both the varmint and my dog, I didn't see any wounds on either. I can't 100% rule out rabies, but the vet doesn't think that's it (he couldn't rule it out either though, although unlikely was his opinion. Any words of wisdom would be greatly appreciated!!!

Answer
Some of these dogs can show weight loss from many different reasons.  Sometimes it is internal like pancreatitis, liver or kidney disease.  Blood tests may help show these.  In other cases there is something wrong with the intestines, like inflammatory bowel disease or tumors that cause weight loss.  In other cases there is a problem with digestion.  Your vet will just need to do a workup to rule in or out all of these possibilities.