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When I hear about a young dog that is losing weight I think of parasites...intestinal and heartworms. It can cause a wasting appearance even before the respiratory signs develop.
Your dog needs a trip to the vet. You can start with a heartworm test and fecal test ($30-$45) and see how it goes. Something is taking your dog's nutrients!
You wouldn't see them in his stool. When we are looking for intestinal worms that can do this...they live up high in the intestinal tract...we look microscopically for the eggs of the worms. Heartworms wouldn't be in the stool, the babies are in the blood.
The two intestinal worms that you could see are roundworms (spaghetti) or tape worms (rice) and these don't usually cause disease in older animals...they might be there..but not as big a concern as Hookworms and Whipworms.