What are the signs of dog depression? Can heart-worm travel into becoming brainworm?

My dog Morph is/was the worlds smartest dog and a most courageous 4 leg animal. He’s mixed pure bred golden retriever and shepard mutt. Understands 3 languages, looks both ways before crossing highway traffic. He defended me by attacking 2 fully grown rottweillers when he was about a year old. He’s nearly 10 now but acting so weird.
He has mild heartworm treated for past 2 years, all of his shots. He has always stalked and eats wild muskrats, beaver and the odd rabbit staying mostly outdoors. Now he’s crazy won’t eat, can’t decide what he wants to do. If there were dog taverns he’d be there sucking beer all winter.