My first dog died of heartworms. My parents were young and not very knowledgeable on how to take care of a dog back then. But it wasn't something we couldn't notice. He was very, very ill and frail, but my dad didn't want to put him to sleep. It was horrifying to watch. I seriously think that scarred me for life. When he was finally in so much agony that he could barely walk, my dad called the vet to take him in. But he never made it, he died there in the grass in front of us. He was 10, so was I. I had him my whole life, and I don't think I ever got over that.
Sorry, just had to share.
His name was Redbone, by the way. Red, for short. He was an Irish Setter/Redbone Coonhound mix.