Many years ago this happened to my poor dog Booger (a border collie, black lab mix). He was 6 months old when we got him from a friend whose dog had puppies. He had heartworms. We boarded him at the vet while the treatment went on- about a week. When he got home, he was very weak and would not eat his food. I made him chicken and rice which he did eat. Eventually got well and lived 10 more years. No apparent bad outcome.