Roundworms migrate throughout the blood into the lungs, are coughed up, and usually re-swallowed. Sometimes the larvae can travel through the liver and brain. You may never see these worms, and one day one may come out in the dog's stool. They can cause bloating, diarrhea and vomiting. Your dog may stop eating, after passing a stage of overeating, and always being hungry. In young puppies untreated roundworms can cause the bowel to rupture.