Signs of gastrointestinal symptoms in dogs include diarrhea, vomiting, nausea, loose or bloody stool, bad breath and gas. These symptoms can be caused by ingesting toxins or spoiled food, poor digestion, diet, parasites or worms, liver disease, autoimmune disease, enzyme deficiencies or food intolerances. Certain autoimmune or inflammatory conditions of the intestinal tract can lead to malabsorption.