Bloody diarrhea in a dog can indicate bleeding in the upper or lower gastrointestinal tract, according to WebMD. The condition can be a symptom of hemorrhagic gastroenteritis, which is often caused by pathogens in food, petMD notes. A dog experiencing bloody diarrhea should be examined by a veterinarian.

Hemorrhagic gastroenteritis, an inflammation of the stomach and small intestine that is also known as stomach flu, is a potentially fatal condition that requires immediate medical care, notes petMD. In addition to bloody diarrhea, symptoms include vomiting, weight loss, dehydration and listlessness. The condition is caused by fungi, parasites, viruses or bacteria. It is treated with antibiotics and other medications, replacement of fluids and electrolytes, and surgery in some cases.

Causes of diarrhea in dogs include changes in diet, allergic reactions, bacterial and viral infections, liver or kidney disease, medications, stress, and ingestion of poisonous or foreign substances, according to WebMD. The condition may begin and end quickly, or it can recur for weeks or months. Dogs experiencing diarrhea should not be fed for 12 to 24 hours but should be given water to avoid dehydration. A dog suffering from diarrhea should be seen by a vet if the condition persists for more than 24 hours, or if stool is bloody or is accompanied by vomiting, lethargy or decreased appetite.