Diarrhoea occurs when a dog has more than five bowel movements in 24 hours. Usually, these bowel movements are a lot larger and more liquid than normal. When blood is included in the diarrhoea, this is a neon sign saying that the dog needs to go to the vet immediately. Bloody diarrhoea is a sign of many potentially lethal health problems in dogs.
Blood in the stool can come in two colours. The first is very obvious: It is the same bright red colour usually seen in blood. Bright red blood in diarrhoea is technically called hematochezia. But black stool that resembles tar also shows the presence of blood. This blood, technically called melena, is black because it has gone through the digestive tract.
According to "Dog Owner's Veterinary Handbook" (Deborah M. Eldrige, DVM, et al, 2007), there are a variety of causes for bloody diarrhoea. One cause is swallowing a foreign object that is cutting the dog's organs. Another is eating decayed garbage or the body of a dead animal. There are also illnesses such as granulomatous enteritis, acute infectious enteritis and histicocytic ulceracitive colitis. The latter disease almost exclusively affects boxers for no known reason. Rat poison can cause vomiting and blood diarrhoea. Cancer can also cause bloody diarrhoea.
In order to help quickly determine the cause of the bloody diarrhoea, the vet will need to know if the dog is experiencing other symptoms such as loss of coordination, vomiting, uncontrollable panting, constantly licking the lips (a sign of nausea), a sudden loss of vision (bumping into objects) or seizures.
Treatment depends on the cause. A foreign object or damaged bowel needs surgical intervention. Enteritis requires antibiotics and corticosteroids to help reduce swelling. In the case of histicocytic ulceracitive colitis, getting the symptoms under control with intravenous fluids is a short-term treatment, but long-term treatment requires a change of diet. With rat poisioning, the dog needs vitamin K injections and, possibly, a blood transfusion.
According to "The Veterinarians' Guide to Your Dog's Symptoms" (Michael S. Garvey, DVM, et al, 1999), foul-smelling bloody diarrhoea that is bright red is the most extreme emergency, indicating that the dog in hemorrhaging internally. This is when the dog is repeatedly passing pools of blood and not much else. The dog may also show some other signs of distress such as whimpering, heavy panting and keeping the ears flattened to the head. The dog needs to be taken to a vet immediately.