The most common cause of dark stool in dogs is the presence of digested blood in the stool. This is a condition known as "Melena" and is potentially serious. Although less common, iron toxicity in your dog's system can also result in dark stools. Consult your veterinarian if your dog is passing black, smelly stools on a regular basis.
Causes of Melena
Melena is caused by bleeding from the dog's upper gastrointestinal tract. This could be a result of a stomach ulcer, bleeding in the small intestine or cancer. Intestinal bleeding can be caused by certain drugs, bleeding disorders, a variety of gastrointestinal conditions, or foreign bodies lodged in the stomach or intestine. If the blood remains in the colon long enough, bacteria breaks it down and makes it turn black. This mixes with the feces and results in the stools turning black and tarry.
Iron toxicity in dogs happens when the dog accidentally ingests iron or other vitamin supplements. Dogs are unable to eliminate excess iron from their bodies, and this can result in bleeding disorders or bloody diarrhea. Either of these conditions will cause the dog's stools to turn black. Iron toxicity needs to be treated as a matter of urgency or the dog may have liver failure or go into a coma.
Testing for Melena
Take the dog to your veterinarian, along with a fresh stool sample. The veterinarian will conduct a series of tests including a urinalysis, a diagnosis of the stool sample and blood tests. He may also take X-rays of the dog's chest and abdomen, and an ultrasound of the abdominal area. Once the results of the tests are available, he will be able to tell you if your dog has any serious conditions that require treatment.
Treatment for Melena
If the Melena is not caused by a more serious condition, treatment for the dark stools is relatively simple. The veterinarian may recommend a special diet of bland or home-cooked food, and provide medication such as Famotidine that will coat the intestines and block stomach acid.