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13 Causes of Black Diarrhea in Dogs

Adrienne is a certified dog trainer and former veterinarian assistant who partners with some of the best veterinarians worldwide.

Help, My Dog Has Black Diarrhea!

Let's face it, dealing with dog poop is not fun. However, knowing what healthy dog poop looks like is a normal part of being a responsible pet owner.

Normal dog poop should be brownish, firm, and with a log-like shape. A dog that suddenly starts passing black and tarry stools is a red flag and a good reason to call the vet.

In this article, veterinarian Dr. Ivana Crnec discusses what can cause dogs to have black diarrhea—from discolorations due to eating something to a health issue such as ingesting rat poison, obstruction or perforation, and stomach ulcer.

Finally, Dr. Ivana will go through the diagnosis process and possible treatments.

Melena: The Presence of Blood in a Dog's Poop

Melena is the medical term for black-tinged stool. In addition to being black-colored, melena often has a unique tarry-like texture similar to coffee grounds. Melena indicates the presence of digested blood in a dog’s poop.

The fact that the blood is black tells us a lot about its source. Namely, it means that the blood has spent some time in the small intestine (the iron in the blood is oxygenated when passing through).

Melena is a sign that something wrong is going on in the upper parts of the dog’s intestines and stomach.

However, some melena causes are not located within the digestive system.

Closely monitor the color of your dog's stools

Closely monitor the color of your dog's stools

13 Causes of Black Diarrhea in Dogs

An array of underlying medical issues can cause black diarrhea in dogs. Some are benign and transient, while others are serious and potentially life-threatening. If your dog has black diarrhea, please play it safe and consult with your vet.

To make things simpler for understanding, let's take a look at the different black diarrhea causes and discuss their treatments.

1. Ingestion of Black-Tinged Foods

Foods like black licorice, blood sausages, and even blueberries can result in black discoloration of the poop. The fact that they can trigger stomach upsets contributes to the diarrhea component. Together, the two factors combined result in black diarrhea.

Of course, it goes without saying that dogs should not be fed human foods and harmful foods should be kept out of reach.

2. Ingestion of Certain Medications

Certain medications have similar effects. For example, if your dog was prescribed activated charcoal due to a stomach upset or dietary indiscretions, chances are it will have black-colored diarrhea.

Another medication that causes black poop in dogs is Pepto Bismol.

3. Ingestion of Blood

Ingesting blood can result in black diarrhea, especially if we are talking about a larger amount of blood. For example, if your dog got a foxtail stuck in its nose and experienced heavy nose bleeds, chances are some of the blood will be swallowed and then pooped.

Alternatively, a dog may be vomiting blood and then eating the vomit. Or for example, the dog may have an actively bleeding wound that it constantly licks.

All of these cases are serious and require veterinary management. And each of these scenarios requires a different approach.

For instance, the underlying reason for vomiting needs to be identified and treated. In the final case, the wound requires proper management – stitching or cleaning and bandaging.

4. Dietary Indigestions

Dietary indiscretions are among the dogs’ favorite things to do. For example, a dog stealing some spicy Chinese food from the countertop classifies as a dietary indiscretion. The spices are too irritating and can damage the lining of the digestive tract resulting in bleeding.

Some dietary indiscretions involve inedible things – branches, toys, rocks. In such cases, the objects may physically scrape and injure the digestive tract as they pass through, once again causing bleeding.

While some dietary indiscretion cases can be treated with symptomatic therapy and supportive care, others may require surgical management.

5. Foreign Bodies in the Digestive Tract

Some dietary indiscretion cases can progress into gastrointestinal blockages and obstructions.

Namely, if the digested item is inedible, the dog will be unable to digest it. To make things worse, it will also be unable to expel it out. In other words, the item will probably block the intestines causing an obstruction.

A dog with a foreign body in the gastrointestinal tract is an emergency. Some foreign objects are visible on x-rays, and others are not, thus making the diagnosis more challenging.

More often not, gastrointestinal obstructions require surgical removal of the foreign body.

6. Gastrointestinal Ulceration

Same as in humans, gastrointestinal ulcers in dogs occur when the gastric acid and digestive juices start damaging the lining of the stomach and intestines. Under normal circumstances, the lining is protected with a mucus layer.

However, many factors can contribute to the weakening of the layer, including prolonged and inadequate use of medications, stress, liver conditions, and even tumors.

A dog with gastrointestinal ulcers shows additional signs and symptoms (abdominal pain, cramping, lack of appetite, lethargy, behavioral changes due to pain) and requires immediate veterinary attention.

7. Rodenticide Poisoning

Anticoagulant rodenticide poisoning in dogs is more common than you might think and occurs when dogs accidentally consume rat baits.

The rodenticide works by inhibiting the activity of the K1 vitamin, which is responsible for synthesizing blood clotting factors. If these factors are lacking, bleeding will develop.

Dogs usually start manifesting signs of toxicity one or two days after ingesting the poison. The signs and symptoms include bleeding from all-natural openings and producing bloody diarrhea and vomit.

The poisoning is a medical emergency and requires long-term treatment with vitamin K supplements.

8. Heavy Metal Poisoning

Heavy metals like lead and arsenic are still commonly used in everyday items despite their potential dangers. A dog exposed to heavy metals will develop neurological and gastrointestinal issues.

In addition to these symptoms, a dog might be showing signs like confusion, loss of balance, seizures, blindness, and unusual behavioral changes.

If you suspect your dog might be suffering from heavy metal poisoning, you need to go to the vet's office as soon as possible.

9. Infectious Agents

Various infectious agents can cause black diarrhea in dogs. Common culprits include bacteria, parasites, viruses, and fungi.

All of these pathogens can cause severe bleeding either in the stomach or intestines. In both cases, the outcome is the same – bloody diarrhea.

Finding the infectious agent behind black diarrhea can be tricky and often requires various tests—blood works, abdominal scans (x-rays and ultrasounds), stool analysis, endoscopy, etc.

Once the culprit is identified, the vet will recommend a treatment strategy. Depending on the pathogen, the treatment will entail antibiotics or, in the case of intestinal parasites—parasiticides.

10. Medication Side Effect

Some medications cause gastrointestinal bleeding as a side effect. The most common medications resulting in such adverse reactions are non-steroid anti-inflammatory drugs (NSAIDs).

In dogs, NSAIDs are the most frequently prescribed anti-pain medication, especially in older dogs with arthritis.

Non-steroid anti-inflammatory drugs work by inhibiting the effects of prostaglandins, and these hormone-like chemicals are responsible for producing the mucus that protects the stomach lining.

Once the lining is unprotected, it will get damaged by the stomach acid. The consequence is bleeding.

11. Metabolic Disease

Metabolic conditions like hypothyroidism and hyperadrenocorticism can result in black diarrhea.

Hypothyroidism is the result of low thyroid levels, and it slows down the metabolism leading to a variety of gastrointestinal issues.

On the other hand, hyperadrenocorticism (Cushing’s disease) is the result of high cortisone levels due to an overactive adrenal gland.

Luckily, these issues are relatively simple to diagnose – all it takes are some blood tests. However, it should be noted that hypothyroidism and hyperadrenocorticism cannot be treated; only managed. Plus, their management is often costly.

12. Blood Clotting Disorders

The most common blood clotting disorder in dogs is Von Willebrand's disease (vWD). The condition is inherited and manifests with a genetic deficiency of a specific protein necessary for platelets formation. Platelets are critical for closing bleeding blood vessels.

The disease can occur in any dog, but it is most common among Doberman Pinschers, Scottish Terriers, Shetland Sheepdogs, and Chesapeake Bay Retrievers.

The most common signs include spontaneous bleedings, the sudden appearance of bruises on the body, and prolonged bleeding after injuries and surgeries.

13. Cancer

Stomach cancer in dogs is not particularly common. However, when it does occur, it is devastating. Stomach cancer is more likely to develop in older dogs (over nine years) of certain breeds—Staffordshire Bull Terriers, Belgian Shepherd Dogs, Rough Collies, and Chow Chows.

One of the first clinical signs indicating a stomach ulcer is black diarrhea. To diagnose the condition, the vet will start with contrast radiography and then, if necessary, perform endoscopy.

Based on the size, location, and type of cancer, possible treatment options include surgery, radiation, and chemotherapy.

Concluding Thoughts

Black diarrhea, or as it is medically termed, melena, is a scary situation for every dog owner. The condition can be triggered by an array of underlying conditions—some more serious than others.

However, the general rule of thumb is to seek veterinary help the moment your dog starts passing black diarrhea.

Dogs are sensitive, and things can progress from bad to worse in no time. The sooner your dog gets examined by a veterinarian and a diagnosis is set, the sooner the condition will be managed.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2022 Adrienne Farricelli