Black Stools in Dogs: What Are They Due To?

A dog's stool tells a lot about its general health. Unfortunately, black stools can indicate a very serious underlying pathology.
Black Stools in Dogs: What Are They Due To?
Samuel Sanchez

Written and verified by the biologist Samuel Sanchez.

Last update: 09 May, 2023

Dogs defecate from one to 5 times a day when they’re puppies, although the number of daily stools depends on the dog’s size, age, physical constitution and diet. The appearance of the stools can tell us a lot about its health, since the color, texture, presence of foreign bodies and many other things indicate digestive processes. So, what are black stools in dogs due to?

As unpleasant as it may sound, knowing the normal colors of your dog’s stools can help you detect an illness or disease before it gets worse. In this article, we’ll tell you why dogs’ stools are sometimes black and what this means. If you want to know more, read on.

The normal color of dog feces

Before exploring the reasons for black stools in dogs, you’ll need to have some basic notions about the normal intestinal transit in dogs. The greatest indication of health in this area is the deposition of normal stools, as a change in color, consistency, and composition may indicate a gastrointestinal or systemic problem.

The normal color of a dog’s stools should be brown, something similar to the shade of milk chocolate. This shade is a sign that the liver is functioning properly, as bilirubin in the bile (excreted by the hepatic system) gives the stool this color. However, some slight changes may take place depending on the diet and hydration of the dog.

In addition to color, professional sources also guide their judgment by the consistency of the stool. There are scales that are numbered from 1 to 7, with 7 being a rocky consistency and 1 being basically water. The perfect balance is found in a stool that is moist, yet compact enough not to lose its shape when handled.

In general, these are the signs of a healthy gastrointestinal tract in dogs if we are talking about their stools:

  1. Chocolate brown-colored stools
  2. Stool size is equivalent to the amount of food ingested.
  3. Unpleasant odor (this is normal), but not extremely nauseating.
  4. Elongated stool form, which remains with movement or transport.
  5. Moist stools, but not too watery.

Although feces in good condition don’t confirm the health of the animal, they’re a clear indication of well-being.

A dog with a poo.

What causes black stools in dogs?

In humans, black stools are indicative of bleeding in the stomach. If the patient bleeds in the upper digestive tract for any reason, the blood passes through the stomach and is digested, hence it turns from a red to a tarry black color. In dogs, this rule is also true, but there are more causes. Let’s look at them in depth.

1. Injury to the digestive tract

As we have said, a black, foul-smelling stool in dogs usually indicates a bleeding injury to some portion of the upper digestive tract, be it the stomach, lower esophagus, or other associated organs. This has most likely been caused by ingestion of a sharp material, such as a hard toy, stone, or needle, or even some pathology such as gastric ulcers.

The reason the stool comes out black in this type of scenario is because of the transit of blood in the body. As it’s digested, the red color of the blood turns into a tar black, plus it can give off an unpleasant odor during the process.

2. Parasites

Some parasitic worms insert themselves into the walls of the intestine and feed on the host’s blood and nutrients. Perhaps the most problematic microorganism of this group for dogs is Toxocara canis, a whitish “worm” 9 to 18 centimeters (3.5 to 7 inches) long that settles in the intestinal area of the animal, causing various symptoms.

Black feces in dogs can appear in this case, although it’s more common that these are reddish, since the bleeding occurs at the intestinal level. Other symptoms of this condition are vomiting, constipation, diarrhea, weight loss, and general malaise. For any of these clinical signs, the veterinarian will perform a coprological culture of the pet to detect the parasite.

3. Hemorrhagic gastroenteritis

This condition is characterized by two clear symptoms: diarrhea and blood in the stools. Its causes aren’t known, although it’s thought that it could be a response to harmful bacteria or the endotoxins they produce. At the pathological level, there’s an increase in the dog’s intestinal permeability, which promotes the filtration of blood and proteins.

Hemorrhagic gastroenteritis requires a visit to the veterinarian in all cases, as it can be lethal. With the indicated fluid therapy and preventive application of antibiotics, the mortality rate is less than 10%. Unfortunately, 10-15% of affected dogs will have recurrent cases.

4. Cancer

Cancer occurs when a group of cells mutate in their DNA, which disrupts their cycles of death and division. Because they don’t die when they should, these cells give rise to the dreaded tumor, which, in turn, can spread to other parts of the body in metastatic cases.

Gastric cancer can cause the dreaded black stools to appear in dogs. This sign is accompanied by weight loss, general listlessness, eating problems, and lethargy. The prognosis depends on the stage of the cancer and the general health of the affected dog.

5. Clotting problems

A low platelet count in the canine’s blood plasma and other conditions can cause their blood not to clot properly. In turn, this causes bleeding to occur anywhere in his body, including the upper digestive tract. Ingestion of rat poison often causes clotting problems in dogs.

6. Diet

Black stools can also be caused by too much iron in the diet. This usually happens when the dog’s food is based on a BARF-type diet or when homemade food is offered to them. Some viscera such as liver contains large amounts of iron, so its consumption modifies the coloration of the defecation.

The solution for this cause is usually easy, since it’s only necessary to reduce the consumption of this type of food. Once the diet is modified, in little more than a month the dog’s stools should return to their usual color.

7. Drugs

Some drugs such as non-steroidal anti-inflammatory drugs (NSAIDs), bismuth, iron sulfate or metronidazole can also cause black stools. However, this effect is harmless and only occurs with long-term treatment.

How are black stools treated?

Treatment for black stools in dogs depends very much on the cause of the problem. If the origin of this comes from some injury or disease, black stools usually disappear after controlling the condition. When the causes are due to the ingestion of drugs or food, the only thing to do is to suspend their consumption and replace them if necessary.

Although it’s true that the most frequent causes are harmless and resolve in a few weeks, it’s necessary to go to the veterinarian to provide a good diagnosis. Keep in mind that in emergency cases surgical intervention may be necessary, so don’t overlook the presence of black stools.

Other colors in dog stools

We have already told you why dog stools can be dark black. By way of closing, we’re going to show you in the following list other fecal shades that you should know about:

  • Red: Red stools are a sign of a bleeding intestine. It may also be due to an anal fissure.
  • Gray and white: These are signs of pancreatic problems.
  • Green: This may indicate a gallbladder problem or that the dog has simply consumed grass.
  • Orange and yellow: Indicates a liver problem.

Of course, not all possible triggers for abnormally colored stools are serious diseases. Sometimes the dog has simply consumed something that sporadically stains the stools, but in that case, the strange color of the stools shouldn’t last for more than 24 hours.

A dog doing a poo.

As you have seen, there are many causes of black stools in dogs. All of them require veterinary attention, so don’t leave this clinical sign unattended under any circumstances. If the atypical stools are accompanied by other symptoms (vomiting, apathy, loss of consciousness and weight loss), the need for urgent help is even greater.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

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This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.