Blood in Stool

Seeing blood in your pet's poop is one of those moments that can make your stomach drop, but it's actually a fairly common finding and doesn't always mean something dire is going on. Vets sort it into two types based on colour, and that colour tells you roughly where in the digestive tract the bleeding is coming from. Melena is black and tarry blood in the stool that happens when bleeding occurs high in the digestive tract, such as the esophagus, stomach, or small intestine, because the blood gets mixed with intestinal contents and partially digested along the way. Hematochezia is red, undigested blood in the stool, and it points to bleeding lower down, in the colon, rectum, or anus.

The tricky part is that both types have a huge range of possible causes, from mild to genuinely life threatening, and the amount of blood you see doesn't tell you how serious the problem is. A single streak of bright red blood on an otherwise normal stool is often linked to something as simple as straining, a bit of stress colitis, or a diet change. On the other end of the scale, hematochezia can also show up with conditions like parvovirus in puppies, hemorrhagic gastroenteritis, colonic tumours, or bleeding disorders. Melena tends to get taken more seriously by vets because upper GI bleeding has a narrower range of causes, several of which are serious, including stomach ulcers, tumours, or ingestion of toxins like rat poison. Worth knowing too: certain medications such as bismuth subsalicylate (Pepto-Bismol), sucralfate, and activated charcoal can turn stool dark and mimic melena without any actual bleeding involved, so mention any recent medications to your vet.

A few signs mean this isn't a wait-and-see situation:

• Large volumes of blood, or blood appearing over and over rather than once
• Vomiting alongside the bloody stool, especially if the vomit looks like coffee grounds
• Pale or white gums, weakness, collapse, or laboured breathing
• A painful, swollen abdomen
• Any blood in a puppy or young, unvaccinated dog, given the risk of parvovirus

Because even a small amount of blood in a puppy's stool should be treated as an emergency given the risk of parvovirus, which is serious and potentially fatal without prompt treatment, don't wait it out in young dogs the way you might with an older, otherwise well pet.

For a healthy adult dog or cat with just a small streak of blood and normal energy and appetite, it's still worth a call to your vet, but it isn't necessarily a middle-of-the-night dash. If you can, jot down when it started, how often it's happening, and whether the stool is otherwise diarrhoea or firm, and bring a fresh sample or a photo to the appointment. Blood work, a stool exam, and sometimes imaging are usually needed to pin down the actual cause, since the stool colour alone only narrows things down, it doesn't give a diagnosis. The guides below can walk you through specific causes and what your vet may do next.