Diarrhoea in Dogs: Causes, What to Do and What Tests

Diarrhoea in a dog is a common problem: sometimes it is a one-off reaction to a food change, and sometimes a sign of infection, poisoning or internal organ disease. Let's break down why a dog has diarrhoea, when it is dangerous, what to do at home and what tests help find the causes.

Common Causes of Diarrhoea in Dogs

• Diet errors: a food change, overeating, spoiled food, table scraps.
• Parasites: worms and protozoa (especially in puppies).
• Infections: viral (parvovirus in puppies — dangerous) and bacterial.
• Stress: travel, a move.
• Organ disease: pancreas, liver, inflammatory bowel disease.
• Toxins and foreign bodies.

When Diarrhoea Is Dangerous (Red Flags)

See a vet urgently if diarrhoea comes with:

• blood (bright red or black tarry stool);
• repeated vomiting, refusing food, severe lethargy;
• lasts more than 1–2 days or is very frequent/watery (dehydration risk);
• in a puppy, an old or small dog — seek help sooner.

Bloody Diarrhoea and Diarrhoea with Vomiting

Blood in the stool always warrants a vet visit: causes range from colitis to infections and foreign bodies. Diarrhoea together with vomiting is dangerous for fast dehydration and may indicate poisoning, infection or obstruction — see also vomiting in dogs.

Blood in the Stool: Bright Red or Black

What the blood looks like tells you where it came from.

Bright red blood — streaks or drops at the end of a stool, sitting on its surface. It has not been digested, so the source is low down: the colon and rectum. Colitis is the usual reason — the dog strains often and long, passes very little, sometimes with mucus. Unpleasant, but usually less worrying than the second pattern.

Black tarry stool — sticky, glossy, with a sharp smell. That colour comes from digested blood, so the source is higher up: the stomach or the small intestine. It is more worrying than bright red, and it warrants a work-up even when the dog looks bright.

What puts blood into the stool:

• haemorrhagic gastroenteritis — a heavy bloody diarrhoea in which fluid is lost within hours; the dog weakens in front of you, and this is an emergency;
• parasites — worms and protozoa, especially in puppies and after scavenging;
• a foreign body — a stick, a bone, fabric: it injures the bowel wall and brings blood along with abdominal pain, and a dog X-ray is what confirms it;
• infections and poisoning, less often tumours and clotting disorders.

⚠️ Blood plus lethargy, pale or greyish gums, vomiting or refusing water means the clinic today, not "let's watch until tomorrow": that combination points to ongoing blood loss or severe dehydration, and a puppy or a small dog reaches a dangerous state within hours. Blood noticed in the urine rather than the stool is a different story, covered separately in blood in dog urine. Once the episode is over, the usual ration comes back gradually — how exactly is described in feeding after poisoning.

What to Do at Home (No Red Flags)

If the dog is active, without blood or vomiting: ensure water (dehydration is the main risk), a light fast of up to 12 hours (not for puppies!), then feed small portions of a bland diet. Do not give human anti-diarrhoea medicines without a prescription. No improvement in 1–2 days — see a vet.

What Tests to Run

Most often: a stool test (parasites, infections), a complete blood count and biochemistry — to assess inflammation, dehydration and organs; their interpretation is in dog blood test. If a foreign body or obstruction is suspected — ultrasound for dogs and cats. If the dog is also not eating — dog not eating.

To understand your pet's tests in plain language, upload the form (PDF or photo) to the pet results interpretation service. This helps you understand the result but does not replace a vet.

If the diarrhoea keeps coming back for weeks, or starts after every change of food, the cause is more often chronic bowel inflammation or a food reaction: how that is confirmed and what to feed is covered in gastritis and food allergy. When the bowel wall itself, the lymph nodes and the pancreas need looking at, an abdominal ultrasound for dogs is ordered — it also explains what echogenicity and free fluid mean in the report. If the dog has turned weak and sleepy alongside the diarrhoea, that state is unpacked in lethargic dog, and the whole picture can be described on the dog symptoms page.

This article is informational. Diagnosis and treatment of your pet are the job of a veterinarian.