Acute colitis in dogs (large intestinal diarrhea)
Acute colitis is a sudden inflammation of the large intestine, the final part of the digestive tract, also called the colon. The colon's main job is to absorb water from the stool and store it before evacuation. When irritated, the dog need...
What is it?
Acute colitis is a sudden inflammation of the large intestine, the final part of the digestive tract, also called the colon.
The colon's main job is to absorb water from the stool and store it before evacuation. When irritated, the dog needs to go outside very frequently, strains, and passes only a small amount of soft stool each time, often covered in mucus, sometimes streaked with bright red blood. The dog may have accidents in the house because the urge comes on suddenly.
Acute colitis is not a disease with a single cause—it is a clinical presentation. It can result from dietary indiscretion, stress, parasites, or infection. The veterinarian's job is to recognize it and then determine its underlying cause.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Very frequent urges to defecate, with only a small amount of stool each time.
Soft or liquid stools, often covered in mucus, a slippery substance that resembles raw egg white.
Streaks of bright red blood on the stool—this is bleeding from the end of the digestive tract, alarming but common in colitis.
Prolonged straining: the dog crouches, gets up, and does it again.
Accidents in the house in a normally housetrained dog, or urgent requests to go outside, including at night.
The dog usually remains alert and continues eating. Weight loss is rare in colitis—if present, it suggests large intestinal disease or a chronic condition instead.
Black stools are not part of colitis—they indicate bleeding higher up in the digestive tract.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Dietary indiscretion: table scraps, garbage, bones, spoiled food picked up on walks.
Too-rapid food change.
Intestinal parasites, particularly whipworms, which live in the large intestine, and giardia.
Bacterial intestinal infections.
Stress: boarding, travel, moving, arrival of a new animal.
In many cases, the episode is brief and no specific cause is identified.
Recent dietary indiscretion or food change.
Stress: boarding, travel, new animal.
Untreated intestinal parasites.
Dog that picks up and swallows everything found on walks.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The vet will ask how long it has been going on, what the dog may have eaten, and whether it is vaccinated and dewormed.
The vet examines the dog, palpates the abdomen, takes the temperature, and checks the anal area.
A stool sample is tested for parasites; it is sometimes repeated because some parasites are not shed every day.
A puppy or young unvaccinated dog that is dull or vomiting is tested for parvovirus.
Further tests, blood work or ultrasound, are reserved for dull or feverish dogs, those losing weight, or those whose diarrhea is not improving.
Treatment
In most cases, treatment is straightforward and aims to keep the dog comfortable while the colon repairs itself.
A highly digestible diet, or conversely one enriched in fibre according to veterinary advice, often helps normalize stools within a few days.
Antiparasitic treatment is given if a parasite is identified or cannot be ruled out.
Antibiotics are not routinely used: most cases of acute colitis do not require them.
The cause is addressed when identified: return to regular food, stress reduction.
Do not give any human anti-diarrhoea medication without veterinary advice: some are dangerous for dogs.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Switch food gradually by mixing the old and new over several days.
Do not give table scraps and prevent access to the bin.
Watch what your dog picks up on walks.
Have your dog regularly dewormed according to lifestyle and veterinary advice.
Pick up droppings promptly in the garden: eggs of certain worms survive a long time in soil.
Possible complications
Dehydration, especially in puppies or older dogs.
Irritation of the anal area from repeated straining and bowel movements.
Progression to chronic disease if the cause persists.
When to see a veterinarian
Your puppy or young dog is unvaccinated, is vomiting and dull: seek emergency care.
You see large amounts of blood or black stools: consult the same day.
Your dog is dull, has a fever, refuses to drink or has pale gums: consult the same day.
The diarrhea has lasted several days without improvement.
It is a puppy or an older dog.
The signs have lasted more than three weeks, or return regularly.
Prognosis
Acute colitis is most often mild: most dogs improve within a few days with straightforward treatment.
If the signs persist or return, the prognosis depends on the underlying cause once identified.
Frequently asked questions
What is the difference with small intestinal diarrhea?
In colitis, the dog passes stool very frequently in small amounts, with mucus and straining. With small intestine disease, such as giardiasis, the dog defecates less often but in large amounts of soft or liquid stool, and may lose weight. This distinction helps the veterinarian direct his diagnostic tests.
Colitis, Parvovirus, or Hemorrhagic Gastroenteritis: How to Tell Them Apart?
In colitis, the dog generally remains alert and eats. Parvovirus primarily affects unvaccinated puppies or young dogs, with repeated vomiting, fever, and lethargy. Hemorrhagic gastroenteritis begins suddenly, with very bloody diarrhea and the dog becoming prostrate within a few hours. Both of these conditions require seeing a veterinarian the same day.
Red blood on stools, is it serious?
Bright red streaks of blood are common in colitis, as the bleeding comes from the end of the digestive tract. This warrants a consultation, but it is not necessarily serious. Large quantities of blood, or black stools, however, require seeing a doctor within the day.
What if diarrhea has been lasting for more than three weeks?
We no longer speak of acute colitis. Diarrhea that persists or recurs prompts investigation of a chronic cause: whipworms, inflammatory bowel disease, food allergy, or in young Boxers and French Bulldogs, a particular form of chronic colitis.
References
Content reviewed by the TunisieVet editorial team · Updated 10 October 2026
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