Acute colitis in cats (large intestine diarrhea)
Acute colitis is a sudden inflammation of the large intestine, the last part of the digestive tract, also called the colon. The colon's main role is to absorb water from stool and store it before evacuation. When it becomes irritated, the c...
What is it?
Acute colitis is a sudden inflammation of the large intestine, the last part of the digestive tract, also called the colon.
The colon's main role is to absorb water from stool and store it before evacuation. When it becomes irritated, the cat visits the litter box very frequently, strains, and passes only a small amount of soft stool each time, often slimy and sometimes streaked with bright red blood.
Acute colitis is not a disease with a single cause: it is a clinical pattern. It can result from a change in diet, a parasite, an infection, or stress. The veterinarian's task is to recognize it and then find 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 trips to the litter box, with only a small amount of stool each time.
Soft or liquid stools, often covered with mucus—a slimy substance resembling raw egg white.
Streaks of bright red blood on the stool: this is bleeding from the end of the digestive tract, striking in appearance but common in colitis.
Prolonged straining, with the cat squatting, standing, and squatting again, sometimes while crying out.
Stool deposited outside the litter box, because the urge comes on suddenly and the cat does not have time to reach the box.
The cat generally remains alert and continues to eat. Weight loss is uncommon in colitis: when present, it more often points to a small intestine problem or a chronic disease.
A sudden onset in a cat that was well just days earlier is typical.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Too-rapid change in diet, table scraps, poorly tolerated food.
Intestinal parasites: worms, coccidia, giardia, tritrichomonas.
Bacterial digestive infections.
Stress: moving house, boarding, arrival of a new animal, home construction.
Ingestion of an object or irritating material.
In many cases, the episode is short-lived and no specific cause is found.
Recent change in diet.
Period of stress: moving, boarding, new pet.
Untreated intestinal parasites.
Young age.
Living in a group setting: shelter, breeding facility, household with multiple cats.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian first checks that the cat is urinating normally: in a cat straining in the litter box, a urinary blockage must be ruled out first, as this is a much more serious condition.
He palpates the abdomen, examines the area around the anus, and takes the temperature.
A stool analysis looks for parasites; it is often repeated, since some parasites are not shed every day.
The veterinarian asks how long this has been going on: a few days suggests acute colitis, while more than three weeks suggests a chronic cause.
More extensive tests—blood work, ultrasound, and screening for bacteria—are reserved for cats that are lethargic, feverish, losing weight, or whose diarrhea is not improving.
Treatment
In most cases, treatment is straightforward and focuses on keeping the cat comfortable while the colon heals.
Very easily digestible food, or conversely food enriched in fiber according to your veterinarian's recommendation, often helps normalize the stool within a few days.
Antiparasitic treatment is prescribed if a parasite is identified or cannot be ruled out.
Antibiotics are not automatic: most cases of acute colitis do not require them.
The cause is corrected once identified: returning to the regular food, reducing stress.
Do not give your cat any human medication for diarrhea: several are toxic to cats.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Change to a new food gradually by mixing the old and new together over several days.
Avoid table scraps and milk.
Have your cat treated regularly for internal parasites, based on his lifestyle and your veterinarian's advice.
Anticipate periods of stress by keeping his familiar surroundings: the same food, the same litter box, and a quiet place.
Possible complications
Dehydration, especially in kittens or elderly cats.
Irritation of the area around the anus from repeated straining and bowel movements.
Progression to chronic disease if the cause persists.
In young cats that strain heavily, prolapse of part of the rectum through the anus.
When to see a veterinarian
In a male cat, straining in the litter box without passing anything can signal a urinary blockage: seek emergency care and check whether he is urinating.
You see a large amount of blood, or black-colored stools: consult your veterinarian the same day.
Your cat is lethargic, has a fever, or is refusing to eat: consult your veterinarian the same day.
Diarrhea has lasted more than a few days without improvement.
It is a kitten or an elderly cat.
Something red is protruding from his anus.
Prognosis
Acute colitis is most often benign: most cats improve within a few days with simple treatment.
If signs persist or recur, the prognosis depends on the underlying cause that is identified.
Frequently asked questions
My cat is straining in the litter box: how can I tell if it's colitis or a urinary problem?
Look at what he's leaving in the litter box. In colitis, you find small soft stools, often mucoid. In a urinary blockage, there are no stools and no wet litter clumps. In a male straining without producing anything, consider it a urinary emergency until proven otherwise.
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 is not necessarily serious. Large quantities of blood, or black stools, however, require seeing a doctor the same day.
What is the difference with small intestine diarrhea?
In colitis, the cat goes to the litter box very frequently in small amounts, with mucus and straining. With small intestine disease, it defecates less often but in large liquid quantities, and it may lose weight. This distinction helps the veterinarian direct their examination.
What if the diarrhea has been going on for more than three weeks?
We no longer speak of acute colitis. Diarrhea that persists or recurs must prompt investigation for a chronic cause: parasites such as tritrichomonosis, inflammatory bowel disease, food intolerance, or, more rarely, tumor.
References
Content reviewed by the TunisieVet editorial team · Updated 11 October 2026
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