A cat has chronic diarrhea when loose stool has gone on for three weeks or more, either every day or on and off. At that point it is no longer an upset stomach. It is a sign of something else, such as a food sensitivity, a parasite, inflammatory bowel disease, an overactive thyroid or intestinal lymphoma, and your veterinarian's job is to find out which.
Your job is the evidence: what the litter box looks like, how often your cat goes, and whether your cat is losing weight. And if your cat is also off food, lethargic or vomiting, don't wait for the three-week mark.
What normal cat poop looks like
Scooping the box is the closest thing owners get to a daily health check, so it pays to know your cat's normal.
- Color: medium to deep brown. Gray, yellow or very pale stool, visible blood, mucus or worms are not normal.
- Texture: formed and firm, but not dry and crumbly. It holds its shape when you scoop it.
- Smell: all poop smells, but a sudden change to a much stronger, foul smell is worth noting.
There is no single "right" number of bowel movements a day. What matters is your cat's own pattern. A cat that suddenly goes more often, strains, or has accidents outside the box has changed, and that change is what your vet will ask about.
When diarrhea counts as chronic
Diarrhea means stool that is softer or more watery than normal, usually passed more often. You can use the same simple grades vets use:
- Normal: moist, with a clear shape that only loses form when picked up.
- Soft: some texture but no real shape, a bit like cottage cheese.
- Diarrhea: watery, with no texture. It puddles and has to be wiped up rather than scooped.
A short bout after a sudden food change or a stressful week often settles within a day or two. Diarrhea becomes chronic when it lasts three weeks or longer, whether it is there every day or keeps coming back (Purcell and Cook, dvm360; ACVIM consensus statement, 2023). You may see slightly different cutoffs elsewhere; three weeks is the one the ACVIM consensus panel uses.
Other signs often come with it: mucus or blood, straining, vomiting, a change in appetite, lethargy, and accidents outside the box or soiled fur under the tail.
Watch the weight, not just the stool. In cats with chronic intestinal disease, weight loss is the most common sign, and the ACVIM panel notes that owners often overlook it. Some cats with serious gut disease, including intestinal lymphoma, have little or no diarrhea at all. A cat that is getting thinner needs a vet check even if the litter box looks fine.
Small bowel or large bowel? The clues in the litter box
Vets sort chronic diarrhea into two broad types, because the part of the gut involved changes which causes and tests come first. Often you can tell which type from the litter box.
What you notice | Small intestine | Large intestine (colon) |
|---|---|---|
How often | Normal or a little more often | Much more often |
How much | Normal or large amounts | Small amounts each time |
Urgency, accidents | Usually not | Often |
Straining | Usually not | Often |
Mucus (slime) | Usually none | Often |
Blood | Dark, black, tarry (digested) | Bright red (fresh) |
Weight loss | Possible | Uncommon |
Vomiting | More likely | Less likely |
Based on the Merck Veterinary Manual table on small vs large intestinal diarrhea and its chapter on colitis.
Plenty of cats show a mix of both. Treat the table as a clue for the conversation with your vet, not a diagnosis.
What causes chronic diarrhea in cats
Food. Some cats react to an ingredient, usually a protein, and have ongoing diarrhea until the food changes. Milk is a common culprit too: Cornell's Feline Health Center points out that many cats are lactose intolerant and can get stomach upsets from dairy, cartoon kittens notwithstanding.
Parasites. These are most common in kittens and young cats, especially from shelters, catteries and multicat homes.
- Giardia causes soft, pale, smelly diarrhea that can come and go. Our guide to Giardia in cats covers the tests and the cleanup that stops reinfection.
- Tritrichomonas (T. blagburni, still often called T. foetus) causes long-running large-bowel diarrhea with mucus or blood and a sore bottom, mostly in young cats from catteries and other crowded homes. According to CAPC, it is often mistaken for Giardia, there is no approved treatment, and the diarrhea can take up to two years to settle.
- Coccidia are common in cats, more so in young ones, and can cause diarrhea with weight loss (CAPC). Worms and other gut parasites also show up on fecal tests.
Inflammatory bowel disease (IBD). The gut lining fills with inflammatory cells and stops working properly. Diarrhea, vomiting, appetite changes and weight loss come and go. Read more in our article on IBD in cats.
Intestinal lymphoma. Low-grade (small cell) lymphoma of the intestine looks very much like IBD from the outside and tends to affect older cats. Only a tissue biopsy tells the two apart. See our post on lymphoma in cats.
Overactive thyroid (hyperthyroidism). Common in middle-aged and older cats. Weight loss despite a big appetite, thirst, restlessness, vomiting, diarrhea and larger stools are classic signs (Merck Veterinary Manual).
Other organs and infections. Problems with the pancreas (including pancreatitis and a pancreas that stops making digestive enzymes), liver disease and kidney disease can all upset the gut. Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) are tested for too, especially in outdoor cats and cats from multicat homes.
How your vet finds the cause
There is rarely a single test for chronic diarrhea. Vets work through it in steps, simple and common causes first, and the order depends on your cat. A typical path:
- History and exam. Questions about diet, treats, deworming, other pets, medicines and supplements, and the stool itself, plus a full physical exam that includes weight and hydration.
- Fecal tests. Stool tests for parasites, often repeated, because one sample can miss them. A Giardia antigen test or a PCR test for Tritrichomonas may be added.
- Blood and urine tests. A blood count, chemistry panel and urinalysis look for kidney, liver and other problems. Cats over 7, or with signs that suggest it, get a thyroid test (T4). FeLV and FIV tests, vitamin B12 (cobalamin) and folate levels, and pancreas tests are common add-ons.
- A diet trial. If your cat is otherwise stable, your vet may prescribe a hydrolyzed or novel-protein diet. The Merck Veterinary Manual says it should be the only food for a minimum of 4 to 6 weeks, with no treats unless your vet approves them.
- Imaging. Abdominal ultrasound shows thickened intestines, enlarged lymph nodes and masses. X-rays help most with blockages or foreign objects.
- Biopsy. When diarrhea or weight loss continues, small tissue samples taken by endoscopy or surgery are the only reliable way to tell IBD from intestinal lymphoma.
Some cats get a firm answer from a fecal test. Others need several rounds, and sometimes the cat's response to treatment is what finally confirms the cause. It can feel slow, but each step rules something out.
Start a stool diary
The step 1 questions are how vets place the problem in the gut, so a few notes on your phone go a long way. Until the appointment, write down:
- The date and the stool grade: normal, soft or diarrhea
- How often your cat went, and any accidents outside the box
- Straining, mucus, or blood (fresh red, or dark and tarry)
- Vomiting, appetite and energy that day
- Food and treats, including anything new, and any medicines or supplements
Your vet tracks weight at each visit, which is one more reason not to skip rechecks. A clear photo of an abnormal stool helps too, and when you book, ask whether to bring a fresh stool sample.
How chronic diarrhea is treated
Treatment follows the cause, which is why the workup matters. Depending on what your vet finds, the plan may include:
- A prescription diet (highly digestible, high-fiber, or hydrolyzed or novel-protein), sometimes long term
- Medicine for parasites, with a follow-up stool test
- Vitamin B12 if levels are low, which can firm up stool in cats with chronic gut disease
- Anti-inflammatory medicine for IBD once other causes are ruled out
- Treatment for an underlying condition such as hyperthyroidism or a pancreas problem
- Fluids and hospital care for cats that are dehydrated or weak
Your veterinarian chooses the medicine and the dose. Please don't try human anti-diarrheal products or home remedies first: some over-the-counter medicines are harmful to cats, and they can hide the very signs your vet needs to see.
Many causes can be managed well, but some are managed rather than cured. IBD is controlled with diet and medicine and can flare up again. Low-grade intestinal lymphoma is usually a slow-moving cancer.
When to call your vet
How fast | What you see |
|---|---|
Call now | Severe or bloody diarrhea; diarrhea with weakness or signs of dehydration; black, tarry stool with weakness or pale gums |
Call today | Black, tarry stool, even if your cat seems fine; a kitten with diarrhea; diarrhea lasting more than a day or two with poor appetite, lethargy or vomiting |
Book a visit soon | Loose stool on and off for weeks; weight loss, even with normal stool; straining or mucus in the stool |
Mention at your next visit | A short bout after a food change that cleared up, so it goes in your cat's record |
Severe or prolonged diarrhea can lead to significant dehydration (VCA). Black, tarry stool can mean bleeding in the stomach or small intestine (Merck Veterinary Manual), and Cornell notes that kittens are at higher risk.
Common questions
Will chronic diarrhea go away on its own? A short bout after a diet change often does. Diarrhea that has lasted three weeks usually has a cause that needs finding, and some causes, like Tritrichomonas, can drag on for months.
Can my other cats catch it? Some causes, like Giardia and Tritrichomonas, spread between cats through stool. CAPC advises keeping infected cats away from others and cleaning the litter box often.
Originally written by Dr. Catharina Hjorth. Updated by the I Love Veterinary editorial team.
Sources
- Merck Veterinary Manual: Differentiation of Small Intestinal from Large Intestinal Diarrhea
- Merck Veterinary Manual: Colitis in Small Animals
- Merck Veterinary Manual: Disorders of the Stomach and Intestines in Cats
- Merck Veterinary Manual: Introduction to Digestive Disorders of Cats
- Merck Veterinary Manual: Disorders of the Thyroid Gland in Cats
- Marsilio S, et al. ACVIM consensus statement on chronic enteropathies in cats. J Vet Intern Med, 2023
- Purcell S, Cook AK. How to manage feline chronic diarrhea, Part I: Diagnosis. dvm360
- Purcell S, Cook AK. How to manage feline chronic diarrhea, Part II: Treatment. dvm360
- CAPC: Trichomoniasis
- CAPC: Giardia
- CAPC: Coccidia
- Cornell Feline Health Center: Diarrhea
- Cornell Feline Health Center: Feeding Your Cat
- VCA Animal Hospitals: Diarrhea in Cats
This article is for general information and is not a substitute for advice from your veterinarian.
