Diarrhea lasting more than 3 weeks is chronic, and the order of tests matters more than the tests themselves. Feces first, because parasites are common and cheap to find. Then a decision about small versus large bowel, because that determines whether a food trial or a colon workup comes next. Food trials run 4 to 8 weeks, not 5 days. Ultrasound and biopsy come last, when the simple steps have failed or when the cat is losing weight fast.
Why the order matters
Chronic diarrhea in cats comes from four broad buckets: parasites, food-responsive disease, chronic enteropathy such as inflammatory bowel disease (IBD), and small-cell lymphoma. Cheap tests come first: a Giardia infection found on day 3 ends the workup and costs a fraction of an endoscopy.
Meanwhile, two things need measuring at every visit: weight and albumin. A cat losing 5% of body weight per month, or with albumin under 2.0 g/dL, has a protein-losing problem and should not be sitting in a 6-week food trial at home. That pattern moves ultrasound up the list. General causes are reviewed in causes of diarrhea in cats.
Step one: the feces, three samples
Collect 3 samples over 3 to 5 days, because parasite shedding is intermittent and one negative flotation proves little. Ask for a zinc sulfate flotation rather than a simple smear, plus a Giardia antigen test or a PCR panel. PCR panels catch Giardia, Tritrichomonas foetus, Cryptosporidium, and toxigenic Clostridioides, and they are worth the cost in a cat under 2 with chronic diarrhea.
Tritrichomonas is the one people miss: large-bowel diarrhea with urgency and gas in young cats, treatable with ronidazole, invisible on a routine float. Shelter and breeder cats are the classic cases. Worms are less common indoors, but a 3 to 5 day fenbendazole course at 50 mg per kg is cheap enough to justify.
Step two: small bowel or large bowel
Where the diarrhea comes from narrows the plan. Small-bowel disease gives large volumes, 2 to 4 times a day, weight loss, and often vomiting; the cat may eat well and still lose weight. Large-bowel disease gives small volumes, 5 or more times a day, mucus, urgency, straining, and blood, with weight often holding steady.
Blood tests sharpen the picture. Cobalamin (B12) is absorbed in the ileum, so a low value points at ileal disease. Folate is absorbed in the proximal small intestine, so a low folate suggests proximal disease, while a high folate suggests bacterial overgrowth. Both low means diffuse small-bowel disease. A low B12 combined with low albumin is a cat that needs imaging, not another diet.
Step three: the food trial
A hydrolyzed-protein diet is the standard trial, and it has to be strict: 4 weeks minimum, 8 weeks for a full answer, no treats, no table food, no flavored medication, no other cat's bowl. Hydrolyzed means the protein is broken into fragments too small for the immune system to recognize. A novel-protein diet works by the same logic when the cat has never eaten rabbit, venison, or insect protein.
Most food-responsive cats improve within 2 to 4 weeks. If there is no change at all by week 4, do not extend to week 12 — move to the next step. If the cat improves and then relapses on re-challenge, that confirms the diagnosis without any biopsy. Switching technique is covered in how to switch cat food.
What deworming and probiotics actually do
Deworming is worth one proper course: fenbendazole 50 mg per kg daily for 3 to 5 days covers roundworms, hookworms, and Giardia partly. Praziquantel covers tapeworms, which rarely cause diarrhea but are common. Repeating the same dewormer every month without a fecal test is the pattern that hides the real problem.
| Intervention | Realistic effect |
|---|---|
| Fenbendazole, 3 to 5 days | Clears roundworms and hookworms; partial effect on Giardia |
| Probiotics | Strain-dependent, modest; may shorten a mild flare, will not fix IBD |
| Psyllium or pumpkin fiber | Helps large-bowel diarrhea with mucus; can worsen small-bowel disease |
| Metronidazole, 5 to 7 days | Reduces inflammation and anaerobic bacteria; not a cure, and repeated courses should be questioned |
Probiotics are not interchangeable: a named strain with a count in the billions is worth 2 weeks, a flavored chew is not, and none replaces a fecal test.
Step four: ultrasound and biopsy
Ultrasound answers questions a blood panel cannot: bowel wall thickness, lymph node size, pancreatic changes, and free fluid. Normal feline small-intestinal wall is under about 3 mm when measured in a relaxed loop; thickening plus enlarged lymph nodes raises IBD or lymphoma. Ultrasound also finds the triaditis pattern — inflammation of bowel, pancreas, and liver together — which is common in older cats.
Biopsy is the only way to separate IBD from small-cell lymphoma, and the two look identical on ultrasound. Full-thickness surgical biopsy is the gold standard; endoscopic biopsies are easier on the cat but miss disease deeper in the wall. A practical middle path many vets use: treat for IBD for 3 to 4 weeks with diet plus prednisolone, and biopsy only if there is no response.
Monitoring while you wait
Weigh the cat weekly on the same scale and write it down. Track stool score on a 1 to 7 scale, where 1 is hard pellets and 7 is liquid, and record how many times a day. Recheck albumin and cobalamin at 4 to 6 weeks. A cat whose weight is stable and stool score improves from 6 to 4 is responding, even if the stool is not normal yet.
Supplement cobalamin when it is low: 250 to 500 μg per cat by injection weekly for 4 to 6 weeks, then monthly. It is cheap, safe, and cats with low B12 often eat better and gain weight once it is replaced. Do not bother with B12 if the level is normal — it does nothing for a cat that is not deficient.
When to refer
- Albumin under 1.5 g/dL, or any cat with fluid in the abdomen.
- Weight loss above 10% despite a strict 4-week diet trial.
- Vomiting plus diarrhea plus jaundice, which suggests triaditis.
- No response to diet and a 3-week prednisolone trial.
- A young cat with severe signs, where intestinal lymphoma or a congenital problem is possible.
Referral means an internal medicine specialist with ultrasound and endoscopy on site. Send records ahead, including every fecal result and the diet timeline.
What to do this week
- Collect 3 stool samples over 5 days and request flotation plus PCR.
- Weigh the cat and start a weekly weight log with stool scores.
- Ask for albumin, cobalamin, and folate on the next blood draw.
- Pick one hydrolyzed diet and remove all other food, treats included, for 4 weeks.
- Book a recheck at 4 weeks even if the cat looks better, so the response is documented.
Frequently asked
How long before I can judge a hydrolyzed diet?
Four weeks is the minimum to see a trend, and 8 weeks gives a full answer. Most food-responsive cats show firmer stool and better appetite within 2 to 3 weeks. If nothing changes at all by week 4, the trial is unlikely to work later, so move on to imaging or a biopsy discussion rather than extending it.
Can probiotics replace a fecal test?
No. Probiotics can shorten a mild flare and may reduce gas, but they do not identify Giardia, Tritrichomonas, or worms, and they do not treat inflammatory bowel disease. A fecal flotation plus PCR costs less than 3 months of supplements and actually tells you what is wrong.
When does chronic diarrhea mean lymphoma?
There is no single sign. Older cats with weight loss, thickened bowel on ultrasound, and enlarged lymph nodes are the ones where small-cell lymphoma is suspected, and only biopsy separates it from IBD. A practical approach is a 3 to 4 week trial of diet plus prednisolone, then biopsy if there is no response.