Why Does My Cat Have Diarrhea

Cat diarrhea causes: dietary changes, parasites, IBD, food allergies, and infections. Home care vs veterinary treatment.

First, Figure Out Which Kind of Diarrhea

Cats are stoic about GI signs, and the details that matter get missed in "my cat has diarrhea." Tell your vet whether the stool is small-bowel origin (large-volume cow-pat or watery stool, maybe vomiting, often weight loss) or large-bowel origin (small amounts, mucus, fresh blood streaks, multiple trips to the box, sometimes straining that looks like constipation). Cornell Feline Health Center notes these two patterns have completely different differential lists, diagnostics, and treatments. Note the color too: black and tarry means digested blood (upper-GI); fresh red streaks usually means lower-colon inflammation.

The Feline-Specific Differential

Cats' shorter GI tract and different immune biology make their diarrhea list look different from a dog's. In rough prevalence order:

1. Dietary — abrupt diet change, cow's milk, high-fat table food. Many adult cats are lactose intolerant; the "saucer of milk" myth is a classic trigger.
2. Parasites — especially Tritrichomonas foetus, Giardia, and Cryptosporidium. Tritrich is under-recognized outside of shelter/breeder populations (especially Bengals, Siamese, multi-cat households) and produces chronic, foul cow-pat stool that bounces back when metronidazole is stopped. Specific PCR testing is needed; it is not found on a routine fecal float.
3. Viral — feline panleukopenia (FPV), FIV, FeLV, feline coronavirus (and FIP).
4. Food-responsive enteropathy / food allergy — a huge portion of chronic cat diarrhea resolves on a strict 8-week hydrolyzed or novel-protein diet.
5. Inflammatory bowel disease (IBD) and small-cell GI lymphoma. These two are on a continuum in middle-aged and senior cats. Ultrasound shows thickened bowel wall with loss of layering; definitive diagnosis requires biopsies (often endoscopic).
6. Hyperthyroidism — the classic senior cat presenting thin, ravenous, vocal, and with soft stool. A simple total T4 screens.
7. Chronic kidney disease — uremic gastroenteritis in late-stage CKD.
8. Pancreatitis and triaditis (concurrent pancreatitis, cholangitis, IBD). Cats hide abdominal pain; often the only clues are anorexia, weight loss, and soft stool. Spec fPL plus abdominal ultrasound is the workup.
9. Bacterial — Campylobacter, Salmonella, Clostridium, enteropathogenic E. coli, especially in raw-fed cats. Zoonotic risk is real.
10. Toxin or foreign body — string foreign bodies are particularly dangerous; always lift the tongue and look for linear material.

When to Monitor, When to Call, When to Go

Monitor at home (up to 24 hours)

Reasonable only for an otherwise well adult cat that is still eating, drinking, using the litter box normally aside from soft stool, and showing no vomiting. Offer a highly digestible canned diet in small portions. Do NOT withhold food from a cat for more than a few hours — feline hepatic lipidosis is a real risk, particularly in overweight cats. Keep a fresh water source (many cats prefer a running fountain).

Same-day or next-day vet

• Diarrhea persisting beyond 24–48 hours
• Any diarrhea accompanied by decreased appetite
• Mucus or blood
• Weight loss, polyuria/polydipsia, or vocalization at the litter box
• Multi-cat household where another cat is also symptomatic

ER tonight

• Kitten, senior, or known CKD cat with any diarrhea plus not eating
• Repeated vomiting plus diarrhea, or visible blood
• Lethargy, hiding, pale/yellow gums, low body temperature, or open-mouth breathing
• Suspected ingestion of a toxin or a piece of string hanging from mouth or anus (do NOT pull)

What the Exam Room Looks Like

A feline GP visit for acute diarrhea usually includes: weight, temperature, careful abdominal palpation (for mass, bowel thickening, pain, or foreign body), hydration assessment, fecal flotation, and often a giardia ELISA. For a chronic or recurring case, expect additional testing: CBC + chemistry (kidney, liver, glucose, albumin), total T4 (senior cats), FeLV/FIV status, Spec fPL, cobalamin and folate, and Tritrichomonas PCR if the pattern fits. Abdominal ultrasound is the single highest-yield next step for chronic feline diarrhea — it is how a thickened, layered small intestine (IBD/lymphoma) is distinguished from a mass or foreign body. If ultrasound suggests infiltrative disease, endoscopic or surgical biopsies are the gold standard; cytology of a fine-needle aspirate can support a diagnosis of intermediate or large-cell lymphoma.

Cost Expectations (2026, US)

• Exam + fecal + giardia ELISA + anti-nausea injection: $130–$300
• Senior/chronic panel (CBC, chem, T4, fPL, cobalamin/folate): $300–$600
• Tritrichomonas PCR: $75–$150
• Abdominal ultrasound by a radiologist: $400–$700
• Endoscopic biopsies with anesthesia and histopath: $1,800–$3,500
• Hydrolyzed prescription diet (Royal Canin HP, Hill's z/d, Purina HA): $55–$95 per month for a single cat
• Panleukopenia hospitalization: $1,500–$4,500

Breed, Age, and Household Risk

• Bengal, Siamese, and purebred multi-cat households: Tritrichomonas foetus is over-represented. Any chronic, foul, cow-pat stool in a young Bengal deserves a PCR test, not just another dose of metronidazole.
• Kittens from shelters / rescues: Coccidia, giardia, roundworm, panleukopenia risk until vaccinated and dewormed with an appropriate protocol.
• Senior cats (10+ years): IBD vs. small-cell lymphoma is the dominant chronic differential, with hyperthyroidism and CKD high on the list.
• Overweight / obese cats: Any anorexia plus GI signs is a hepatic lipidosis risk — do not delay.
• Outdoor or indoor/outdoor cats: Campylobacter, Salmonella, plant / rodent-bait exposure.

Owner Mistakes We See Weekly

• Withholding food overnight in an overweight cat. The dog "NPO and re-feed" rule is dangerous in cats — hepatic lipidosis can set up shop in 48 hours of anorexia.
• Giving Imodium or Pepto-Bismol. Loperamide can cause severe neurologic toxicity in cats; bismuth subsalicylate contains a salicylate (aspirin-relative) that cats metabolize very poorly.
• Treating "one more round of metronidazole" as the answer to chronic diarrhea. Metronidazole can improve signs briefly but does not cure Tritrichomonas or IBD, and it disturbs the microbiome. The 2022 ACVIM consensus for chronic enteropathy in dogs increasingly informs feline practice too: sequenced dietary trial first, then targeted workup.
• Feeding "grain-free" or boutique diets as an elimination trial. Those still contain multiple novel proteins from prior foods. A true food trial uses a single hydrolyzed protein fed exclusively for 8 weeks, with no treats or flavored meds.
• Ignoring a second symptomatic cat in the house. Giardia, tritrich, panleukopenia, and parasites all spread through shared litter boxes.
• Not collecting a fresh sample. Giardia cysts degrade quickly; some labs want a sample within 30 minutes for best yield.

Home Care That's Reasonable

• Switch to a highly digestible canned food (Royal Canin Gastrointestinal, Hill's i/d, Purina EN). Wet food adds water and makes portion control easier.
• Cat-specific probiotic (FortiFlora feline, Proviable) for 10–14 days.
• Offer water in multiple ceramic or glass bowls, or a fountain; many cats will double their intake simply with a running water source.
• Psyllium (1/4 tsp per meal mixed into wet food) can help firm up large-bowel diarrhea under your vet's guidance.
• Keep litter-box hygiene meticulous; scoop multiple times daily and disinfect to prevent reinfection from Giardia or Tritrichomonas cysts.