Chronic diarrhea in cats almost always points to one of a handful of causes: a food sensitivity, an inflammatory condition in the gut, a parasite that standard tests miss, or an underlying disease like hyperthyroidism or kidney problems. A single episode of loose stool is rarely concerning, but if your cat has had diarrhea for more than two or three weeks, or it keeps coming back, something specific is driving it.
The Most Common Causes
When diarrhea persists in an adult cat and isn’t clearly tied to something they ate, the problem is usually rooted in the gastrointestinal tract itself. The three big categories are inflammatory conditions, infections or parasites, and food reactions. Less commonly, the cause sits outside the gut entirely: an overactive thyroid gland, kidney or liver disease, or intestinal lymphoma (a type of cancer that develops in the lymph nodes).
What makes chronic diarrhea tricky is that many of these causes look identical from the outside. A cat with inflammatory bowel disease and a cat with a food allergy can both have the same soft, messy stool for months. That’s why identifying the actual cause often requires a process of elimination rather than a single test.
Food Sensitivities and Allergies
Food reactions are one of the most common and most fixable causes of ongoing diarrhea. Cats can develop sensitivities to proteins they’ve eaten for years, so the fact that your cat has been on the same food “forever” doesn’t rule this out. The immune system can start reacting to a previously tolerated protein at any point in life.
The only reliable way to diagnose a food allergy is an elimination diet trial, where your cat eats a single protein source they’ve never been exposed to before (or a specially processed food where the proteins are broken down small enough that the immune system doesn’t recognize them). This trial needs to run at least six to eight weeks to be accurate. At eight weeks, the test catches more than 90% of food allergies in cats. During this period, your cat can eat nothing else: no treats, no table scraps, no flavored medications. Even small exposures can keep the immune reaction going and make the trial look like it failed when it didn’t.
If the diarrhea clears up on the new diet, your vet will typically have you reintroduce the old food to confirm the reaction. When symptoms return, you have your answer.
Inflammatory Bowel Disease
Inflammatory bowel disease (IBD) is a catch-all term for conditions where the immune system sends too many inflammatory cells into the wall of the intestine. The cause isn’t fully understood, but it appears to involve a dysfunctional interaction between the immune system, gut bacteria, diet, and environmental factors. It’s one of the most common diagnoses in cats with chronic diarrhea that doesn’t respond to dietary changes alone.
A definitive IBD diagnosis requires a biopsy of the intestinal wall, examined under a microscope to confirm elevated levels of inflammatory cells. This means either endoscopy or surgery, which is why vets often try less invasive approaches first, like food trials and bloodwork. A blood panel measuring vitamin B12 and folate levels can offer clues: low B12 suggests disease in the lower part of the small intestine (where B12 is absorbed), while high folate levels point to bacterial overgrowth in the upper small intestine. Neither result is conclusive on its own, but together with other findings, they help narrow things down.
Parasites That Standard Tests Miss
If your cat’s fecal tests have come back clean but the diarrhea continues, a parasite called Tritrichomonas could be responsible. This single-celled organism is surprisingly common, particularly in cats from catteries or multi-cat households. One survey of cattery cats found it in 31% of those tested. It causes chronic large-bowel diarrhea that can contain blood or mucus, and the stool often has a particularly foul smell.
The frustrating part is that Tritrichomonas is frequently misidentified as Giardia under a microscope because the two organisms look similar. They move differently (Tritrichomonas darts around erratically while Giardia drifts like a falling leaf), but telling them apart requires an experienced eye. A PCR test, which detects the organism’s DNA in a stool sample, is the most reliable way to get an accurate diagnosis.
Many cats infected with Tritrichomonas stay active, playful, and maintain their weight despite having loose stools for months or even years. The diarrhea sometimes resolves on its own, only to relapse later. This pattern of a seemingly healthy cat with persistent soft stool is a hallmark of the infection.
Causes That Differ by Age
Kittens and young cats are far more likely to have infectious causes: parasites like Giardia, coccidia, or Tritrichomonas, along with viral infections. Their immune systems are still developing, and exposure in shelters, catteries, or multi-cat environments is extremely common. Dietary indiscretion (eating things they shouldn’t) also plays a bigger role in younger cats.
In middle-aged and older cats, the list shifts toward chronic inflammatory conditions, food sensitivities that develop over time, hyperthyroidism, kidney disease, and cancer. Hyperthyroidism in particular is worth flagging because it’s very common in cats over 10 and causes diarrhea alongside increased appetite, weight loss, and restlessness. A simple blood test can detect it, and it’s highly treatable.
How to Assess Your Cat’s Stool
Before your vet visit, paying attention to what the stool actually looks like will help your vet narrow the possibilities faster. Veterinarians use a seven-point scale to grade stool consistency. Ideal cat stool scores a 2 or 3: firm, log-shaped, and holds together when you scoop it. A score of 4 (log-shaped but soggy, loses form when picked up) is mildly abnormal. Scores of 5 through 7 represent progressively worse diarrhea, from shapeless moist piles to completely watery puddles with no texture at all.
Also note where your cat is straining, whether there’s mucus or blood, and how frequently they’re going. Large-bowel diarrhea (from the colon) tends to involve small, frequent amounts with mucus or bright red blood and straining. Small-bowel diarrhea produces larger volumes less frequently, sometimes with weight loss. This distinction helps your vet decide which part of the digestive tract to focus on.
The Role of Probiotics
Probiotics won’t replace proper diagnosis, but they can genuinely help as part of a treatment plan. One well-studied strain, Enterococcus faecium SF68, significantly reduced the incidence of diarrhea lasting two or more days in a controlled study of shelter cats given the supplement for eight weeks. Other strains, including certain Lactobacillus and Bacillus species, have been shown to firm up stool, reduce harmful bacteria, and lower fecal moisture in cats.
Look for veterinary-specific probiotic products rather than human formulations, since the strains and doses are tailored to cats. Probiotics work best alongside other interventions (like a diet change) rather than as a standalone fix for chronic diarrhea with an unknown cause.
Signs That Need Urgent Attention
Chronic diarrhea on its own warrants a vet visit, but certain combinations of symptoms move the timeline up significantly. Weight loss that’s visible within days rather than weeks, complete refusal to eat for more than 24 hours, yellowing of the gums or eyes, extreme lethargy, or collapse all require immediate veterinary care. Rapid weight loss paired with not eating is especially dangerous in cats because their livers are uniquely vulnerable to damage when they stop taking in calories, a condition called hepatic lipidosis that can become life-threatening within days.
Gradual weight loss alongside ongoing diarrhea in an otherwise alert and eating cat is still a red flag, but one that can typically wait for a scheduled appointment rather than an emergency visit. The key question is whether your cat is still eating and drinking. If yes, you have time to pursue a methodical diagnostic workup. If not, that changes the urgency.