Why Does My Cat Have Diarrhea? Causes & When It’s Serious

Cat diarrhea usually traces back to something straightforward: a sudden diet change, a mild infection, or stress from a disrupted routine. In a large UK shelter study, roughly 12% of cats had diarrhea at any given time, with kittens being about two and a half times more likely to be affected than adult cats.¹ Most episodes resolve on their own within a day or two, but the causes range widely, and some of them demand veterinary attention quickly.

Acute Versus Chronic Diarrhea

Veterinarians draw a practical line at about three weeks. Diarrhea lasting 21 days or fewer is classified as acute; anything beyond that is chronic.² This distinction matters because acute diarrhea is often self-limiting and tied to a single identifiable trigger, while chronic diarrhea usually signals an ongoing disease process that won’t clear up without investigation. If your cat has had loose stools for more than a couple of weeks, treating it as a “wait and see” situation is no longer a good strategy.

The appearance of the stool itself gives clues about where in the gut the problem lives. Small-intestinal diarrhea tends to produce large volumes of watery or fatty stool, sometimes with weight loss but no straining. Large-intestinal diarrhea looks different: smaller amounts, often with mucus or fresh blood, and your cat may strain or rush to the litter box more frequently. Paying attention to these details before a vet visit can help your veterinarian narrow down the cause faster.

Diet Changes and Food Reactions

The single most common trigger for a short bout of diarrhea is a sudden change in food. Cats’ digestive systems adjust slowly to new proteins or fat levels, and swapping brands overnight can overwhelm the gut bacteria that handle digestion. Research has confirmed that changes in both diet and environment are linked to acute diarrhea in cats, with measurable shifts in the gut microbiome appearing alongside the loose stool.³ This is why veterinarians recommend transitioning to a new food over seven to ten days, gradually increasing the proportion of the new diet.

Food reactions go deeper than a simple adjustment period, though. Some cats develop true adverse food reactions, which can be either allergic (immune-mediated) or non-allergic intolerances. In cats, these reactions most commonly show up as diarrhea, vomiting, or skin problems like itching and hair loss.⁴ Food allergy in cats can also be associated with respiratory, neurologic, and even behavioral signs, making it trickier to pin down than you might expect.⁵ The exact prevalence of feline food allergy remains unknown, partly because diagnosing it requires an elimination diet trial rather than a simple blood test. If your cat’s diarrhea keeps returning and doesn’t respond to standard treatments, a food allergy is worth investigating with your vet.

Parasites and Protozoal Infections

Parasites are a leading cause of diarrhea in kittens and cats that spend time outdoors, though indoor cats in multi-cat households are not immune. The usual suspects include roundworms, hookworms, and the single-celled organisms Giardia and Tritrichomonas foetus. Giardia in particular can cause persistent watery diarrhea that doesn’t respond to standard deworming medications, because Giardia is a protozoan, not a worm.

Tritrichomonas foetus deserves special mention because it often flies under the radar. This organism is most commonly found in young cats from catteries and multi-cat environments. Catteries where T. foetus has been identified tend to have a recent history of diarrhea among their cats, along with crowded living conditions.⁶ To make matters more complicated, cats can carry both T. foetus and Giardia at the same time, which has been documented in kittens suffering from chronic liquid diarrhea that failed multiple rounds of treatment.⁷ Standard fecal flotation tests can miss these organisms entirely. Diagnosis often requires a fresh fecal smear examined within minutes or a specialized PCR test, so if your vet’s initial fecal check comes back clean but the diarrhea persists, ask about more targeted testing.

Coccidia are another protozoal culprit, especially in kittens. They tend to cause watery, sometimes bloody diarrhea and are typically picked up through fecal testing. The good news is that most parasitic causes are treatable once correctly identified. The frustrating part is that identification sometimes takes multiple rounds of testing.

Viral Infections

Several viruses can cause diarrhea in cats, but two stand out for their severity. Feline panleukopenia virus (FPV), sometimes called feline distemper or feline parvovirus, attacks the rapidly dividing cells in the intestinal lining and bone marrow. This leads to severe enteritis, a dangerous drop in white blood cells, and a high mortality rate, particularly in unvaccinated kittens.⁸ The hallmarks are profuse, often bloody diarrhea combined with vomiting, lethargy, and fever. This is one of the reasons the core kitten vaccination series is so important.

Feline coronavirus (FCoV) is far more common and usually far less dramatic. Most cats infected with FCoV show only mild gastrointestinal symptoms or none at all. The virus typically stays confined to the gut, causing transient soft stool at most.⁸ The concern with FCoV is the small percentage of cases where the virus mutates inside the cat’s body and triggers feline infectious peritonitis (FIP), a serious immune-mediated disease. FIP itself doesn’t primarily present as diarrhea, but the initial FCoV infection can, and multi-cat environments are hotspots for transmission.

Inflammatory Bowel Disease

When diarrhea persists for weeks or months despite standard treatments, inflammatory bowel disease (IBD) is one of the more common diagnoses in cats. Feline IBD is characterized by persistent or recurrent gastrointestinal signs driven by chronic inflammation of the intestinal lining.⁹ It is a diagnosis of exclusion, meaning your vet has to rule out parasites, infections, food reactions, and other identifiable causes first. Confirming IBD requires intestinal biopsies to examine the type and severity of the inflammatory cells in the gut wall.

This is where things get particularly tricky for cat owners and veterinarians alike. The symptoms of IBD overlap heavily with those of low-grade intestinal lymphoma, a form of cancer that can look almost identical on the surface. Cats with IBD tend to be younger (a median age around 8 years) than those with low-grade intestinal lymphoma (median around 13 years), but there is significant age overlap, with both conditions appearing anywhere from early adulthood to old age.¹⁰ Distinguishing between the two often requires advanced diagnostics beyond a standard biopsy, including special staining techniques or molecular testing. If your cat is diagnosed with IBD but doesn’t respond well to treatment, lymphoma should be reconsidered.

When Chronic Diarrhea Points to Something Else Entirely

Not all chronic diarrhea comes from a problem within the intestines themselves. Exocrine pancreatic insufficiency (EPI) occurs when the pancreas fails to produce enough digestive enzymes. In cats with EPI, food passes through the gut without being properly broken down, leading to loose, voluminous, greasy stools, weight loss despite a good or even ravenous appetite, and sometimes a greasy coat around the tail area.¹¹ EPI is less common in cats than in dogs, but it’s worth testing for in any cat with chronic diarrhea and progressive weight loss, especially when the appetite seems paradoxically normal or increased.

Hyperthyroidism, which is extremely common in older cats, can also cause diarrhea or soft stool. The overactive thyroid speeds up metabolism and gut motility, pushing food through too quickly for proper absorption. Kidney disease and liver disease can produce loose stools as well. The point here is that your vet may need to look well beyond the gut to find the real culprit, particularly in a middle-aged or senior cat.

Stress and Environmental Disruption

Cats are creatures of routine, and disruptions to their environment can trigger genuine gastrointestinal upset. Moving to a new home, introducing a new pet, boarding, a change in household schedule, or even rearranging furniture can set off a bout of diarrhea. Research has confirmed that environmental changes are a recognized trigger for acute diarrhea in cats, accompanied by measurable alterations in gut bacteria and metabolic pathways.³

Stress-related diarrhea is usually acute and resolves once the cat acclimates. However, in shelter environments, the combination of stress and close contact with other cats creates a perfect storm. That UK shelter study found that cats housed in multi-cat pens had higher odds of diarrhea than those in single housing.¹ If you’ve recently adopted a cat from a shelter and they have loose stools, stress is a very plausible explanation, but it’s still worth a vet check to rule out infectious causes that are common in group housing.

Age Matters More Than You Might Think

Kittens and senior cats sit at opposite ends of a risk spectrum. Kittens are roughly two and a half times more likely to develop diarrhea than adult cats, largely because their immune systems are still developing and they’re more susceptible to parasites and viral infections.¹ In kittens, even common infections like Giardia or coccidia can escalate quickly because of their smaller body reserves.

Senior cats face a different risk profile. That same study found that severe diarrhea was over four and a half times more likely in senior cats than in younger adults.¹ In older cats, chronic diarrhea is more likely to reflect systemic disease: hyperthyroidism, kidney disease, IBD, or intestinal lymphoma. A senior cat with persistent loose stool deserves a thorough workup, including blood tests and possibly imaging, rather than a simple course of deworming medication.

When Diarrhea Becomes an Emergency

Most cat diarrhea doesn’t warrant a midnight trip to the emergency clinic, but a few situations call for urgent care. The biggest danger with diarrhea, especially in kittens and small cats, is dehydration. A case report of an 8-month-old cat with Giardia-related diarrhea illustrates how quickly things can deteriorate: the cat arrived at the clinic lethargic with an estimated 10% dehydration, rapid heart rate, dry gums, and poor skin turgor after about seven days of watery diarrhea.¹² That level of dehydration is life-threatening and required intravenous fluid therapy to correct.

Call your vet or go to an emergency clinic if you notice any of the following alongside diarrhea:

Blood in stool: Bright red blood or dark, tarry stool suggests bleeding somewhere in the digestive tract.
Lethargy or collapse: A cat that won’t get up, seems weak, or is unresponsive needs immediate attention.
Vomiting plus diarrhea: The combination accelerates fluid loss dramatically, especially in kittens.
Refusal to eat or drink: A cat that goes more than 24 hours without eating is at risk for hepatic lipidosis, a dangerous liver condition.
Known toxin exposure: If your cat has ingested a household chemical, a toxic plant, or a human medication, diarrhea may be just one symptom of poisoning.
Kittens under 6 months: Young kittens can dehydrate within hours. Any diarrhea lasting more than a day in a kitten should be treated as urgent.

You can do a rough dehydration check at home by gently pinching the skin between your cat’s shoulder blades and releasing it. In a well-hydrated cat, the skin snaps back immediately. If it returns slowly or stays tented, your cat is already significantly dehydrated.

What to Do at Home and What to Avoid

For an otherwise healthy adult cat with a single episode of soft stool or mild diarrhea and no other symptoms, a brief period of dietary simplification is reasonable. Offering a bland, easily digestible food (plain cooked chicken or a veterinary gastrointestinal diet) for a couple of days can help the gut settle. Keep fresh water available at all times, since cats with diarrhea lose fluid faster than normal.

What you should not do is reach into your medicine cabinet. Many human over-the-counter medications, including ibuprofen and acetaminophen, are toxic to cats even in small doses. Loperamide (the active ingredient in some human anti-diarrheal products) is not recommended for cats without veterinary guidance, as it can mask symptoms and slow down the elimination of infectious organisms. Even well-intentioned home remedies like milk can backfire, since most adult cats are lactose intolerant and dairy will make diarrhea worse.

Diets with specialized fiber sources have shown promise for managing diarrhea in cats. A study found that cats fed a therapeutic diet with fiber sources rich in antioxidant and anti-inflammatory compounds had better clinical outcomes than those on traditional fiber sources, with all cats on the therapeutic fiber showing positive or complete resolution by day 28.¹³ Your vet can recommend specific gastrointestinal or fiber-enriched diets if dietary management seems appropriate for your cat’s situation.

The Diagnostic Puzzle

One of the frustrating realities of feline diarrhea is that figuring out the cause can be harder than it looks. Interpreting the clinical relevance of organisms found in a cat’s stool is often misunderstood and can lead to inappropriate treatment or overtreatment.¹⁴ A cat can test positive for Giardia, for example, and that Giardia may have nothing to do with the diarrhea. Many cats carry organisms like feline coronavirus or low levels of various bacteria without ever showing symptoms. Just because a pathogen shows up on a test doesn’t mean it’s the culprit.

This is why veterinarians sometimes use a stepwise approach rather than throwing every test at the problem on day one. For a first episode of acute diarrhea in an otherwise well cat, a vet may recommend dietary adjustment and monitoring before running extensive diagnostics. For chronic cases, the workup typically escalates: fecal testing, blood panels, possibly imaging with ultrasound, and ultimately biopsies if less invasive tests don’t yield an answer. Research into the gut microbiome of cats with diarrhea has identified distinct bacterial patterns in sick versus healthy cats, with cats suffering from diarrhea showing increases in certain bacterial groups and decreases in others.² These findings are still largely in the research phase, but they hint at a future where microbiome testing could become a more routine part of workup.

Probiotics and Microbiome-Based Approaches

Probiotics have gained popularity in feline medicine, though the evidence base for cats specifically is still catching up to what’s available for dogs and humans. Much of the current understanding of how probiotics might benefit cats with digestive issues is extrapolated from research in other species.¹⁵ That said, some veterinarians do recommend feline-specific probiotic formulations as a supportive measure for cats with diarrhea, particularly after antibiotic use or during dietary transitions. These products are generally safe, but their effectiveness varies and they’re unlikely to resolve diarrhea caused by an underlying disease. Think of them as a supplement to proper veterinary care, not a replacement for it.

Fecal microbiome transplant, in which stool from a healthy donor cat is processed and given to a sick one, is an area of active research interest. It’s been used in dogs and humans with some success for certain conditions, and the concept is being explored for cats as well. For now, this remains a niche intervention rather than something your average vet clinic offers.

Medications That Cause Diarrhea

If your cat is already on medication for another condition, the drug itself could be the source of loose stools. Antibiotics are the most common offenders, as they disrupt the balance of gut bacteria. Cats given broad-spectrum antibiotics for an unrelated infection, like a respiratory illness or wound, frequently develop soft stool or diarrhea as a side effect. Non-steroidal anti-inflammatory drugs (NSAIDs), when used in cats, can irritate the gastrointestinal lining. Chemotherapy agents, corticosteroids at high doses, and certain dewormers can also trigger diarrhea.

If your cat develops diarrhea shortly after starting a new medication, contact your vet. They may adjust the dose, switch to a different drug, or add a protective measure for the gut. Never stop a prescribed medication without veterinary advice, as abruptly discontinuing certain drugs can cause problems of its own.

1
PubMed Central. Faecal consistency and risk factors for diarrhoea and constipation in cats in UK rehoming shelters
2
PLOS ONE. The Fecal Microbiome in Cats with Diarrhea
3
PubMed Central. Identification of Gut Microbiome and Metabolites Associated with Acute Diarrhea in Cats
4
PubMed. Adverse food reactions: Pathogenesis, clinical signs, diagnosis and alternatives to elimination diets
5
PubMed Central. Food allergy in the cat: a diagnosis by elimination
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PubMed Central. Prevalence of and risk factors for feline Tritrichomonas foetus and giardia infection
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PubMed Central. Coinfection with Tritrichomonas foetus and Giardia duodenalis in Two Cats with Chronic Diarrhea
8
Theoretical and Natural Science. An Integrated Review of Feline Calicivirus, Panleukopenia Virus, Herpesvirus, and Coronavirus: From Viral Entry to Disease Control
9
PubMed Central. Feline idiopathic inflammatory bowel disease: what we know and what remains to be unraveled
10
Journal of Veterinary Internal Medicine. ACVIM consensus statement guidelines on diagnosing and distinguishing low‐grade neoplastic from inflammatory lymphocytic chronic enteropathies in cats
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PubMed. Exocrine pancreatic insufficiency in the cat
12
Buletin Veteriner Udayana. CASE REPORT: TREATMENT OF GIARDIASIS COMPLICATED BY SEVERE DEHYDRATION IN A DOMESTIC CAT
13
The FASEB Journal. Food with Specialized Dietary Fiber Sources Improves Clinical Outcomes in Adult Cats with Constipation or Diarrhea
14
SAGE Journals (Journal of Feline Medicine and Surgery). Feline enteropathogens and molecular diagnostics: benefits, limitations and clinical applications
15
PubMed Central. Probiotics and Cat Health: A Review of Progress and Prospects