Cat diarrhea is most often caused by a sudden diet change, a mild infection, or stress, and it typically resolves within a day or two. But when it persists, looks unusual, or comes with other symptoms like vomiting, it can signal something more serious. Understanding the likely cause helps you figure out whether to wait it out or call your vet.
Dietary Triggers
The most common reason for a short bout of diarrhea is something your cat ate. Switching food brands or flavors too quickly is a classic trigger. A cat’s digestive system needs time to adjust to new proteins and fat levels, so any food transition should happen gradually over five to seven days, mixing increasing amounts of the new food with the old.
Some cats develop true food allergies, which cause chronic or recurring diarrhea rather than a single episode. If your vet suspects a food allergy, the standard approach is a diet trial lasting three to eight weeks. During this time, your cat eats only a diet containing proteins they’ve never been exposed to before (called a novel protein diet) or a specially processed formula where the proteins are broken down small enough that the immune system doesn’t react to them. No treats, no table scraps, nothing else during the trial period.
Cats that get into garbage, eat houseplants, or swallow small objects can also develop diarrhea. Foreign material in the gut sometimes causes a partial obstruction, which is a more urgent situation.
Parasites and Infections
Intestinal parasites are extremely common in cats, especially those that go outdoors or live in multi-cat households. Roundworms are the most frequently seen parasite and can cause diarrhea, vomiting, and appetite loss, particularly in kittens. Hookworms feed on blood in the intestinal wall, leading to diarrhea and weight loss in mild cases and anemia in severe ones.
Two single-celled parasites deserve special mention. Coccidia can destroy the intestinal lining in kittens, causing mucousy diarrhea, though adult cats usually carry them without symptoms. Giardia infects the small intestine and can cause acute or chronic diarrhea, but most infected cats actually show no signs at all.
A less well-known parasite called Tritrichomonas foetus is sometimes misidentified as Giardia on routine stool exams. If your cat has been treated for Giardia and isn’t improving, this parasite is worth testing for. It requires a specific PCR test on a fresh stool sample (free of cat litter, which can interfere with the test).
Kittens Face Extra Risks
Kittens are more vulnerable to diarrhea for several reasons. Their immune systems are still developing, the stress of weaning and rehoming disrupts their gut, and they’re more susceptible to parasites they encounter for the first time. In most cases, diarrhea in a kitten that is otherwise bright, alert, eating, and drinking points to diet changes, stress, or parasites.
The more dangerous possibility is feline panleukopenia, a highly contagious virus that causes vomiting, severe diarrhea, and can kill rapidly. Kittens under four months old are at highest risk, even if vaccinated, because maternal antibodies can interfere with vaccine effectiveness at that age. A kitten with watery diarrhea and vomiting who seems lethargic or refuses food needs veterinary attention the same day.
Chronic Conditions Behind Ongoing Diarrhea
When diarrhea lasts weeks or keeps coming back, the cause is usually something beyond a simple stomach upset. Inflammatory bowel disease (IBD) is one of the more common culprits. In IBD, inflammatory cells build up in the walls of the digestive tract, thickening them and interfering with normal digestion and nutrient absorption. Cats with IBD often have diarrhea alongside vomiting, weight loss, bloody stools, low energy, and reduced appetite.
Diagnosing IBD requires ruling out a long list of other possibilities: parasites, bacterial infections, hyperthyroidism, and cancer. Intestinal lymphoma, a form of cancer that affects the gut, can look nearly identical to IBD on imaging and blood work. Distinguishing the two usually requires biopsies of the intestinal wall. Hyperthyroidism, which is common in older cats, speeds up metabolism and can cause persistent soft stools or diarrhea along with increased appetite, weight loss, and restlessness.
What Your Cat’s Stool Is Telling You
Not all diarrhea is the same, and the appearance of your cat’s stool provides real clues. Veterinarians use a seven-point fecal scoring system that ranges from hard, dry pellets (score 1) to completely watery puddles with no texture at all (score 7). Healthy cat stool falls around a 2 or 3: firm, log-shaped, and holds its form. Scores of 5 through 7 represent true diarrhea of increasing severity.
Color matters too. Dark red or black stool suggests bleeding higher up in the digestive tract, such as the stomach or small intestine, where blood gets partially digested before passing through. Bright red blood on or in the stool points to bleeding lower down, in the colon or rectum. Mucus coating the stool can indicate inflammation, dehydration, or parasites. Yellow stool is potentially the most urgent, as it can relate to liver disease, certain poisonings, or dangerous bacterial overgrowth.
Signs That Need Immediate Attention
A single episode of soft stool in an otherwise healthy adult cat is rarely an emergency. But certain combinations of symptoms change the picture quickly:
- Vomiting plus diarrhea together: this combination accelerates fluid loss and is always considered urgent.
- Bloody or black stool: indicates active bleeding somewhere in the digestive tract.
- Yellow stool: possible liver involvement requiring prompt evaluation.
- Lethargy, refusal to eat, or unusual behavior: suggests your cat’s body is struggling beyond a simple gut issue.
For kittens, senior cats, pregnant cats, or cats with existing health conditions, diarrhea that doesn’t resolve within 24 hours warrants a vet visit. These groups dehydrate faster and tolerate illness less well than healthy adults.
How to Check for Dehydration at Home
Diarrhea pulls water out of your cat’s body quickly, so monitoring hydration matters. You can check two things at home. First, gently lift the skin over your cat’s shoulders and release it. In a well-hydrated cat, the skin snaps back into place almost immediately. In a dehydrated cat, it returns slowly or stays “tented” in a pinched position. One caveat: older cats naturally lose skin elasticity, so this test is less reliable in senior pets.
Second, look at your cat’s gums. They should be moist and slick. Dry or tacky gums are a sign of dehydration. If you notice either of these signs alongside diarrhea, your cat likely needs fluids that go beyond what a water bowl can provide.
Managing Mild Diarrhea at Home
For an otherwise healthy adult cat with soft stool and no alarming symptoms, a short period of dietary simplification often helps. A bland diet of boiled white rice mixed with plain boiled chicken breast (no skin, no bones, no seasoning) can give the gut a chance to settle. The typical ratio is about 75% rice to 25% chicken. Feed small amounts several times a day rather than one or two large meals.
Probiotics designed for cats can also make a difference. A large controlled study of shelter cats found that those given a specific probiotic strain (Enterococcus faecium, the active ingredient in products like FortiFlora) were significantly less likely to have diarrhea lasting more than two days compared to cats given a placebo. These come as powdered sachets you sprinkle on food.
Keep fresh water available at all times. Some cats drink more readily from a running water fountain than a still bowl. If your cat tolerates it, you can also offer low-sodium chicken broth (with no onion or garlic, which are toxic to cats) to encourage fluid intake.
What Happens at the Vet
If diarrhea persists or your cat shows warning signs, your vet will likely start with a stool sample analysis. A fecal flotation test catches most common parasites like roundworms, hookworms, and coccidia by floating their eggs to the surface of a prepared solution. For harder-to-detect organisms like Tritrichomonas or to confirm Giardia, a PCR test is more accurate. The standard fecal smear only catches Tritrichomonas about 14% of the time, so a negative smear doesn’t rule it out.
Beyond stool tests, blood work can screen for metabolic problems like hyperthyroidism or kidney disease, and abdominal imaging (X-rays or ultrasound) helps identify masses, obstructions, or thickened intestinal walls that suggest IBD or lymphoma. For chronic cases, the diagnostic process takes patience. Your vet is systematically crossing conditions off a list, and the workup can span several visits before landing on a clear answer.