Why Does My Cat Have Diarrhea? Investigations & Treatment


Nov 14, 2023
Why Does My Cat Have Diarrhea? Investigations & Treatment
Although this article has been written by a veterinarian and is based on scientific research and expert knowledge, it is intended for informational purposes only. It does not replace consulting your own veterinarian and establishing a patient-doctor relationship. To support our efforts, this site contains affiliate links to products we recommend and find helpful, which may earn a small commission at no extra cost to you. Read more.
Table of Contents


Diarrhea is the passage of stools with an increased water content, ranging from a soft to liquid consistency.

It's important not to confuse diarrhea with discharge from the anal glands or vagina, or with vomit you find. Also, constipated cats may pass small amounts of liquid feces that leak around the impacted stools, which can be mistaken for diarrhea.

Many owners at some point find themselves asking, 'Why does my cat have diarrhea?' Since diarrhea is a common symptom of many diseases, this article shares helpful ways to determine the most likely cause, and you will discover:

  • All the causes of diarrhea in cats (keep in mind they may have more than one problem contributing to their diarrhea)
  • How to tell if the problem is with the small or large intestine and why that matters
  • How to narrow down the likely causes of diarrhea based on how long it lasts (acute vs. chronic), your cat's age (young vs. older), and their overall health (well vs. showing other signs of illness)
  • A plan for investigating and treating diarrhea in cats

Purina fecal / diarrhea scoring chart
Purina Fecal Scoring Chart
An easy-to-use tool that helps you to describe your cat's stools and monitor their response to treatment. Stools should ideally be score 2.

Causes of Diarrhea in Cats

Cause Description
Food-Related Issues (Transient) Sudden changes in diet, new treats, spoiled food, eating table scraps, or consuming small prey (bugs, reptiles, etc.)
Infectious Bacteria, viruses, protozoa and worms
Stress Travel, boarding, hospitalization, or significant changes in routine or environment
Foreign Material Ingestion of non-food items that become stuck and obstruct their intestines or irritate their intestines as they pass through
Inflammatory Dietary-responsive, fiber-responsive, antibiotic/probiotic-responsive, or steroid-responsive
Cancer Lymphoma (most common), adenocarcinoma, or mast cell tumor
Toxins and Drug Side Effects Toxins such as chocolate and plants or medications such as amoxicillin-clavulanate
Disorders Outside the Intestines Kidney disease, liver disease, pancreatitis, exocrine pancreatic insufficiency (EPI), and hyperthyroidism

1. Food-Related Issues (Transient)

Cats eating something that temporarily aggravates their intestinal tract is a common cause of diarrhea. Signs should start to improve within 2-4 days, and potential triggers include:

  • Sudden diet changes. Switching to a new brand or flavor of food without a gradual transition.
  • Treats and table scraps. Introducing a new treat or feeding table scraps or titbits.
  • Lactose intolerance. Feeding adult cats milk, as most are lactose intolerant.
  • Spoiled food. Consumption of wet food left out for too long or scavenging from the garbage.
  • Raw food. Feeding cats raw food that contains bone shards can irritate the intestines.
  • Prey. Hunting and consuming birds, rodents, insects, or small reptiles.
  • Weaning kittens. Transitioning kittens from milk to solid food.

Things cats can eat that give them diarrhea

2. Infectious Causes of Diarrhea

The majority of infectious cases of diarrhea resolve on their own or with supportive care, such as fluids, a high-quality diet, anti-diarrheal oral paste (e.g. Pro-Pectalin), and probiotics (e.g. Proviable). Practicing good hygiene is also important in the management of infectious diseases, such as wearing gloves, promptly cleaning up accidents, removing poop stuck to your cat, optimizing litter box cleanliness, and handwashing.

However, some infections may require a specific treatment, such as tritrichomonas, giardia, and worms, or can be life-threatening and require intensive supportive care, such as panleukopenia in kittens and unvaccinated adults.

Infectious causes of diarrhea are usually more prevalent in cats who are younger, immunocompromised (e.g. FeLV/FIV positive or receive immunosuppressive medications), eat raw food, hunt, live in a multicat household, have access to the outdoors, lack preventative care, or were recently exposed to other cats (such as fostered kittens).

If the cause of your cat's diarrhea is zoonotic (an infection that can potentially be transmitted to people), such as campylobacter, clostridium, salmonella, giardia, or cryptosporidium, and there are children or immunocompromised individuals in the household, it's important to discuss a treatment plan with your physician and veterinarian to reduce the risk of transmission. The Worms & Germs Blog is also a helpful resource on zoonotic diseases in pets.


  • Panleukopenia. Also known as feline parvovirus, it is a highly contagious and life-threatening disease, especially in young kittens who often deteriorate rapidly. Clinical signs include loss of appetite, vomiting, and weakness. If diarrhea is present, it is typically liquid, relatively large in volume, and, in severe cases, bloody (reddish-brown). Core feline vaccinations protect against panleukopenia.
  • Feline enteric coronavirus. A common intestinal virus that occasionally causes diarrhea, which is usually mild and resolves without treatment.1 Rarely, the virus mutates and is able to infect macrophages (a type of white blood cell), allowing it to leave the intestines and travel around the body, causing feline infectious peritonitis (FIP). Clinical signs of FIP include lethargy, anorexia, fever, fluid buildup in the belly and around the lungs, and eye and neurological abnormalities. Detecting feline coronavirus in the feces does not mean a cat has FIP.
  • Other viruses, such as rotavirus, astrovirus, torovirus, and reovirus, may cause diarrhea that resolves without treatment or with supportive care.


  • Campylobacter, clostridium, and salmonella may trigger diarrhea, however, it should be noted that they can also be found in healthy animals without diarrhea, so identifying them in the feces does not confirm them as the cause.2-10
  • Diarrhea caused by bacteria usually resolves on its own or with supportive care.
  • The use of antibiotics needs to be carefully considered as they may further unbalance the natural gut flora (there are trillions of beneficial bacteria in a cat's guts) and promote bacterial resistance, which in the long term is detrimental to both animal and human health.
  • Antibiotics may be considered if a pathogenic bacteria has been identified in the feces or is strongly suspected and the patient is severely unwell (fever, lethargy, etc.), showing signs or at risk of sepsis (infection and inflammation spreading throughout the body), has a weak immune system, or is not responding to other treatments.
  • Review the ACVIM Consensus Statement for further information on antibiotic use in the treatment of intestinal bacteria in cats and dogs.


  • Giardia. It can be found in healthy cats who do not have diarrhea.7-10 If diarrhea occurs, it may resolve after a few days, although if your cat is unwell or the diarrhea is persistent, they may be treated with fenbendazole and/or metronidazole. Diarrhea is often mild but, in some cases, can become watery.
  • Tritrichomonas. It typically affects kittens and young cats. Diarrhea is often intermittent, particularly foul-smelling, frequent, urgent, and may contain blood and mucus. In severe cases, fecal incontinence may be present, in which cats leak liquid feces, and their anus may become swollen and painful. Although cats are typically otherwise well and the diarrhea eventually resolves, it can take up to two years. Therefore, it is often treated with ronidazole, a medication that is used off-label, handled with disposable gloves, and discontinued if there are neurological side effects.
  • Cryptosporidium. Diarrhea usually resolves on its own or with supportive treatment. Many cats infected with cryptosporidium have no clinical signs, so identifying it in the feces does not mean it is the cause of the diarrhea.8-12
  • Cystoisospora. Also known as coccidiosis, it can cause diarrhea in kittens or cats with weakened immune systems. Infections usually resolve with supportive care. However, a veterinarian may prescribe antiprotozoal medications in severe cases or when the condition persists.13


  • Roundworms, hookworms, whipworms, and tapeworms can potentially cause diarrhea.
  • Diarrhea is more likely to be seen in kittens with worms than adult cats, however, for any cat experiencing diarrhea, ensuring their worming is up-to-date with an appropriate dewormer (e.g. Drontal) is recommended.

Haw's Syndrome in cats is when both third eyelids are elevated and become visible. While the cause is unknown, it is often associated with gastrointestinal issues, such as intestinal parasites or a mild virus known as torovirus. Diarrhea may be present and underlying gastrointestinal issues should be addressed. Ensuring worming is up-to-date is often a good idea. Resolution of the third eyelids usually occurs within several weeks, but unwell or persistent cases may require further diagnostics.

Haw's Syndrome in Cats

3. Diarrhea Due to Stress

Undoubtedly, stress impacts health and could potentially be the trigger for your cat's diarrhea if it only occurs after an event they find unpleasant and lasts a few days. Stressful events include car rides, boarding, visiting the groomers, hospitalization, surgery, storms, or significant changes in their routine or environment, such as guests staying or an owner traveling.

However, stress as the sole cause of a cat's diarrhea is considered uncommon. A veterinarian's assessment is necessary to rule out other potential underlying causes, and depending on the severity and duration of the symptoms, further investigations may be recommended.

Feliway (a calming pheromone spray) and Zylkene (a calming milk protein supplement) may help cats cope with stressful events.

4. Diarrhea Due to Ingesting Foreign Material

If a cat consumes foreign material, such as rubber, foam, or plastic, it can obstruct their intestinal tract, which is referred to as a foreign body obstruction and is a surgical emergency. Typically, a sudden onset of moderate-severe vomiting would also be expected, although a partial obstruction of the intestines could potentially lead to ongoing diarrhea and weight loss without vomiting.

A cat's belly would normally be painful, and they would start to appear dull and quiet. A veterinarian may be able to feel a foreign object and would use imaging (radiographs and ultrasound) to help make a diagnosis.

Occasionally, eating foreign material doesn't cause an obstruction but irritates the guts as it passes through, leading to diarrhea.

Cat abdominal radiograph showing foreign body

5. Inflammatory Causes of Diarrhea

Cats may suffer from chronic (long-term) inflammation of their intestines, also referred to as chronic inflammatory enteropathy or inflammatory bowel disease (IBD). A cat's intestines may become thickened and their abdominal lymph nodes may become reactive and enlarged. Potential clinical signs include:

  • Vomiting (more than twice a month)
  • Diarrhea (lasting more than three weeks or intermittently over one month or more)
  • Weight loss

The inflammation is suspected to be caused by an overreaction of the immune system to intestinal contents that are usually well-tolerated, such as certain foods or normal bacteria, and tends to improve in response to diet changes, antibiotics, or steroids.

  • Diet-responsive. Some cats develop a hypersensitivity or allergy to their food, which is not necessarily associated with a recent diet change and may be food they consumed previously without problems. Skin itchiness occasionally presents with food allergies, and a cat may over-scratch or over-groom.14 Since the protein source is usually the culprit, such as chicken or tuna, switching to a diet containing hydrolyzed protein (broken down so that it doesn't cause a reaction) or a novel protein (one they have not been exposed to before) may improve their stools. Blood, saliva, and hair tests are unreliable for identifying food allergies and likely only indicate previous exposure.15
  • Fiber-responsive. Diarrhea in some cats may resolve in response to a fiber-enriched diet. Dietary fibers may help bind excess water and improve fecal consistency, as well as feed and promote the growth of beneficial bacteria.
  • Antibiotic-responsive. Occasionally, a cat's diarrhea appears to improve in response to antibiotic treatment, suggesting it may have been caused by an infection or dysbiosis (an unbalanced population of bacteria in their guts). However, antibiotics don't discriminate between beneficial and problematic bacteria in the intestines, and their use can further unbalance the microbiome.16 Overuse of antibiotics is also a concern for creating antibiotic-resistant microorganisms, a serious threat to global health. Instead, the focus is shifting toward supporting gut health by providing small, frequent meals of a highly digestible diet supplemented with prebiotics, probiotics, fibers, and B12. As research develops, fecal microbiota transplants (FMT) may become more common, which involves transferring feces from a healthy donor cat into the patient's gastrointestinal tract to restore beneficial gut microbes.
  • Steroid-responsive. In some cats, diarrhea improves in response to steroids, drugs that reduce inflammation and suppress the overreactive immune system. Oral prednisolone is the most commonly used steroid and is usually slowly tapered to the lowest effective dose and continued long-term.

Cat abdominal ultrasound showing normal vs thickened intestines
A cat with thickened intestines observed on ultrasound (imaging), most commonly seen with inflammatory bowel disease or lymphoma.

6. Intestinal Cancer

Intestinal cancer leading to diarrhea is more likely to occur in older cats. Other clinical signs include vomiting and weight loss. Effects on the appetite are variable, often initially increasing and later on decreasing. A veterinarian may be able to feel thickened intestines, a mass, or enlarged abdominal lymph nodes on a physical examination, but not always.

Lymphoma (cancer of the lymphatic system) is the most common cancer of the intestinal tract in cats and can occur as diffuse thickening of the intestinal wall or discrete masses. It may be confined to the gastrointestinal tract or also affect other organs, such as the liver and spleen.

Small-cell lymphoma is low-grade and the clinical signs are similar to inflammatory bowel disease (IBD). Treatment includes steroids (oral prednisolone) and chlorambucil, which is a chemotherapy (anti-cancer) medication.

Large-cell lymphoma is high-grade and tends to present with more severe clinical signs and has a poor prognosis. It is often treated with COP/CHOP protocols, which require several months of intravenous and oral chemotherapy medications, as well as oral prednisolone.

Adenocarcinoma and mast cell tumors are less common cancers of the intestinal tract in cats.

7. Drug Administration and Toxin Ingestion

Various human foods, plants, household chemicals, and human medications can cause diarrhea in cats if ingested. Depending on the toxin, other symptoms would usually be expected, such as vomiting, lethargy, inappetence, tremors, or seizures. If you suspect your cat has ingested a toxic substance, it's crucial to contact your veterinarian or an emergency clinic immediately. They can provide guidance on what steps to take and may recommend bringing your cat in for evaluation and treatment.

Some drugs commonly administered to cats may also cause diarrhea as a side effect, such as amoxicillin-clavulanate (antibiotic), meloxicam (pain relief), or chemotherapy. If your cat is experiencing diarrhea after starting a medication, contact your veterinarian.

Toxins and drug side-effects can cause diarrhea in cats

8. Disorders Outside the Intestines

If the cause of diarrhea is due to a problem with the intestines, it is referred to as a 'primary gastrointestinal cause' of diarrhea. Disorders of other organs, such as the liver, kidneys, pancreas, and thyroid, can also cause diarrhea and are referred to as a 'secondary gastrointestinal cause' of diarrhea.

Secondary gastrointestinal causes of diarrhea do not usually present with diarrhea as the primary complaint, and usually, the cat has other signs of illness as well, such as vomiting, lethargy, weight loss, an increased/decreased appetite, or increased thirst and urination. Often, the diarrhea is normal or increased in volume with a normal to slight increase in frequency (2-4 times per day).

Secondary gastrointestinal causes of diarrhea include:

  • Liver disease. Cholangiohepatitis (inflammation of the liver and biliary tract), hepatic lipidosis (fatty deposits in the liver that occur after a period of not eating), or liver cancer can all potentially cause diarrhea, as well as lethargy, vomiting, a reduced appetite, weight loss, and jaundice (yellow skin).
  • Pancreatic disease. The pancreas, located next to the stomach and small intestine, helps with digestion by releasing enzymes. Pancreatitis (an inflamed pancreas) and pancreatic cancer (more common in older cats) can potentially cause diarrhea, vomiting, a reduced appetite, abdominal pain, lethargy, and weight loss. Exocrine pancreatic insufficiency (EPI) is when the pancreas does not produce enough digestive enzymes and can occur as a consequence of chronic pancreatitis. Possible clinical signs of EPI include diarrhea, vomiting, and weight loss despite an increased appetite.
  • Kidney disease. Advanced chronic kidney disease (CKD) or acute kidney injury (AKI) resulting in a significant buildup of waste products in the body may cause diarrhea as well as vomiting, increased thirst, reduced appetite, lethargy, and bad breath. CKD is more common in older cats and has a gradual onset. AKI can affect cats of any age, has a sudden onset, and may occur due to toxins, infections, urinary tract obstructions, or shock.
  • Hyperthyroidism. The thyroid gland, located in the neck, can become enlarged (known as a goiter) and produce too much thyroid hormone, usually resulting in weight loss despite an increased appetite. Other potential symptoms include diarrhea, vomiting, increased thirst and urination, and behavioral changes, such as aggression and increased vocalization. Hyperthyroidism typically affects older cats.

Disorders in other organs can cause diarrhea in cats (kidneys, liver, pancreas and thyroid)

Acute vs. Chronic Diarrhea

Diarrhea is acute if it's short-lived and lasts less than 2-3 weeks. Many cases of acute diarrhea will resolve on their own or with supportive care, although cats that are showing other signs of illness, such as lethargy, vomiting, or dehydration, may require more intensive treatment or investigations.

Diarrhea is considered chronic if it lasts more than 3 weeks or occurs intermittently over one month or more. More extensive investigations and/or treatment trials are usually required.

The table below shows causes of diarrhea that are acute, and causes of diarrhea that are chronic.

Causes of Acute Diarrhea Causes of Chronic Diarrhea
Sudden diet change

New treats

Spoiled food

Prey (bugs, reptiles, rodents)



Drug side effects

Acute pancreatitis

Foreign body obstructions


Inflammatory bowel disease (diet-responsive, steroid-responsive)



Foreign body obstructions (partial)

Disorders of other organs (kidney disease, liver disease, chronic pancreatitis, exocrine pancreatic insufficiency, or hyperthyroidism)

Small Bowel vs. Large Bowel Diarrhea

When food is swallowed, it travels down the esophagus and into the stomach. From the stomach, it enters the small intestine, also known as the small bowel. Next, the ingesta moves into the large intestine, also known as the large bowel, which consists of the colon (longest part), rectum (short part located at the end), and anus. Although the small bowel is much longer than the large bowel, it has a much narrower diameter, hence the names.

cat anatomy - stomach, large intestine (bowel), and small intestine (bowel)

Diarrhea will have different characteristics depending on whether it is caused by a problem in the large or small bowel. If fresh blood or mucus (shiny, slimy, or jelly-like bits) are present, and your cat is frequently passing small amounts of feces, it is like large bowel diarrhea. If these characteristics are absent, or your cat is passing larger volume stools, has an altered appetite, or is losing weight, small bowel diarrhea is more likely.

Diarrhea may also have features of both small and large bowel (mixed bowel diarrhea), indicating widespread disease or that a disorder in one part is starting to affect the other.

The table below lists the characteristics of small vs. large bowel diarrhea (adapted from 'Clinical Reasoning in Veterinary Practice: Problem Solved!').

Characteristic Small Bowel Large Bowel
Volume of Feces Increased Decreased
Frequency of Defecation Normal or mildly increased Increased (more than 3x/day)
Blood If present, usually digested (black melaena) or semi-digested (reddish brown) If present, it's fresh and undigested (red)
Mucus Absent Common
Weight Loss Common Uncommon (unless severe inflammation or cancer)
Appetite Often decreased or increased Usually normal
Vomiting Common Uncommon
Urgency Uncommon Common
Straining Uncommon Common
Flatulence (Farting) Possible Uncommon
Dehydration Can occur if severe Uncommon
Activity Often quiet or lethargic Often normal

Localizing diarrhea to the small or large intestine can help narrow the list of likely underlying causes and help focus diagnostic testing. Large bowel diarrhea usually indicates a problem with the intestinal tract, whereas small bowel diarrhea can be due to a problem with the intestinal tract (primary intestinal disease) or an organ outside the intestinal tract (secondary intestinal disease).

The table below categorizes diseases capable of causing either small bowel diarrhea or large bowel diarrhea, or both small and large bowel diarrhea (adapted from 'Clinical Reasoning in Veterinary Practice: Problem Solved!').

Small Bowel Large Bowel Small and/or Large Bowel
Sudden diet change

Spoiled food



Feline coronavirus


Foreign body obstructions

Coccidia (cryptosporidium, cystoisospora)

Disorders of other organs (kidney disease, liver disease, pancreatitis, EPI, or hyperthyroidism)




Inflammatory bowel disease (diet-responsive, steroid-responsive)

Irritation (bony or foreign material passing through)

Bacteria (clostridium, salmonella, campylobacter)

Giardia (more commonly small bowel)


Feline infectious peritonitis

Cancer (intestinal lymphoma)

Young vs. Older Cats

Kittens-young cats have less developed immune systems and are more likely to develop infectious causes of diarrhea. They may also be more likely to eat inappropriate foods or objects that could cause diarrhea.

Young-middle-aged cats are more likely to develop a food allergy (food-responsive chronic enteropathy) or inflammatory bowel disease (steroid-responsive chronic enteropathy). Pancreatic and liver disease may also occur.

Senior cats are more likely to develop chronic kidney disease, hyperthyroidism, and intestinal cancer.

Please note that there will always be exceptions to the rules, and your cat's age should not be relied upon to make a diagnosis.

Senior cat and kitten

Well vs. Unwell Cats

The cause of diarrhea in a well cat, who is bright and eating, is more likely to be related to food-related issues (such as the sudden introduction of a new food or treat), stress, or an infection, such as worms or a mild bacterial or viral infection. Some cats with inflammatory bowel disease may have mild but recurring signs.

The cause of diarrhea in an unwell cat, for example, who is frequently vomiting, lethargic, has a decreased or increased appetite, or is losing weight, could be due to an infection, such as panleukopenia, a complete or partial foreign body obstruction, inflammatory bowel disease, intestinal cancer, toxins, or a disorder of another organ (the liver, kidneys, pancreas or thyroid).

All causes of diarrhea in a kitten can potentially lead them to rapidly deteriorate and feel unwell. Kittens are tiny and prone to dehydration from losing water in stools and vomit, hypoglycemia (low blood glucose) if they stop eating, and hypothermia (low body temperature). They are also more likely to develop sepsis (the movement of intestinal bacteria through their leaky/damaged guts to other parts of the body) due to their immature immune systems.

Please note that any cat having diarrhea, especially after 24-48 hours, should be examined by a vet, even if they seem bright and well, because cats often hide signs of illness, and a professional assessment is essential.

Investigating and Treating Diarrhea in a Cat

The following information provides general guidelines for the workup and treatment of a cat with diarrhea. However, it does not replace a veterinary assessment, and if your cat is unwell, always seek and follow the advice of your vet.

Acute Diarrhea

Scenario 1: A cat has a sudden onset of mild clinical signs, such as soft stools, but is still bright and eating well.

If a cat is well, the diarrhea is mild, has only occurred for a few days, and no abnormalities are found on their physical examination, a vet may not perform any diagnostics initially. However, your vet will want to recheck your cat and perform further investigations if the diarrhea persists or your cat deteriorates (starts vomiting, becomes lethargic, the diarrhea worsens, etc.). Initial supportive care to help speed up the resolution of the diarrhea may include:

  • Feeding small, frequent meals of a bland, easily-digestible diet, such as unseasoned boiled, skinless chicken without bones, until the diarrhea resolves, then slowly transitioning back to their regular food. Intestinal cells get nutrition to heal from the gut lining, so it's important to keep feeding. If suddenly changing their diet may cause further problems, you may opt to continue providing their normal food.
  • Ensuring worming is up-to-date with an appropriate dewormer, such as Drontal for Cats.
  • Administering a kaolin and pectin-based oral paste to help bind the poop and protect the gastrointestinal tract lining, such as Pro-Pectalin.
  • Feeding probiotics (beneficial bacteria) and prebiotics (to feed and promote the growth of beneficial bacteria). Commonly recommended probiotics include Fortiflora and Proviable.
  • Ensuring easy access to fresh water to prevent dehydration, or if they are mildly dehydrated, fluids may be administered subcutaneously (under the skin) by your vet.
  • There is no evidence to support the use of antibiotics for mild, non-complicated cases of acute diarrhea, even if there is a small amount of fresh blood (which can occur quite commonly if the colon is inflamed or due to straining).17, 18

Scenario 2: A cat has a sudden onset of moderate-severe clinical signs, such as profuse diarrhea, large amounts of blood in the feces (red or black), lethargy, fever, dehydration, frequent vomiting, or abdominal pain.

This group of cats requires immediate veterinary attention and intensive supportive care, such as intravenous fluid therapy, anti-nausea medication, and nutritional support. Antibiotics may be required if they are significantly unwell due to an infection (fever, lethargy) or at risk of sepsis. Kittens can also quickly become hypoglycemic (low blood glucose) or hypothermic (cold) and may require glucose supplementation and warming. Certain tests may be prioritized, for example, a panleukopenia test in a vomiting kitten with profuse diarrhea, imaging if your cat was recently chewing plastic and now has vomiting and diarrhea, or bloodwork to evaluate for pancreatitis if a cat has abdominal pain.

Scenario 3: Diarrhea has just started and the cat is fairly well but has a history of other clinical signs, such as intermittent vomiting, weight loss, or increased thirst.

This group of cats will likely prompt more immediate investigations, such as bloodwork to assess kidney and thyroid function, especially in an older cat, or imaging, such as X-rays and ultrasound, especially if your vet feels thickened intestines (thickening can be subtle and hard to judge) or an abdominal mass, to assess for inflammatory bowel disease or cancer.

Chronic Diarrhea

Diarrhea in a cat may warrant further investigation if it continues despite supportive treatment, especially lasting for more than three weeks, or has occurred intermittently over a month or longer, even if the symptoms are mild.

Below is a general step-by-step diagnostic pathway, starting with inexpensive and non-invasive tests, to more costly and invasive diagnostics. Which diagnostics are performed and in what order will be decided between you and your vet, depending on the most likely cause of diarrhea, its duration, the severity of your cat's illness, and the finances available.

If the diarrhea is ongoing, but your cat is otherwise well, a vet may suggest treatment trials such as deworming, diet changes, probiotics, and B12 supplementation rather than proceeding with too many diagnostics.

1. Fecal Analysis and Deworming Trial

Since the majority of infectious causes of diarrhea resolve on their own or with supportive care, the following infectious agents are of most interest since they can cause life-threatening or ongoing (chronic) diarrhea.


  • A fecal test is recommended for kittens and unvaccinated adult cats showing symptoms such as lethargy, fever, vomiting, or diarrhea.
  • An in-house SNAP Parvo (ELISA) test can be used, but false negative results (if they are no longer shedding large amounts of virus in the feces) and false positive results (due to receiving a modified-live vaccination within the previous two weeks) may occur, so clinical signs and other abnormalities, such as a low white blood cell count on bloodwork, should guide diagnosis.
  • There is no specific treatment for the virus and supportive care includes intravenous fluids, glucose supplementation, anti-nausea medication, nutrition, and antibiotics.


  • Ensuring a cat is up-to-date with deworming treatment is recommended, including indoor-only cats, since worm eggs could be brought inside on shoes or by dogs, or from eating bugs and reptiles that come in the home.
  • Even though worm-induced diarrhea is not common in adult cats, deworming is a simple and effective way to rule them out as a cause. It would be a shame to pursue diet changes or more expensive diagnostics if it were merely a case of intestinal worms.
  • A fecal flotation test is commonly performed by a vet to check for worms, in which a small amount of fresh feces (ideally less than 2 hours old) is mixed with a solution that allows the eggs to float to the surface, where they are collected on a coverslip that's placed on a slide and examined under the microscope. Fecal flotation may also recover coccidia (cystoisospora) oocysts, which are usually more significant in kittens, and giardia cysts, although they can be challenging to identify.
  • Worm eggs are shed intermittently and not always present in the sample, so regardless of the results, deworming may still be advised.
  • Drontal for Cats is a commonly used dewormer (2 doses 2-3 weeks apart).

parasite eggs cat feces
Parasite eggs identified in cat feces: a) Hookworm b) Roundworm c) Whipworm d) Cystoisospora (Coccidia)


  • Giardia infections can cause chronic diarrhea, which is variable and ranges from soft to watery, and intermittent to continuous.
  • An in-house SNAP (ELISA) test can be used to detect the presence of giardia in a sample of fresh feces collected while they are having diarrhea.
  • Treatment typically involves febendazole +/- metronidazole.

Giardia are protozoa (single-celled organisms) that can cause acute or chronic diarrhea.


  • If a young cat is experiencing large bowel diarrhea (frequent, small-volume liquid stools, sometimes with straining and fresh blood), tritrichomonas testing is recommended.
  • A fresh fecal sample that ideally contains mucus and bloody diarrhea but no cat litter can be sent to a reference laboratory for PCR testing.
  • In addition, a very fresh fecal smear (ideally sampled directly from the rectum) can be placed onto a slide with a drop of saline and looked at under the microscope to observe for movement (giardia may also be seen with this technique and look similar to tritrichomonas, however, tritrichomonas tend to wiggle forward, whereas giardia move in a swirling or 'falling-leaf' pattern).
  • Tritrichomonas can take up to two years to clear, so it is often treated with ronidazole, a medication used off-label, handled with disposable gloves, and discontinued if there are neurological side effects.

Video: Tritrichomonas Observed in a Fresh Fecal Smear

Video: Tritrichomonas vs. Giardia

PCR testing for cryptosporidium and PCR testing +/- culture for salmonella, campylobacter, and clostridium may be considered in certain situations, such as when an infection is suspected and an animal is systemically unwell (fever, lethargy, etc.), lives with an immunocompromised person, or there is an outbreak of diarrhea in a population of cats. However, these microbes can be found in healthy cats without diarrhea, so results must be interpreted carefully.

2. Blood Tests

Based on your veterinarian's examination and clinical assessment, they may recommend blood tests to assess your cat's overall health status and check for disorders outside the intestinal tract that can potentially cause diarrhea, such as kidney disease, liver disease, pancreatitis, exocrine pancreatic deficiency (EPI), and hyperthyroidism.

Basic Blood Tests:

  • CBC (Complete Blood Count). Assesses the number of red and white blood cells. Abnormal white blood cells could indicate inflammation or infection. Low red blood cells (anemia) could be due to chronic disease or intestinal blood loss, and an increased red blood cell count can occur due to dehydration.
  • Biochemistry. Raised liver enzymes (ALT, ALP, GGT) could indicate liver disease, however, mild increases can also be seen with intestinal disease and hyperthyroidism. Raised kidney parameters (creatinine and urea/BUN) need to be interpreted with a urine sample to see if the urine is concentrated (dehydration) or dilute (kidney issues).

Viral Testing:

  • Feline Leukemia Virus (FeLV) / Feline Immunodeficiency Virus (FIV). If a cat is very unwell or has been sick for some time, establishing their FIV/FeLV status is advisable since these viruses can cause a variety of problems, including weakening their immune system and making them more prone to infections or certain types of cancer.

Specialized Tests:

  • Thyroid Test (T4/fT4). Elevated levels indicate hyperthyroidism.
  • fPL (Feline Pancreas-Specific Lipase). Screens for pancreatitis.
  • TLI (Trypsin-like Immunoreactivity). Screens for Exocrine Pancreatic Insufficiency (EPI).
  • Cobalamin (B12). Small intestinal disease and EPI can reduce cobalamin absorption, resulting in low blood levels. Cats who are deficient often do not respond to other treatments until they are appropriately supplemented.

Cats with cobalamin (B12) deficiency due to small intestine disease or EPI may not respond to other therapies until they are appropriately supplemented with weekly subcutaneous (under the skin) injections.

3. Imaging

A vet might recommend non-invasive imaging techniques that allow the internal organs to be visualized. Although they often don't provide a conclusive diagnosis, they help gather information that guides clinicians toward a likely diagnosis, helping to formulate a treatment plan or identify organs that need further assessment. Imaging may also look completely normal, even though disease is present.

Radiography involves the use of X-rays to create pictures of the abdomen's internal structures. While it might not always be beneficial in cases of chronic diarrhea, it can help assess for chronic foreign bodies, intussusceptions (telescoping of the intestines into itself), masses, gas accumulation in the gastrointestinal tract, and organ size.

Ultrasound utilizes sound waves to generate real-time images of the internal organs. Clinicians skilled in ultrasound can identify intestinal obstructions caused by foreign bodies or intussusceptions. Additionally, ultrasound can reveal masses, lymphadenopathy (enlarged lymph nodes due to inflammatory bowel disease or cancer), thickening of the intestinal walls (due to inflammatory bowel disease or lymphoma), free abdominal fluid, and allows for assessment of other abdominal organs, such as the liver, stomach, spleen, pancreas, and kidneys.

Abdominal ultrasound in a cat
A cat with chronic diarrhea having an ultrasound to visualize his intestines and other abdominal organs.

4. Food Trials

Food trials, also known as elimination diets, help establish if food allergies, intolerances, or sensitivities are a cause of chronic diarrhea in cats. Food allergies are common in cats and occur when the immune system mistakenly identifies a specific protein, such as fish or chicken, in the intestines as harmful and mounts a response against it that triggers inflammation. Cats usually develop allergies to proteins they have been previously exposed to in their diet. Therefore, potential diets to trial include a novel (new) or hydrolyzed (broken down) protein.

A novel protein is one that your cat has never been exposed to before and their immune system has not been sensitized to, minimizing the chances of an allergic reaction. Your cat's diet history is crucial in choosing a new protein, and common choices for a novel protein diet include lamb, kangaroo, venison, and rabbit. It's essential to check the ingredients carefully to ensure the food contains a single protein source and doesn't list 'meat' in the ingredients, making it impossible to know which proteins it contains.

A hydrolyzed protein diet is made from proteins that have been broken down into smaller components (amino acids and peptides), making them less likely to be recognized by the immune system as an allergen and trigger an immune response. If your cat has been exposed to a variety of foods and selecting a new protein is challenging, a hydrolyzed diet might be the best choice.

There is no perfect diet that will help all cats with chronic diarrhea due to food allergies, so 2-3 different diets may need to be trialed. A diet trial should last at least two weeks to see if there is any improvement and must be fed exclusively (no treats, flavored probiotics, or toothpaste). When switching diets, it is best to do so gradually over a week to avoid causing further gastrointestinal upset.

When performing a food trial, nothing else must pass your cat's lips except the food you are trialing and water, otherwise, the trial may be ruined. Treats, table scraps, pill pockets, toppers, flavored water, soups, and toothpaste are not allowed. If your cat takes flavored medications or supplements, check with your vet if they might affect the food trial, but do not discontinue or switch them without your vet's advice.

Recommended Diets for Feeding Trials in Cats with Chronic Diarrhea



Royal Canin Hydrolyzed Protein

  • Made with hydrolyzed (broken down) protein to a size less likely to be recognized by the immune system and trigger an allergic response
  • Strict manufacturing procedures avoid cross-contamination
  • Contains a blend of fibers and prebiotics to support digestion
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Hill's Prescription Diet z/d

  • Formulated with highly digestible hydrolyzed animal protein to reduce gastrointestinal upset in cats with food sensitivities
  • Contains antioxidants and a blend of prebiotics to support digestion
  • Wet and dry formulations
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Royal Canin Selected Protein Rabbit (PR)

  • Made with limited ingredients, including rabbit (a common novel protein), hydrolyzed soy and omega-3
  • Highly palatable and easily digestible
  • Available in wet and dry as well as duck (PD), although if your cat is sensitive to chicken there may be crossreactivity between poultry proteins
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ZIWI Peak Venison

  • Made with 92% free-range, grass-fed venison meat, organs, and ground bone, with additional green-lipped mussel
  • A single-protein diet that's ideal as a food trial for cats with sensitive stomachs
  • Also available in lamb and rabbit & lamb (common novel proteins) as well as wet and air-dried formulations
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Feline Natural Lamb

  • Made with 98% free-range, grass-fed lamb meat, organs, and ground bone, with additional green-lipped mussel
  • A single-protein diet made with a common novel protein that's ideal as a food trial for cats with sensitive stomachs
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Rayne Nutrition Kangaroo

  • Contains a single protein, kangaroo, which is a common novel protein and appropriate for elimination diet trials
  • Made with highly digestible ingredients as well as algae as a source of omega-3 and prebiotics to help restore beneficial intestinal flora
  • Also available in rabbit as well as pate, stew, and freeze-dried formulations
Rayne Nutrition

5. Probiotics and/or Fibre Trial

Probiotics and/or fiber in combination with an easily digestible, high-quality diet, may help promote gut health and improve fecal consistency in some cats.

Hill's Gastrointestinal Biome Cat Food is a fiber-enriched, anti-inflammatory, and easily digestible diet that has been clinically shown (Clinical Evidence Report) to promote regular healthy stools by nourishing the gut microbiome.

Alternatively, a source of fiber, such as unflavored psyllium husk, can be sprinkled on top or mixed with your cat's wet food (or a squeezable treat like Churu). A starting dose of 1/8-1/4 of a teaspoon twice a day is typically used, gradually increasing the amount to achieve the desired effect. Please consult your vet first to ensure there are no contraindications to adding fiber to your cat's diet, such as poor appetite, pancreatic disease, or end-stage megacolon, and discontinue its use if it's not helping or there are adverse effects.

Recommended to Increase Fiber Intake

GI Biome

GI Biome

Hill's Gastrointestinal Biome

  • A highly digestible and complete diet formulated by Hill’s nutritionists and vets
  • Contains a unique blend of prebiotic fibers to support beneficial gut bacteria and promote healthy digestion
  • Clinically proven to improve stool quality in as little as 24 hours and reduce the risk of recurrence
  • Wet and dry formulations are available
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Psyllium Husk

Psyllium Husk

Organic Psyllium Husk

  • Additional fiber may help bind excess water and improve fecal consistency
  • It can be sprinkled on top of wet food, mixed with a squeezable treat, or alternatively fill empty gelatin capsules
  • Start with 1/8-1/4 of a teaspoon twice a day and gradually increase if necessary
  • Consult your vet first to ensure there are no contraindications and discontinue if there are adverse effects
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Probiotic supplements can help repopulate the gut with beneficial bacteria to aid digestion and reduce diarrhea. It's important to use products from reputable companies that are high-quality and active within the gastrointestinal tract. Fortiflora is a single-strain probiotic with a guaranteed level of viable bacteria. Proviable is a multi-strain probiotic, and in a study, 72% of owners perceived an improvement in their cat's chronic diarrhea following a 21-day course.19

Recommended Feline Probiotics




  • Beneficial bacteria and antioxidants to support your cat's digestive health and immune system
  • Designed for the dietary management of kittens and adult cats with diarrhea
  • Each packet contains a guaranteed level of live microorganisms
  • Microencapsulation enhances the probiotics stability and survival until they reach the intestinal tract
  • Tasty liver flavor for easy administration
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  • Each capsule contains 5 billion colony-forming units (CFUs) comprised of 7 beneficial bacteria strains
  • Help support a healthy intestinal balance in young and adult cats
  • Contains prebiotics that support the growth of beneficial bacteria
  • The capsule contents can be easily sprinkled on food
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6. Antibiotic Trial?

If a cat is not responding to diet changes and probiotics, a veterinarian may prescribe antibiotics if the cause of diarrhea is suspected to be due to an infectious bacteria, an over-reaction of the gut to normal bacteria, or dysbiosis (an unbalanced number and variety of bacteria present in the gut).

However, antibiotics should be used cautiously and only when necessary, as they can further unbalance the gut microbiome and potentially lead to antibiotic resistance.

7. Biopsies

Biopsies involve the collection of small tissue samples via endoscopy or surgery while an animal is under general anesthesia. Samples are examined under a microscope, and specialized tests (IHC and PARR) may also be performed on the tissues to help distinguish between inflammatory bowel disease, small-cell (low-grade) lymphoma, and large-cell (high-grade) lymphoma.

Usually, a food trial is performed before collecting biopsies unless there is a need for more urgent interventions, for example, if there is a high suspicion of cancer.

Endoscopy is minimally invasive and involves inserting a thin, flexible tube through the mouth or anus that is equipped with a camera to visualize the gastrointestinal tract and biopsy tools to collect samples. However, there are limitations as only the inner layers of the intestines can be sampled, and not all parts of the small intestine can be reached. Therefore, an ultrasound is often performed first to determine whether it is a suitable option.

Surgery to collect biopsies involves making an incision into the abdomen to access the gastrointestinal tract. The advantage is that full-thickness tissue samples can be collected to examine all layers of the intestines, and all parts of the small intestine can be accessed. Lymph nodes and other organs, such as the liver and pancreas, can also be visualized and sampled if necessary. However, surgery is more invasive, more prone to complications, and there is a longer recovery period.

If biopsies are the next step in your cat's workup, it's important to discuss the pros and cons of endoscopy vs surgery with your vet.

Biopsies may not always be feasible. For example, if the risk of anesthesia is high (due to severe illness or an underlying heart condition), there is a lack of access to facilities offering endoscopy, or the owner cannot afford the procedure. In these cases, steroids may be trialed, which help with inflammatory bowel disease and somewhat with lymphoma. However, without a probable diagnosis, it is unknown if chemotherapy agents should also be started and the likely prognosis.

Intestinal endoscopy
An endoscope can be used to visualize the inside of the gastrointestinal tract and collect biopsy samples.

8. Steroid Trial

If the cause of a cat's diarrhea has been localized to the intestinal tract, based on bloodwork and imaging, and they have failed other therapeutic trials, steroids (most commonly oral prednisolone) are usually the next step.

Steroids have anti-inflammatory and immune system suppressant effects, which help with inflammatory bowel disease since it likely occurs due to an intolerance of the immune system to food or normal bacteria in the intestines, resulting in an inappropriate inflammatory reaction. Steroids are typically slowly tapered to the lowest effective dose and continued long-term. A second immunosuppressant drug may also be added if the steroids alone are not effective enough.

Steroids can also improve the clinical signs associated with intestinal lymphoma, however, the effects are unlikely to be long-lasting without the addition of chemotherapy. Small-cell (low-grade) lymphoma is usually treated with oral prednisolone and oral chlorambucil, a chemotherapy (anti-cancer) medication. Large-cell (high-grade) lymphoma is usually treated with oral prednisolone and CHOP or COP protocols, which are long courses of both intravenous and oral chemotherapy drugs.

Steroids may cause increased appetite, thirst, urination, and risk of infection, while chemotherapy may cause vomiting, diarrhea, and bone marrow suppression. The severity of side effects varies between cats, and not all patients will necessarily experience them. Your veterinarian will discuss the potential risks and benefits for your individual cat and help monitor and mitigate adverse effects. In addition, if a cat is receiving chemotherapy, strict safety protocols must be followed, such as wearing gloves to handle the medication and clean the litter box, as well as washing your hands afterward.


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Catastrophic Creations