Cat diarrhoea: when to watch at home, when to go in, and why never to fast a cat

Quick answer

Most of the useful information about feline diarrhoea comes from the litter tray, the scale and the cat's behaviour, not from examining the cat itself.

One soft stool in a bright cat that is eating, drinking and behaving normally is worth watching for a day or two. Diarrhoea with any of blood, repeated vomiting, hiding, refusal to eat, or a kitten involved is worth a same-day appointment.

Before anything else, decide which end of the gut you are dealing with. Large volumes of watery stool with weight loss is small intestine. Small amounts passed often, with urgency, straining, jelly-like mucus and streaks of fresh blood, is large intestine. They have different causes and different tests, and telling them apart at home is genuinely useful.

• Never fast a cat with diarrhoea. Dogs tolerate a day without food, cats do not, and an overweight cat that stops eating can develop life threatening liver disease within days.
• Never give human anti-diarrhoeal medicines. Cats process salicylates poorly and opioid-type products can cause severe effects at doses that are trivial in a dog.
• A kitten with diarrhoea is a different animal from an adult with diarrhoea. They dehydrate in hours.
• Weigh the cat daily on a kitchen scale. In the short term, weight is a hydration measure, and it beats every subjective impression.
• Diarrhoea lasting more than about three weeks is chronic by the definition vets use, and it needs investigation rather than another food change.

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A cat straining repeatedly in the tray, producing little or nothing, and crying or hiding, may not have a gut problem at all. In male cats especially, this is how a blocked bladder looks, and owners regularly describe it as constipation or diarrhoea.

A blocked cat dies within a day or two without treatment. If your cat is straining and you are not certain something is coming out, treat it as a urinary emergency and go immediately.
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Decide in 60 seconds

Why the gut produces diarrhoea

The intestine handles a great deal of fluid every day. Most of it is not what the cat drank; it is secreted into the gut to digest food and then reabsorbed further down.

Diarrhoea is what happens when that balance breaks, and there are only a few ways it can break.

Absorption fails.

The lining is damaged or inflamed, so water and nutrients stay in the gut. This is the pattern in infections, chronic inflammatory disease, and infiltration by tumour cells.

Something osmotic holds water in.

Undigested sugars and poorly digested food draw water into the gut lumen. Lactose does exactly this in an adult cat, which is why milk causes diarrhoea rather than curing it.

Secretion increases.

Some bacterial toxins actively drive fluid into the gut.

Transit speeds up.

The contents move through too fast to be absorbed. Stress does this, and it is the mechanism behind the diarrhoea that appears after a house move, a cattery stay or a new cat.

Most real cases combine two or more. What matters practically is that "diarrhoea" is a sign, not a diagnosis, and the useful question is which part of the gut is producing it.

Small bowel or large bowel

A cat showing both patterns at once usually has more extensive disease, and that combination should shorten the time before you go in.

This distinction is not academic. Fresh blood and mucus with urgency point the vet toward the colon, toward parasites such as Tritrichomonas and Giardia, and toward colitis. Large volume watery stool with weight loss points to the small intestine, and to blood tests including B12 and thyroid rather than only a faecal sample.

What normal looks like

The comparison that matters is behavioural. A well cat rests loose and alert, grooms, and comes out for food. A cat with significant gut disease hides, sits hunched, and stops grooming.

Normal cat stool is formed, segmented, holds its shape when lifted, and is mid to dark brown. Most cats pass once or twice a day.

Colour varies more than owners expect. Diet changes it, and a very dark stool is not automatically blood. What is never normal is black and shiny and tarry, which is digested blood, or a stool that has fresh red streaks on the surface.

A healthy adult cat grooms after eating, sleeps stretched out rather than curled tight, and does not change its trips to the tray.

Take a weight now, while the cat is well. A kitchen scale with the carrier on it, and the cat in the carrier, works if the cat will not sit still. Subtract the carrier. That single habit gives every later illness a baseline.

The causes, by age

Kittens.

Parasites dominate: roundworms, hookworms, coccidia, Giardia, and Tritrichomonas foetus, which is common where kittens come from crowded breeding or rescue environments and which does not respond to routine wormers.

Panleukopenia, the feline parvovirus, causes profuse and often bloody diarrhoea with vomiting, fever and collapse in unvaccinated kittens, and it kills quickly. Any unvaccinated kitten with diarrhoea and lethargy is an emergency.

Diet errors matter too: cow's milk, overfeeding milk replacer, or an abrupt switch of food.

Kittens have very little fluid reserve. A kitten that is quiet, cool to the touch and not nursing or eating is in trouble now, not tomorrow.

Adults.

Dietary intolerance and food responsive enteropathy, an abrupt food change, scavenging, and hunting. Parasites, including Giardia. Bacterial infections such as Salmonella and Campylobacter, which are more likely with raw diets and hunting. Toxins. Stress after a move, a new pet or a cattery stay. Chronic inflammatory enteropathy. Pancreatitis, which in cats is often quiet and vague rather than dramatic.

Older cats.

Hyperthyroidism, which typically produces weight loss with a good appetite, sometimes vomiting, sometimes hyperactivity, and which is a simple blood test.

Chronic enteropathy and small cell lymphoma, which look almost identical from the outside and are separated by ultrasound findings and biopsy. Both are manageable, and the distinction changes treatment, which is why the workup is worth doing rather than trialling diets indefinitely.

Chronic kidney disease and hyperthyroidism often coexist with gut signs in this age group.

The look-alikes that are not diarrhoea

A blocked bladder.

Repeated squatting with nothing produced, in and out of the tray, vocalising, licking at the back end. Male cats especially. This is the emergency at the top of this article and it is mistaken for gut trouble constantly.

Constipation with overflow.

A cat impacted with hard faeces can leak brown liquid around the mass, which looks exactly like diarrhoea. The tell is straining that produces liquid but never a formed stool, and a cat that has not passed a proper stool for days.

Anal sac disease.

Scooting, licking, a smear of foul brown fluid, and normal stool otherwise.

A linear foreign body.

String, tinsel, dental floss or thread swallowed and caught, often at the base of the tongue. Causes vomiting, diarrhoea and rapid deterioration, and it needs surgery. Look under the tongue if the cat will let you, and tell the vet if any string is missing from the house.

Hairballs.

Retching and bringing up a tube of hair is a different problem from diarrhoea, although chronic gut disease makes both worse.

The first 48 hours at home

This applies only to an adult cat that is bright, eating, drinking, and has no blood, no vomiting and no straining.

Keep feeding.

Small, frequent meals of the food the cat already eats, or a highly digestible diet if your vet has recommended one. Do not starve the cat and do not restrict water.

Add water to the food.

Wet food with extra warm water added, or plain unsalted broth without onion or garlic, increases fluid intake in a species that drinks poorly by nature.

Do not change the food repeatedly.

Each change destroys the information. If the diarrhoea started with a new food, go back to the old one and stop there. Otherwise keep everything constant so that improvement or deterioration means something.

Count and record.

Number of stools, consistency, blood or mucus, appetite, and a daily weight. Photograph anything abnormal against a plain background.

Manage the tray.

Add a second tray, use an uncovered one with low sides so a sore cat can get in, and switch to plain unscented litter for the moment. Clean promptly, because several causes of feline diarrhoea can infect people.

Separate if there are other cats.

If an infection is possible, separate trays and food bowls at minimum. Wash hands after every tray change, and take extra care if anyone in the house is pregnant, elderly or immunosuppressed.

What means go now

Blood in any quantity, or black tarry stool.

Vomiting alongside the diarrhoea, especially if the cat cannot keep water down.

A cat that has not eaten for a day, or that is hiding, hunched, or sitting with its head low over the water bowl without drinking.

Pale or white gums, cold ears and paws, weakness, or collapse.

Any kitten that is quiet, cool or not feeding.

Suspected access to a toxin, a plant, antifreeze, or human medication. Lilies and antifreeze are lethal to cats and both cause vomiting early.

Straining with nothing produced.

What the vet will do, and what to bring

The four things that make the appointment productive: a weight, a written record, the exact food, and a fresh sample from the tray.

Expect a full physical examination, a temperature, an abdominal palpation feeling for thickened loops, enlarged lymph nodes, a mass or a foreign body, and an assessment of hydration and body condition.

Faecal testing.

Flotation for worm eggs, and specific tests for Giardia and, in the right circumstances, Tritrichomonas. Bring a fresh sample. Because shedding is intermittent, vets often ask for samples from three consecutive days, kept sealed and refrigerated rather than frozen.

Blood work.

A general profile, and in an older cat a thyroid level. In chronic small bowel disease, B12 and folate are measured because low B12 is common, is treatable, and its correction is often what finally lets the cat gain weight. Pancreatic tests where pancreatitis is suspected.

Retrovirus testing.

FeLV and FIV status, particularly in a cat with an unknown history or outdoor access.

Imaging.

Ultrasound to assess the thickness and layering of the intestinal wall, the lymph nodes and the pancreas. Radiographs if a foreign body or an obstruction is possible.

Biopsy.

For chronic cases, this is what separates inflammatory disease from small cell lymphoma. It is done either endoscopically or surgically, and the choice depends on which part of the gut is affected.

Bring with you:

• A fresh stool sample, and photographs of the abnormal ones
• The daily weights, with dates
• The exact food, including brand, variety, treats, supplements and anything the cat has stolen
• The date the diarrhoea started and whether it has been continuous or intermittent
• Any recent change: new food, new pet, move, cattery, new cleaning product
• Parasite treatment dates and products, and the vaccination record
• Whether other cats or people in the house have been unwell

Chronic diarrhoea, the long game

Once diarrhoea has been present for weeks, the useful frame is not "what settles the gut" but "what is driving it".

Three things account for most chronic feline cases: a food the cat reacts to, a parasite that has not been found, and chronic inflammatory disease or low grade lymphoma. Hyperthyroidism sits alongside them in older cats.

A properly run food trial is a diagnostic test, and it fails when it is run loosely. One food, nothing else at all, for the number of weeks the vet specifies, in every cat in the household if they share bowls, with no treats, no flavoured medication and no hunting.

Repeatedly switching diets in the hope of stumbling on the right one is the single most common way that owners spend a year without an answer.

What never to do

Do not fast a cat.

Do not give human anti-diarrhoeal medicines. Products containing salicylates are poorly handled by cats, and opioid-type anti-diarrhoeals can cause marked sedation or excitation in this species.

Do not give paracetamol or ibuprofen for any reason. Paracetamol is lethal to cats even in small amounts.

Do not offer cow's milk or cream.

Do not use leftover antibiotics from a previous illness or another animal. Most acute feline diarrhoea does not need antibiotics, and unnecessary courses disturb the gut population further.

Do not assume a single wormer has cleared the parasites. Different parasites need different drugs, and Giardia and Tritrichomonas are not covered by routine products.

Do not change food every few days to find one that suits. Change once, deliberately, and give it time.

When to get help

Recovery is a formed stool plus a stable weight plus normal behaviour. Two out of three is not recovery.

Go immediately for collapse, pale gums, straining with nothing produced, suspected poisoning, a swallowed string, or a kitten that has gone quiet and cool.

Go the same day for blood in the stool, black tarry stool, vomiting alongside diarrhoea, refusal to eat for a day, hiding and hunching, or any diarrhoea in a kitten.

Book within the week for soft stools that have gone on for more than a few days in an otherwise well adult cat, for any weight loss, and for a cat that has had intermittent diarrhoea for weeks.

If you have already been treating at home, set a limit before you start. If the cat is not clearly improving within two days, or is worse at any point, that is the moment to stop watching and go in. Waiting for a cat to look obviously ill wastes the window in which most of these problems are cheap and simple to fix.