You open the door, gently sniff the air … yep, the cat’s had diarrhoea again

If this happens on the odd occasion, it’s gag-worthy but not really a medical issue. When it goes on and on, that’s a problem. Take a deep breath because we’re now going to explore diarrhoea in cats.

What is diarrhoea?

Diarrhoea is an abnormal increase in the liquid (water) in the stools. Of course, you knew that, but what a lot of people forget is that diarrhoea isn't a disease itself – it's a symptom of an underlying condition. And there are so many different underlying conditions!

Whatever the cause, there are four basic ways that stools become loose. These are known as four the 'mechanisms' of diarrhoea: osmotic, secretory, increased mucosal permeability and abnormal motility. In most cases of cat diarrhoea, there are multiple mechanisms at work. 

We'll look at the mechanisms here, but if you're not into the sciency bit, skip ahead past the next picture.

OSMOTIC DIARRHOEA

Osmotic diarrhoea occurs when too many solutes (ie nutrients/components of the food) stay in the intestine. The nutrients stay in the intestine due to problems with digestion and absorption. A good example of this is lactose intolerance – if you're missing the enzyme that breaks down lactose, lactose particles will remain in your gut.

If you remember back to your school days, water moves through permeable membranes (via osmosis) along a concentration gradient. So if the intestine contains a high concentration of solutes, water is drawn in making the stools more liquid.

This is the only sort of diarrhoea that responds to fasting. 

SECRETORY DIARRHOEA

Secretory diarrhoea is more recognised in people than cats. It occurs when the body secretes electrolytes (ie salts such as sodium, potassium and chloride) into the intestine. Then we get an osmotic type process with water being drawn to the concentrated electrolytes.

The abnormal electrolyte secretion can be caused by lots of things such as:

• bacterial infection such as E. coli or Clostridia (which produce toxins that affect gut cells)

• parasites such as cryptosporidium and Giardia

• viral infection such as coronavirus

• inflammatory bowel disease (IBD)

INCREASED MUCOSAL PERMEABILITY

The bowel wall consists of four layers: mucosa, submucosa, muscularis and serosa. The mucosa is the inner part of the bowel wall. It's where absorption happens.

If the mucosal layer is damaged, it can become leaky. Minor damage causes leakage of mostly water from the body into the gut. More severe damage allows proteins to leak out too. This is called protein-losing enteropathy and often results in significant weight loss.

Damage to the mucosal layer is caused by:

• inflammatory bowel diseases (these are immune-mediated)

• infections (bacterial, viral, parasitic)

• cancer of the intestines

ABNORMAL MOTILITY

If the intestinal contents move through the intestines too fast, normal absorption of water can't occur. The increased motility (hypermotility) is due to an intestinal response to a disease, such as the one mentioned in the other mechanisms of diarrhoea as well as conditions such as hyperthyroidism.

When it comes to cat diarrhoea, more than one mechanism is usually involved.

Is all diarrhoea the same?

No.

We classify diarrhoea in two ways:

• acute or chronic

• small bowel or large bowel

Determining what sort of diarrhoea your cat has helps us figure out the cause and a treatment plan.

Acute v chronic

The terms acute and chronic have nothing to do with severity. They just refer to how long the problem has been going on.

• acute diarrhoea – is common and affects nearly all of us at some point. It usually starts abruptly and goes away within a few days. All cats (and people) are different so it might last a bit longer in some

• chronic diarrhoea – once diarrhoea has been persistent for 2 weeks or more, it's considered chronic. This sort is less likely to go away on its own

With acute diarrhoea, we usually just have to make sure that we prevent dehydration and allow it to resolve on its own with doing much in the way of diagnostics. Chronic diarrhoea often needs work up so we can find the specific cause and treat it accordingly. 

 SMALL BOWEL V LARGE BOWEL

The other important classification when it comes to diarrhoea is the location of the problem:

• small bowel diarrhoea – due to problems in the duodenum, jejunum or ileum. This may be referred to as 'enteritis' or an 'enteropathy'. Small bowel diarrhoea may also be due to related organs such as the exocrine pancreas (ie the bit that makes digestive enzymes)

• large bowel diarrhoea – due to problems in the colon. This may be referred to as 'colitis'

• mixed bowel diarrhoea – which involves the small and large bowel

The following are 'features' of the different types of diarrhoea:

We’re going to focus on chronic diarrhoea from here on.

What causes chronic diarrhoea in cats?

There are lots of potential causes. Here are the broad categories:

• infectious disease – bacteria, viruses, protozoa

• metabolic disease – hyperthyroidism, liver disease, kidney disease

• dietary – intolerance or allergy

• exocrine pancreatic insufficiency

• small intestinal disease

  • structural issues – partial obstruction

  • inflammatory bowel disease (IBD) – lymphoplasmacytic, granulomatous, eosinophilic

  • neoplastic disease – lymphosarcoma, adenocarcinoma, leiomyoma, fibrosarcoma

  • functional problems – motility disorders

How do we diagnose the cause of chronic diarrhoea?

As there are so many causes, it can be very difficult to make a diagnosis in the consult room.

To narrow down the list of possible causes (ie the differentials) we use:

• your cat’s signalment (age, breed, sex)

• history (including the features of the diarrhoea seen at home)

• physical examination

Based on the narrowed list, we might recommend to you further testing or trial therapy. We’re more likely to start with trial therapy if your cat has large bowel diarrhoea than small bowel diarrhoea. Testing could include lab tests, imaging or even biopsies.

Lab testing

This includes:

• faecal analysis – there are various of poo tests, from looking for worm eggs, to culturing bacteria, to viral DNA testing

• routine blood tests – looking for signs of blood loss/protein loss through the gut, liver disease, kidney disease, thyroid disease

• special blood tests for pancreatic function and/or intestinal dysfunction

• urinalysis – to further assess for kidney and liver disease

imaging

X-rays aren’t particularly useful when it comes to chronic diarrhoea but ultrasound can be good to look for intestinal lesions (masses, foreign bodies) and intestinal wall thickness. We can other organs on ultrasound too, such as the pancreas, kidneys, liver and spleen.

Intestinal biopsy

The only real way to diagnose inflammatory bowel disease and cancer is by taking a small piece of affected tissue and sending it to a pathologist (ie take a tissue biopsy). While we can do needle biopsies of many areas/organs, it is tricky to get a bowel wall sample this way. We generally get them one of two ways:

• with a scope (camera) – this only allows tiny samples

• surgically – this is the best way but requires opening the abdomen

Both of these require a general anaesthetic.

How is chronic diarrhoea treated?

Obviously, this depends on the underlying cause.

One thing you might consider doing at home is changing your cat’s diet. It might not help, but is very unlikely to make things worse. Options include a sensitivity diet, a hypoallergenic diet or ‘diarrhoea diet’ (eg Hill’s Microbiome).