Diarrhoea: It happens!

Just about every dog will get diarrhoea at some stage. Most of the time, it's mild and goes away on its own. Sometimes it's severe and/or doesn't clear up.

Here we're going delve into diarrhoea (!!) and see what we can do about it.

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 dog 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 to the next picture.


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 is more recognised in people than dogs. 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)


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


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 dog diarrhoea, more than one mechanism is usually involved.

Classifications of diarrhoea

While you might have skipped over the mechanisms of diarrhoea, this bit is important because the classification of diarrhoea affects our decision making about diagnostics and treatment.


The first classification of diarrhoea depends on how long it lasts:

  • 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 dogs (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 often 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. 

Note that the terms acute and chronic have nothing to do with severity.


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'

  • 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

Knowing where the problem is helps us determine what the underlying cause is. Small bowel diarrhoea and large bowel diarrhoea have different causes and require different approaches.

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



  • watery

  • large volume

  • not usually urgent

  • light coloured

  • soupy or greasy

  • not bloody


  • mucoid

  • small volume (but lots of amounts)

  • often urgent (gets you up at night)

  • darker colour

  • jelly like

  • often bloody


Approach to acute small bowel diarrhoea (acute enteritis)

Dietary indiscretion (ie eating something bad or too much of something good like treats) is one of the most common reasons for a dog to develop acute small bowel diarrhoea. In most cases, we only see gastrointestinal signs (diarrhoea and maybe some vomiting) and the dog is otherwise ok. As long as she remains pretty bright and can maintain hydration through drinking, a few days of bland food will usually get her through.

Other causes of acute small bowel diarrhoea are:

  • bacterial infection (Salmonella, E. coli, Clostridium, Campylobacter)

  • viral infection (Parvo, Corona)

  • toxins (snail bait, some drugs/medications)

  • haemorrhagic gastroenteritis

  • acute pancreatitis

These other causes often require more intensive therapy (eg intravenous fluids) as well as investigation/monitoring.

Approach to chronic small bowel diarrhoea (chronic enteritis)

This type of diarrhoea almost always requires investigation because we're not just looking at intestinal issues here. Chronic small bowel diarrhoea can also be due to pancreatic disorders, liver and gallbladder disorders, and sometimes hormonal disorders too.

The list of intestinal causes alone is quite long:

  • inflammatory bowel disease

  • lymphangiectasia

  • cancer

  • bacterial overgrowth

  • Giardia

  • lactose or other dietary intolerances/allergies

  • partial obstruction

Investigation usually starts with basic blood work (to look for signs of protein loss, liver disease, inflammatory changes). While this is an important step, it would be unusual to come to a definitive diagnosis with this alone.

We often need to move onto imaging (ultrasound) and possibly sending blood and faeces away to the lab for more specific testing.

Ultimately, we might need to obtain samples of the small bowel wall to make a diagnosis. This requires surgery.

Approach to acute large bowel diarrhoea (acute colitis)

The two most common causes of acute colitis are:

  • stress-related (common with environment changes like boarding, moving, severe weather) – this is also known as 'spastic colitis' due to spasmodic nature of the diarrhoea

  • dietary indiscretion

While infection with Giardia usually causes small bowel signs, it can cause acute colitis too. Whipworms can also cause acute colitis but aren't common in Melbourne.

Acute colitis is treated symptomatically in most cases. Treatment may include dietary therapy and/or a short course of medication to reduce the bowel wall inflammation (eg metronidazole). 

Approach to chronic large bowel diarrhoea (chronic colitis)

If the symptoms of colitis have been going on for a month or more or if they keep recurring and resolving over and over, then a medical workup is generally needed. But we must always weigh up the disadvantages of testing and the quality and quantity of information that we can obtain.

We typically start with faecal testing. This will look for infectious causes (parasitic, bacterial and viral infections). It the results are negative, blood testing and urinalysis are in order to assess the patient's general health.

If the cause remains elusive, we may recommend as a colonoscopy. This is done under general anaesthetic and often begins with an enema (it's hard to get animals to drink the stuff we have to before a colonoscopy and fasting for a couple of days can be tough). A scope (camera on the end of a tube) is passed into the colon and the lining is examined. Biopsies are taken via the scope. These are sent off to an external laboratory to be examined under a microscope. The inflammation will then be classified depending on the type of cells infiltrating the colon lining. The colitis can be classified as:

  • lymphocytic/plasmacytic (a form of inflammatory bowel disease)

  • histocytic (which tends to stem from a type of E. coli infection)

  • not truly colitis at all (eg the psychosomatic condition known as irritable bowel syndrome)

Before we head down the invasive road, we often trial various therapies including the following.



These medications have anti-inflammatory properties in the large intestine as well as the ability to kill harmful organisms.



This medication consists of a sulfa antibiotic bound to a salicylate anti-inflammatory. The sulfa bond protects the anti-inflammatory medication until it gets to the large bowel, thus saving the anti-inflammatory effect for the disease of the large bowel. This is an effective medication but is typically given 3 times a day, which is an inconvenience.



The role of fibre in colitis is not clear. In general, colitis is considered to be a fibre-responsive disease but there are different types of fibre and it's hard to know what to use. Insoluble fibres, like cellulose, bulk up the stool and are stimulating to the colon lining. An already irritated colon might not enjoy this, although giving some structure to diarrhoea may be a good thing. Soluble fibres, like psyllium, are fermented by the colon bacteria into nutrients for the colon cells, which helps them heal. 



Some of the commercial therapeutic diets highlight the addition of FOSs to their formulations. FOSs are carbohydrates involving fructose (fruit sugar) units attached to glucose (starch sugar) units. Most carbohydrates are digested by the bacteria of the small intestine, leaving only the undigested fibres and other dregs for the teeming masses of the large intestine. FOSs are not fibres but they are digested in the large intestine (not the small intestine) in the same way that fibres are, yielding the same biochemicals that fibres do. This can help remove pathogenic bacteria from the large intestine and promote the growth of helpful bacteria. 



A probiotic is a protected culture of live helpful bacteria that can colonise the patient's intestine. The bacteria must be protected from the acid of the stomach so as to survive to the lower intestine. Once there, the bacteria make a home and make by-products that are nourishing to the intestinal and local immune system cells.

There are numerous products on the market for both humans and animals. Yes, you can use human products for pets – there are some that say 'but the bugs are different!' which is interesting since we don't actually know much about the natural flora in dogs and cats. In human trials, there appears to be little difference in efficacy of the different strains of bacteria. You're probably better of choosing one made by a well-established company and in a formulation you can get into your pet.

Yoghurts do contain good bacteria although, as they are foods and not medications, there is no one checking that they actually contain any reasonable quantities.



Colitis can result from a food intolerance or food allergy. The way to work out if this the problem in your pet is by feeding a diet consisting of carbohydrates and proteins that are novel or new to the patient. This could be a homemade diet (see BalanceIT for ideas) or a commercial hypoallergenic diet. An 8–10 week diet course is typically needed and no other chews or treats can be eaten during the trial. 



Clostridial organisms are a group of anaerobic bacteria responsible for such unpleasant conditions as tetanus, botulism and gangrene. There are Clostridial organisms that normally live in the large bowel where they don't cause trouble unless some stressful event or diet change allows them to overgrow. If there are a lot of them, the toxins they produce become significant and can cause colitis. The diagnosis of Clostridial disease is complicated.