Responsible owners should always pick up after their dog. This means that changes to a dog’s toileting are often the first sign something is wrong that you notice. One of the most common changes is having stools like water. What does this mean? What could be going on? Let’s discuss.

Distinguishing Diarrhoea

A dog having poo like water is a symptom of diarrhoea. Diarrhoea is defined as “looser, watery or more frequent stools than… normal.” This means that diarrhoea isn’t always like water, but can vary in consistency. Depending on the disease, symptoms seen alongside diarrhoea commonly include vomiting, going off food, dehydration, abdominal pain, lethargy and general unwellness. Diarrhoea may be acute or chronic. 

We often categorise diarrhoea into small intestinal (SI) diarrhoea or large intestinal (LI) diarrhoea, depending on where it originates. Identifying the type of diarrhoea can help vets diagnose what the cause is. SI diarrhoea is often high in volume, watery but at a normal frequency. Abdominal discomfort, vomiting, gassiness and bloating may be seen too. Any blood will be digested and black (melaena). LI diarrhoea tends to be normal or decreased in volume, but increased frequency. There may be straining, mucous on the stools, and any blood will be fresh (haematochezia). Not all diarrhoea will fit these patterns, though, and some diarrhoea can be a mixture of both kinds. 

Common Causes of Diarrhoea


You might be surprised to hear that stress can result in diarrhoea. This could be anything from a stressful event out on a walk, to a change around the house, a concurrent illness, or even moving to a new environment. The latter is especially common in puppies when they first move to a new home. We believe stress-diarrhoea is partly caused by a fight-or-flight response resulting in the release of chemicals that increase intestinal transit speed. Thankfully, stress-associated diarrhoea tends to be mild and quick to resolve, particularly if the stressful event is short-lasting. 

Dietary Indiscretion

This is probably the most common cause of diarrhoea in dogs. Dietary indiscretion means eating anything that causes an upset stomach. This may be your dog eating something he shouldn’t on a walk, eating too much at once, or even a change in diet (always introduce new food slowly). Again, puppies are quite prone to this form of diarrhoea when they receive a new food at their new home. Vomiting commonly precedes diarrhoea with dietary indiscretion. Food does not necessarily have to be toxic itself to cause dietary indiscretion, though some toxic foods do cause diarrhoea. Any food contaminated with bacteria or parasites could also lead to infection and further diarrhoea, as we will discuss below. 

Foreign Body

A foreign body is anything your dog has swallowed that shouldn’t be there. This could be literally anything from pieces of toys or bedding, to stones and sticks, to socks and pants (always an embarrassing one to remove!), or even big pieces of food or bone. A foreign body can cause multiple issues, including diarrhoea. The most serious is perforation, where a hole is made in the gut wall that allows bacteria to leak into the abdomen; or a blockage where food and blood supply cannot get past the foreign body. Both can be life-threatening and result in a dog needing surgery. Always be careful where your dog puts their nose, especially if they are a known scavenger. Muzzles that prevent scavenging can help in these naughty cases. 


Infections of roundworms, tapeworms and protozoan parasites are all common causes of diarrhoea, particularly very young puppies. There are many kinds of parasites that all cause different forms of disease. The most serious infections can result in weight loss or lack of weight gain, dehydration, blood loss and the spread of infectious diseases. Parasites can be hard to diagnose, but eggs may be spotted in faecal samples under a microscope. Worms aren’t always visible with the naked eye. Luckily, most broad-spectrum worming drugs will eliminate and prevent parasite infestations. Protozoan parasites sometimes need more powerful prescription-strength medication to treat effectively. 

It is important to remember that some parasites can be transmitted to humans (zoonotic disease), so protecting your dog protects us too – children, pregnant women and the elderly are especially vulnerable. Good toileting hygiene will also minimise risk (clean up after your dog!). Dogs that eat faeces, slugs or snails, or are fed raw meat are more likely to get parasites so should usually be treated more regularly

Bacterial and Viral Infections

There are a huge number of bacteria and viruses that can cause watery stools in dogs. They range from simple infections your dog may recover from quickly, to life-threatening illnesses. Let’s go over some of these now.

Coronaviruses (not the same as the COVID-19 virus), rotaviruses, and Cryptosporidium protozoan parasites are common infections that cause diarrhoea, especially in puppies living in unhygienic conditions. Infections tend to be mild, though puppies can become dehydrated and unwell with serious infections. The diseases tend to be self-limiting, and puppies often recover with supportive treatment. Cryptosporidium is zoonotic, so always exercise strict hygiene around puppies with diarrhoea. 

One of the most serious is parvovirus

It is also transmitted quickly between puppies in dirty environments, but causes severe bloody diarrhoea, rapid dehydration, depression, collapse and even death in some cases. Parvovirus cases often need days of hospitalisation to help the animal fight the infection. Thankfully, parvovirus is not as common as it once was, as we have a good vaccine that protects dogs. Always ensure your dogs are vaccinated against parvovirus, especially if they are breeding bitches (the maternal protection will protect the puppies until they are old enough to be vaccinated). There is a similar disease called haemorrhagic gastroenteritis that presents very similar to parvovirus, but is not caused by a virus – the symptoms are the same but the prognosis tends to be better than parvovirus. 

Bacterial infections with Salmonella, Campylobacter, E. coli and other similar organisms can cause diarrhoea in dogs as they can in humans. Many of these bacteria are commensals, meaning they are found in normal numbers in the intestines of healthy dogs. However, in certain situations they can become pathogenic and cause disease. Generally this occurs in very young, old or immunocompromised dogs. Infections vary from mild to severe, and may require antibiotics to treat. It is important to note that dogs fed raw food are much more likely to excrete these bacteria in their faeces, and have infections as a result.

There may also be evidence that humans in the house are more likely to have infections when their dogs are raw fed. Thus, we do not recommend raw feeding your dog unless for a specific medical reason guided by a vet, particularly if you have young children or immunocompromised adults in the house. 

On the topic of antibiotics, they can sometimes cause diarrhoea as a result of dysbiosis

Dysbiosis means an animal gut flora are abnormal. Antimicrobials can cause dysbiosis by killing off populations of commensal bacteria your dog needs to have a healthy GI tract. This may even result in an overgrowth of pathogenic bacteria like Salmonella. Diarrhoea is thus a common symptom. This is one of the reasons why vets avoid over-prescribing antibiotics, as by solving one problem we could be creating another. Dysbiosis can be linked to other conditions too, such as stress, an over- or under-active immune system, and poor diet. Dysbiosis can be treated via (ironically) selective antibiotics, anti-inflammatories, pre- and pro-biotics, and faecal transplantation where a sample of faeces is taken from a healthy dog and put into the stomach of a dog with dysbiosis to restock their gut flora. 

Food Intolerance and Allergies 

Food intolerances and allergies are common owner-reported causes of diarrhoea in dogs, though the actual numbers are not clear. True allergies are a consequence of the dog’s immune system having an extreme reaction to allergens from certain foods. A food intolerance is more of a chemical reaction to certain foods, and is similar to a dietary indiscretion. Any food, or even non-foodstuffs, can cause intolerance or allergies. It is important to note that both are very difficult to diagnose without formal blood testing – don’t assume your dog is allergic to, say, chicken, because they get diarrhoea eating chicken-flavoured kibble. There are many other ingredients that may be the cause, or your dog may not even be allergic but have another cause of diarrhoea. 

As well as diarrhoea, food allergies often also result in skin disease such as itchy paws, belly and ears, eye and ear infections and inflammation, anal gland issues, and other symptoms. Treatment can be very difficult, but may require select diet trials, immunotherapy, anti-inflammatory medication or a special home cooked diet.

Other less well-known causes

A number of organs are closely linked with the gastrointestinal system, and if they have a problem it can result in diarrhoea. Let’s list some of these now.

Pancreatitis, or inflammation of the pancreas, is often linked to diarrhoea

Pancreatitis can be acute or chronic (or both). There are many causes of pancreatitis, including infection, inflammation, nearby liver or stomach disease, a sudden intake of fat, or cancer. Dogs that are overweight, on certain drugs, have concurrent diseases, have had pancreatitis before, and certain breeds, are more likely to get pancreatitis. Pancreatitis may be very mild or can be severe. 

As well as diarrhoea, pancreatitis can cause vomiting, abdominal pain, a praying stretched-out posture, pain after eating, going off food, dehydration, weakness, a yellow colour, a swollen abdomen and collapse. Chronic cases may result in EPI (more on this later) or diabetes mellitus if the pancreas is severely damaged. Treatment may involve treating the primary disease if present, supportive fluids, pain relief, a low fat diet, anti-sickness medication, antibiotics and more. Some dogs may need to be hospitalised. Any dog prone to pancreatitis should be fed a low fat diet to prevent recurrence. 

Inflammatory bowel disease, or IBD, is a generalised term for any condition where the immune system is causing inflammation within the intestines

It can be caused many of the conditions already listed above, as well as having no obvious underlying cause. It tends to cause chronic diarrhoea and associated symptoms. Diagnosis can be tricky, and requires ruling out all other causes of diarrhoea first. Often definitive diagnosis can only be made via biopsy of the intestines. Treatment is similar to other causes of diarrhoea, but often needs anti-inflammatory steroids too. 

Liver and gallbladder diseases of all kinds are also commonly linked to diarrhoea. 

Nutritional Deficiencies 

Protein-losing enteropathies describe any condition where a dog loses protein through their intestines into their faeces. This often leads to chronic diarrhoea that is very smelly and like a cow-pat. Other symptoms include weight loss, general unwellness and eating strange things like soil. PLEs can be caused by any other severe GI disease, such as IBD, heavy parasite burdens, pancreatitis, liver disease, cancer and more. They can also be caused by congenital disease, such as congenital liver disease or cobalamin (vitamin B12) deficiency. Dogs with PLEs need to have the primary disease treated, as well as supplements to restore their protein and cobalamin levels.  

Exocrine pancreatic insufficiency (EPI) is a disease where the pancreas does not produce digestive enzymes that are needed to help digest proteins in food. Without, a dog will often lose weight as they don’t absorb much nutrition from food, and have smelly cow-pat diarrhoea. It is most common in German Shepherds but can occur in any dog – severe pancreatitis or pancreatic cancer can cause EPI. EPI can be treated by supplementing the dog with oral enzymes, a low fat diet, cobalamin supplements and other medications. 

Other Conditions

There are a few other conditions that can sometimes cause diarrhoea, though this list is not exhaustive.

Addison’s disease, or an underactive adrenal gland, is a complicated disease with varying disease processes and symptoms. Addison’s can be seen as a chronic disease with varying symptoms such as anorexia, vomiting, diarrhoea, weight loss, increased urination and thirst, weakness, shivering and depression, or as an acute episode presenting as collapse, shock, a slow heart rate and abdominal pain. Diagnosis can be tricky due to its vague symptoms, but generally requires blood tests. Treatment requires daily corticosteroids and monthly mineralocorticoid injections.

Finally, there are a number of cancers that can lead to diarrhoea in dogs. We’ve already discussed some of those that indirectly cause diarrhoea, such as cancers of the pancreas, liver and thyroid gland. Cancers of the guts themselves, such as lymphoma, adenocarcinomas, mast cell tumours, haemangiosarcomas and many others have all been linked to diarrhoea by causing damage to the gut wall and protein loss. Some large masses can also cause foreign body blockage symptoms. Treatment of cancer varies from chemotherapy, radiotherapy, surgery, or palliation. Many cancers causing diarrhoea require symptomatic medication such as cobalamin supplements, pre- and pro-biotics, pain killers, high-protein diets and more.

The bottom line?

Diarrhoea isn’t a single disease, it’s a symptom – and it can be a symptom of many, many conditions! Most cases are maild and will self-resolve over 2-3 days, but if your dog is lethargic, has any other symptoms, or there’s blood in the stool, call your vet sooner for a check over.

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