Diarrhea Again? Let's Explore Why It's Happening

Diarrhea Again? Let's Explore Why It's Happening

In This Article

Diarrhea – Unfortunately, we have all been there with our pets. It is no fun for anyone, but what exactly causes diarrhea in pets? Diarrhea is defined as loose unformed stools that result from increased movement through the gastrointestinal tract (1).

As you can imagine, increased motility through the GI tract can lead to decreased nutrient and water absorption. Diarrhea is often a clinical sign that occurs from an underlying cause.  

There are many causes of diarrhea, and it would be impossible to list every single one in this blog. However, further classifying diarrhea into one of the three categories below can help clinicians narrow down possible differentials.  .

Types of Diarrhea 

  1. Small bowel diarrhea is defined as larger volumes of stools with a normal or slight increase in frequency. There is often no straining involved but you may see additional clinical signs such as vomiting, weight loss, or tarry dark stools, indicating digested blood. Viral infections, such as canine distemper, parvovirus, feline panleukopenia, and coronavirus can cause this type of diarrhea (2). Other local gastrointestinal issues that can result in this type of diarrhea include bacterial infections in the GI tract and primary gastrointestinal cancer. Of course, there are other causes not originating in the gastrointestinal tract that can result in diarrhea too, such as some endocrine disorders, medication side effects, and toxins (2). 
  2. Large bowel diarrhea is defined as smaller volumes of diarrhea with an increased frequency of production. Oftentimes, you will notice that your pet is having to go outside way more than usual. Straining is often a clinical sign (2). The stool may have bright red blood or mucus present. Possible causes of large bowel diarrhea include intestinal parasites, foreign bodies, dysbiosis, toxins, dietary indiscretion, and stress colitis.
  3. Lastly, there is mixed bowel diarrhea which as you can guess, demonstrates signs of both small and large bowel diarrhea. This usually results when disease processes affect both the small and large intestine categories (3). Inflammatory bowel disease (IBD) can technically fit into all three categories depending on the specific case and where it affects the GI tract.

Chronic vs. Acute Diarrhea 

Diarrhea can be further classified as chronic or acute. Generally, if diarrhea persists for three weeks or more, it is considered chronic. Now that we know the different classifications of diarrhea, how do we determine when a case is mild or severe? A lot of this depends on additional clinical signs and the circumstances that may have contributed.  

Chronic cases should always be addressed to avoid weight loss and malnutrition. If an animal experiencing diarrhea is also showing signs of inappetence, lethargy, dehydration, or continued vomiting, these are likely more severe cases that need medical attention. Stools that are dark and tarry or those which contain excessive amounts of red blood should also be addressed immediately. If an animal has diarrhea but is otherwise acting normally and eating/drinking normally, then I would consider this milder in form. 

Identifying specific circumstances which may have contributed to the development of diarrhea is also helpful. Was your pet just transitioned to a new food? This can cause possible dietary indiscretion. Did your pet just experience a stressful situation such as boarding at a facility or changes to their home environment? Offering a bland diet in these situations can be helpful. The good news is cases like these tend to be self-limiting, meaning they usually resolve on their own.  

As you can see, there are so many different causes of diarrhea in our pets. Classification of the type of diarrhea helps narrow down possible causes and direct diagnostic selection which in turn helps guide treatment plans for each individual. Diet changes, fiber additions, medications, and probiotics are all often used depending on the cause. If you are in doubt about whether your pet needs medical attention for diarrhea, do not hesitate to call your veterinarian as they can help guide you in making decisions. Stay tuned for our next blog which focuses on IBD and its management in canine patients. 


  • “Diarrhea In Dogs | VCA Animal Hospitals.” VCA Hospitals, vcahospitals.com/know-your-pet/diarrhea-in-dogs. Accessed 12 Feb. 2022.
  • “Diarrhea.” Veterinary Teaching Hospital, 4 Jan. 2022, hospital.vetmed.wsu.edu/2022/01/04/diarrhea.
  • “Dealing with Diarrhea in Dogs – and Why Can’t They Just Go on the Wood Floor?” Morris Animal Foundation, www.morrisanimalfoundation.org/article/causes-of-diarrhea-in-dogs. Accessed 12 Feb. 2022
Author's Name

Dr. Dana Hog

Dr. Dana Hogg graduated in 2015 from North Carolina State University. She grew up in Wilson, North Carolina. Growing up with several animals, Dr. Hogg was drawn to the field of veterinary medicine at a young age. She completed her undergraduate degree at NCSU in 2009 and her master's degree in 2011.