Why Does My Dog or Cat Have Diarrhea? What Their Gut May Be Telling You

Every pet parent eventually has that moment.

Your dog squats on a walk—or you clean the litter box—and immediately think:

Well... that doesn't look right.

Diarrhea is common in both dogs and cats, and sometimes the explanation is fairly simple. Maybe they ate something unusual, their diet changed too quickly, or something disrupted their normal digestive routine.

Other times, there's more going on.

The important thing to understand is that diarrhea isn't one specific disease. It's a sign that normal digestive function has been disrupted.

So what could your pet's poop be telling you?

What Causes Diarrhea in Dogs and Cats?

There's no single answer.

Loose or watery stool can occur for many reasons, including:

Sudden diet changes • eating unusual or spoiled food • food sensitivities • stress • parasites • infections • medications • intestinal inflammation • other gastrointestinal conditions

Research also shows that diet and environmental changes can be associated with acute diarrhea in companion animals. https://pmc.ncbi.nlm.nih.gov/articles/PMC10434016/ 

So one episode doesn't automatically mean there's a major problem.

But repeated diarrhea deserves more attention.

1. Sometimes, It's Something They Ate

Dogs have a reputation for eating first and asking questions later.

Cats may be more selective, but their digestive systems can still react to changes in food, treats, or ingredients.

Diet is particularly important because food doesn't only feed your pet.

It also feeds the microorganisms living in their gut.

Research shows that diet can influence both the composition and metabolic activity of the gastrointestinal microbiome in dogs and cats. https://pmc.ncbi.nlm.nih.gov/articles/PMC7329990/

That's one reason sudden dietary changes can be more significant than simply putting something different in the bowl.

2. Your Pet Has an Entire Ecosystem Inside Their Gut

Here's where poop gets surprisingly interesting.

The digestive tract contains a huge community of microorganisms known collectively as the gut microbiome.

These microorganisms are involved in nutrient metabolism, intestinal function, immune interactions, and maintaining the intestinal environment. https://pmc.ncbi.nlm.nih.gov/articles/PMC9039956/

When that microbial ecosystem becomes disrupted, it's called dysbiosis.

Research has found dysbiosis associated with several gastrointestinal disorders in both dogs and cats, including diarrhea and chronic intestinal disease. https://pubmed.ncbi.nlm.nih.gov/39545596/

That doesn't mean every messy poop is caused by dysbiosis.

But it does tell us that healthy digestion involves much more than simply breaking down dinner.

3. Cats Have Their Own Gut Story

Cats aren't simply small dogs when it comes to digestion.

They're obligate carnivores, and their nutritional requirements and gastrointestinal biology differ from dogs. Those differences can also influence their intestinal microbiome.

Researchers studying cats with acute diarrhea have found measurable differences in their intestinal microbial communities compared with healthy cats. https://pmc.ncbi.nlm.nih.gov/articles/PMC10434016/

Earlier research comparing healthy cats with cats experiencing acute or chronic diarrhea also found significant differences in their fecal microbiomes. https://pmc.ncbi.nlm.nih.gov/articles/PMC4437779/

So feline gut health deserves just as much attention as canine gut health.

4. What Can Your Pet's Poop Tell You?

We're not suggesting you turn every litter-box visit into a scientific investigation.

But poop is useful information.

Pay attention to changes in:

Consistency • frequency • colour • mucus • blood • unusual material

Also look at the pet attached to the poop.

A single softer-than-normal stool in an otherwise energetic pet is a very different situation from persistent watery diarrhea accompanied by vomiting, significant lethargy, loss of appetite, pain, dehydration, or blood.

Young, elderly, and medically vulnerable pets can also be affected more quickly by fluid loss.

In other words:

Don't only look at the poop. Look at the whole pet.

5. Where Do Probiotics, Prebiotics & Postbiotics Fit In?

This is where microbiome science becomes especially interesting.

Probiotics

Live microorganisms that, when provided appropriately, can support the microbial ecosystem.

Prebiotics

Substrates that are selectively utilized by microorganisms and can support beneficial microbial activity.

Postbiotics

Preparations of inanimate microorganisms and/or their components that can provide a health benefit.

Research into these collectively—sometimes called biotics—is growing in companion animals.

A 2024 evidence review concluded that emerging evidence supports potential roles for prebiotics, probiotics, and postbiotics in restoring gut homeostasis and supporting gastrointestinal disorders in dogs and cats, while also emphasizing that the evidence is still developing. https://pubmed.ncbi.nlm.nih.gov/39545596/

And that's an important distinction.

Not every probiotic is the same, and probiotics aren't a universal treatment for every cause of diarrhea.

The strain, formulation, dose, and individual animal all matter.

Why Myco Pet Uses a 3P Approach

This is exactly why Probiotics 3P is the Myco Pet product I'd recommend for this topic.

Rather than focusing on probiotics alone, it combines:

PREBIOTICS + PROBIOTICS + POSTBIOTICS

The goal is to provide nutritional support for the intestinal ecosystem from multiple directions—supporting microbiome balance, digestive health, and healthy stool quality in dogs and cats.

Research into probiotics and microbiome-targeted nutrition in companion animals continues to grow, with studies investigating effects on microbial balance, intestinal function, and gastrointestinal health. https://pmc.ncbi.nlm.nih.gov/articles/PMC10609632/

But it's important to keep the role clear:

Probiotics 3P supports digestive and microbiome health. It's not a substitute for determining why persistent or severe diarrhea is happening.

Because if your dog swallowed a sock...

the microbiome is not the biggest problem in the room.

The Bottom Line

Your pet's poop isn't glamorous.

But it's one of the easiest ways to notice when something in their digestive system has changed.

Diarrhea can have many causes, and the gut microbiome is only one part of the picture. What science increasingly shows, however, is that dogs and cats both have complex intestinal ecosystems closely connected with digestive function and health. https://pmc.ncbi.nlm.nih.gov/articles/PMC11188957/

So next time you encounter a questionable poop, don't only think about what's coming out.

Think about what's happening inside the gut, too.