Paws and Poo: Understanding Acute Diarrhea in Dogs
By Tara Ghormley, DVM, DACVIM
Acute diarrhea is one of the most common reasons dogs visit a veterinarian. While often self-limiting, it can also be a sign of serious illness. Understanding the physiology of the gastrointestinal tract, the role of the microbiome, and the mechanisms behind acute diarrhea is crucial for deciding when intervention is necessary and what treatments are most appropriate.
How the Intestine Normally Works
The intestinal tract has two major roles: absorbing water and nutrients while forming a barrier against pathogens. This barrier is made up of epithelial cells, mucus, bacteria, and immune cells tightly linked together. Goblet cells produce mucus to keep harmful microorganisms separated from the epithelium, while the immune cells provide additional protection.
When this barrier is disrupted (by infection, inflammation, or toxins), proteins, electrolytes, and fluids leak into the lumen. In severe cases, pathogens can cross into the bloodstream, triggering systemic illness or setting the stage for chronic enteropathies later in life.
The Microbiome’s Role
The trillions of bacteria in a dog’s gut are not passive passengers. They compete with pathogens for space and nutrients, aid digestion, ferment carbohydrates into short-chain fatty acids (fuel for intestinal cells), and help train the immune system.
When the microbiome is thrown off balance, a state called dysbiosis, the risk of diarrhea increases. Researchers can now measure this imbalance with a dysbiosis index, where positive values indicate microbial disruption, and normal values are 0 or -1.
What Counts as Acute Diarrhea?
Acute diarrhea is sudden onset of loose stools, usually defined as three or more watery bowel movements in 24 hours or stools with visible blood. Most episodes resolve within 3–7 days, though some persist up to two weeks. Dogs are more frequently affected than cats.
Veterinarians classify diarrhea as:
Small bowel: increased volume, often with weight loss.
Large bowel: small amounts, straining (tenesmus), mucus, or blood.
Mixed: features of both, which is common.
Why Dogs Get Diarrhea
Causes range from infections to diet and stress:
Infectious agents: parvovirus, coronavirus, distemper, Salmonella, Campylobacter, Clostridium perfringens, E. coli, Giardia, hookworms, and whipworms.
Dietary factors: ingesting table scraps (“garbage gut”), sudden food changes, and hypersensitivities.
Drugs and toxins: antibiotics, non-steroidal anti-inflammatories, chemotherapy.
Stress or systemic illness.
In many cases, no cause is identified.
Mechanisms Behind the Mess
Diarrhea arises through several mechanisms:
Osmotic diarrhea: Nutrients are not absorbed (e.g., pancreatic insufficiency), drawing water into the intestine.
Secretory diarrhea: Enterocytes actively secrete fluid into the lumen, often due to infections like E. coli.
Increased permeability: Ulceration or inflammation allows protein, fluid, and blood to leak across the mucosa.
Altered motility: Inflammation stimulates cytokines, speeding intestinal contractions and reducing nutrient absorption.
Clinical Picture
Affected dogs show varying degrees of dehydration and electrolyte shifts (hypokalemia, hypochloremia, hyper- or hyponatremia). Hemorrhagic diarrhea points to mucosal damage, most often with parvovirus or acute hemorrhagic diarrhea syndrome (AHDS).
Interestingly, sick dogs often have less bacterial diversity in their microbiomes, with higher dysbiosis index values compared to healthy controls.
How Severe Is It?
The Canine Acute Diarrhea Severity Index (CADS) scores activity, appetite, vomiting, stool consistency, and stool frequency to provide an objective measure of disease severity. Monitoring the severity index is a way to quantify any improvement or worsening of the dog’s symptoms.
Treatment Strategies
General Support
Most cases require only supportive care:
Rehydration and electrolyte correction
More severe cases with vomiting or anorexia may also benefit from:
Antacids, antiemetics, gastroprotectants, or analgesia
Deworming
Broad-spectrum deworming is recommended based on age and exposure risk.
Diet
Nutritional management remains the first-line therapy with a highly digestible diet. These diets contain an easily digestible protein with smaller amounts of soluble and insoluble fiber and low or moderate fat content. Increased fiber is typically avoided if a highly digestible diet is required. However, patients with large bowel diarrhea may benefit from an increase in fiber content.
Probiotics, Prebiotics, and Synbiotics
Probiotics: Live microorganisms intended to restore the microbiome. Studies show mixed results, especially in acute diarrhea. They may be more useful in chronic disease.
Prebiotics: A non-digestible food that promotes the growth of beneficial bacteria already present. These may be more effective than probiotics if those beneficial bacteria are present.
Synbiotics: A combination of a probiotic and prebiotic. These may promote microbiome recovery.
Antibiotics
Here lies one of the biggest mismatches between practice and evidence.
Metronidazole and tylosin are commonly prescribed, but evidence shows they do not improve clinical outcomes and worsen dysbiosis for weeks or even years.
ENOVAT Guidelines (2024) state antibiotics should be reserved for severe cases such as those not improving with fluids, showing systemic illness, or with marked inflammation on blood work. First-line options include ampicillin, Clavamox, or trimethoprim-sulfa. These should be administered only until clinical resolution (3–7 days).
What Owners Expect
Owner expectations often drive overtreatment. In one survey:
51% expected diarrhea to resolve in 5 days.
27% expected recovery in just 2 days.
Many expected antibiotics if blood or mucus was present.
Yet most cases normalize with supportive care alone. Education about the natural course is vital for satisfied clients.
Acute Hemorrhagic Diarrhea Syndrome (AHDS)
Previously called hemorrhagic gastroenteritis (HGE), AHDS is a severe and sudden condition characterized by hematochezia, anorexia, lethargy, and sometimes hypovolemic shock. Vomiting precedes diarrhea in 80% of cases and Miniature Schnauzers appear overrepresented.
The condition involves necrotizing enterocolitis with destruction of intestinal mucosa. Research suggests involvement of Clostridium perfringens type A and its NetF toxin, though other factors may be required for disease development.
Clinical Management
Baseline diagnostics: CBC, chemistries, fecal Giardia test, possible ultrasound.
Supportive care with IV fluids and antiemetics usually stabilizes dogs within 12–24 hours.
Soft stools may persist up to 10 days.
Most do not require antibiotics.
Prognosis
Survival rates are high (96%). Duration of hospitalization is typically short, and prognosis is good if fluids are provided. However, some dogs later develop chronic GI disease, possibly due to long-term microbiome disruption.
Key Takeaways
Acute diarrhea in dogs is usually self-limiting.
Owners often expect antibiotics, but studies consistently show they don’t improve recovery and contribute to resistance.
Supportive care—rehydration, diet, and time—is the best treatment in most cases.
AHDS is a severe but distinct syndrome; despite its dramatic presentation, it responds well to fluids and supportive therapy.
Antibiotics are rarely indicated, and when used, they should be narrow-spectrum and short-term.