42 Diarrhea

Diarrhea is typically due to  one or more of  two main forces: secretory and osmotic. However, there are other grouping possible.   Infectious agents are involved in many diarrhea syndromes. Many of these organisms also impact the nervous system, causing hyperactive peristalsis. Hyperactive peristalsis can also limit digestion and absorption through decreased transit time.

Deranged motility can co-exist with all types of diarrhea. Faster transit time decreases absorption.

Osmotic diarrhea

Osmotic diarrhea occurs when excessive solutes in the intestine draw fluid back into lumen in an attempt to balance concentration gradients.

There are three main versions:

We feed an animal something that we know can’t be digested

    • This substance is often designed to pull water into the GI tract. Ex: epsom salts
    • this is more useful in the LI
    • we often just get loose feces
Feeding an animal epsom salts is used to treat impactions. The goal is stool softening, not really diarrhea

Animals eat something that can’t be digested

    • Example – lactose intolerance
    • Example – maldigestion/malabsorption*
    • When food stuffs cannot be digested, they stay in the lumen, again creating a concentration gradient and stimulating flow of water into the lumen.

With malabsorptive diarrheas, the enterocytes are destroyed or damaged. Water and food components cannot be resorbed properly.  Since food components are not being absorbed and stay in the lumen, this creates a concentration gradient that pulls water from the blood into the gut lumen, creating diarrhea

The tight junctions and/or the epithelial cells in the gut are damaged

    • Water can leak across the gut wall
    • This also means the concentration gradients are lost
    • Damage also leads to increased solute in the lumen, directly related to parts of the damaged cells

.Often the causes of malabsorptive diarrheas (infection, inflammation) lead to disruption of the tight junctions. Tight junctions between intestinal cells are supposed to maintain active transport functions and concentration gradients. If damaged, these capacity is lost. Besides being unable to regulate water flow, this can cause weight loss and lead to immune system compromise and response. Bleeding can be seen related to loss of barrier function and/or cell damage. Loss of tight junctions also allows lots of things to cross the gut barrier – bacteria, large proteins, electrolytes.

Overall, malabsorptive diarrhea leads to decreased water absorption and increased solute leakage.

Examples – rota/coronavirus diarrhea.

Secretory diarrhea

Secretory diarrhea occurs when secretion of water into the lumen exceeds the absorptive capacity. This is generally due to overactivity of the crypt cells and extra release of chloride ions. This release then leads to water flow into the gut lumen due to the change in concentration gradient.

Examples – cholera, giardia, cryptosporidia, and ETEC (enterotoxigenic E coli)

Small intestinal issue only – no crypt cells in the LI!


Etiologic agents

General causes of diarrhea include

  • infectious agents (viral, bacterial, fungal, protozoal, parasitic)
  • dietary (coprophagy, grain overload, lactose, sand)
  • antibiotic associated (changes in normal gut flora)
  • changes in blood or lymphatic flow (hypertension, lymphatic obstruction)
  • infiltrative disease (immune cells, tumor cells)
  • intestinal accidents (ischemia,intussusceptions, short bowel syndrome)
  • endocrine disorders (hyper/hypothyroidism, Addisons disease)
  • toxins and drugs (NSAIDs)
  • pancreatitis, malabsorption
  • dysmotility -stress, nervous system dysfunction

Many cases of diarrhea are due to infectious agents – bacteria, viruses and parasites. These are important to identify to minimize spread between animals. Examples include parvovirus, Salmonella, African swine fever, and giardia.

SI diarrhea vs LI diarrhea

Most times we have both SI and LI inflammation- enterocolitis  – particularly due to infectious agents that don’t stay in one area of the gut.

Most digestion and absorption occurs in the SI meaning malabsorptive diarrheas typically involve the small intestine. Secretory diarrheas are only found in the SI as crypt cells are only found in the SI. As the LI doesn’t have secretory or absorptive capabilities,  colitis related diarrhea is primarily osmotic.

While the signs of diarrhea are pretty easy to recognize, it can sometimes be useful to differentiate small intestinal from large intestinal diarrhea. Small intestinal diarrhea often causes fever, dehydration and eventually weight loss.

Many cases of  enteritis are also associated with bloating as the food stuff may actually be digestible by the colonic bacteria.

Small intestinal diarrhea Large intestinal diarrhea
Large volume Small volume
Normal to slightly increased frequency Increased frequency
Melena Hematochezia
Weight loss
Fatty stool (steatorrhea) Mucus

Due to the extensive colon and opportunity for water absorption, we often don’t even see diarrhea due to localized SI enteritis in horses – all the fluid is resorbed. Most diarrhea in horses will be colitis or enterocolitis.

It is possible to have both SI and LI diarrhea in the same patient!

Diarrhea therapy

Therapy is generally supportive – fluids and good nursing care.

  • Fluids for dehydration
  • Correct electrolyte and acid- base abnormalities
  • Bland diet (I/D; no grain; etc)
  • Provide oncotic pressure
  • Deworm most everyone
  • Antibiotics for some cases
  • NSAIDs or other pain relief
  • Bind endotoxin
  • Protect gut
  • Ice feet to prevent laminitis in horses
  • Isolate any animals with potentially infectious conditions!


Many cases of bacterial diarrhea are actually not responsive to antibiotics. Metronidazole (antibiotic) has been used at high levels; however, it disrupts the normal bacterial flora and has not been shown to be useful in the treatment of diarrhea. In a study in dogs, metronidazole resulted in dybiosis (abnormalities in the microbiome), including decreases in microbiome diversity and secondary bile acids levels. The changes lasted at least 4 weeks after the drug was stopped.

Other studies have shown a lack of response to amoxicillin/clavulanic acid, even in dogs with hemorrhagic diarrhea and white blood cell changes.

Treatment of young animals with antibiotics may permanently disrupt their gi flora.

Probiotics and prebiotics

Probiotics have been shown useful in the treatment of diarrhea in dogs. These populations of “good” bacteria provide the short chain fatty acids and other bacterial products. The effect of probiotics is generally short lived; the added bacteria do not seem to take up permanent residence in the gut.

Prebiotics are compounds that provide nutrition to the gut bacteria. Psyllium is commonly used as a prebiotic and is added to some diets designed to optimize gut health.

Prebiotics and probiotics can be used in combination!

Motility agents

Peristalsis may be slowed to permit more water absorption (eg using loperamide) if the cause is dietary – or basically as long as the cause is not infectious.  We avoid slowing motility with infectious agents as we want to get rid of the infectious agent.

Oral Rehydrating Agents

Water follows sodium.  If we can push sodium back into cells, the cells will tend to pull water back into themselves and out of the lumen.  If the pumps  are still working, we can use particular ratios of sodium and glucose to rehydrate our patients. These solutions work well in secretory diarrheas. They would not work as well in malabsorptive diarrheas when the pumps are not working

See this video at the 2min mark

Key Takeaways


  • Osmotic diarrhea occurs when fluid is drawn into the lumen; secretory diarrhea occurs when fluid is forced into the lumen. Inflammatory diarrhea leaks fluid and solute into the lumen.
  • Oral rehydration fluids with the appropriate sodium and glucose levels force fluid back into the blood by recreating the concentration gradient.
  • SI diarrhea is high volume and may be associated with signs of maldigestion/malabsorption such as weight loss, flatulence and fatty stool. Blood is digested (melena).

Oral rehydrating solutions work by enhancing the function of the sodium-glucose transporter.


Symptomatic Management of Primary Acute Gastroenteritis,  Todays Vet Pract 2015- SA general; lots of info about therapies

Diarrhea – all species overview, WSU

Diarrhea in beef and dairy calves – nice high level overview

Foal diarrhea – another high level overview


Advanced physiology

Lactose intolerance, humans but generally applies

Management of chronic enteropathies with a dietetic food, Vet Rec 2019

An overview of calf diarrhea – J Vet Sci, 2014 – much more detailed

Acute colitis in adult horses – Vet Quart 1997, detailed pathophys

Diarrhea pathophysiology in peds – similar categories, slightly different approach

Constipation and diarrhea 3:diarrhea – video series- well done

Pathophysiology of diarrhea – CSU web pages

SI vs LI diarrhea in pets – WSU page pages

Intestinal diseases in cattle – Merck Vet Manual

Diarrhea in horses – The Horse

Clinical effects of probiotics in dogs– JVIM, 2019

Efficacy of a prebiotic-probiotic supplement, shelter dogs– JVIM, 2017

Just for fun

Approach to diagnosis and therapy of acute diarrhea in dogs-spans physio to disorders, TVP

Swine diseases – diarrhea is important! By Dr. Zhitnitskiy (UMN)

Chronic diarrhea in dogs, JVIM 2017

Feline diarrhea, Cornell

Puppy diarrhea – AKC overview

Challenge quiz – UGA


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Vet Med: Applied GI Physiology- Supplemental Notes Copyright © by Erin Malone DVM PhD is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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