Equis ISSN 2398-2977

Diarrhea: idiopathic

Contributor(s): Jamie Prutton, Jarred Williams


  • Cause: by its very nature the exact etiology behind idiopathic diarrhea is unknown. The diagnostic evaluation should revolve around ensuring all common differential diagnoses are ruled out.
  • Signs: often mild in nature it can range to severe. Owner will often complain about increased fecal water or diarrhea during exercise.
  • Diagnosis: this is a diagnosis of exclusion and all tests associated with other causes of diarrhea should be performed including hematology, biochemistry, fecal analysis, ultrasonography.
  • Treatment: challenging without a known etiology with efforts made towards symptomatic treatment.
  • Prognosis: good to excellent if signs are mild and will often be perceived as a more significant problem by the client.



  • See the differentials list for causes of diarrhea.
  • An etiology for idiopathic diarrhea cannot be stated due to its very nature.

Predisposing factors


  • Unknown but diarrhea is often associated with dietary changes, stress, drug administration (in particular antibiotics and nonsteroidal anti-inflammatory drugs) and in specific cases can be due to an infectious etiology.


  • Dysbiosis can lead to disruption of the normal flora of the intestine leading to:
    • Abnormal carbohydrate metabolism.
    • Decreased metabolism of bile acids.
    • Abnormal GI motility.
    • Blunting of intestinal villi.
  • Altered carbohydrate metabolism leads to reduction in the production of short-chain fatty acids (SCFA). SCFAs are intrinsically linked with fluid and electrolyte absorption and therefore a sudden drop leads to intraluminal accumulation of electrolytes and carbohydrates thus leading to an oncotic, secretory diarrhea.
  • Bile acids are colonic secretagogues and therefore, when accumulating, can lead to increased luminal fluid.
  • Rapid dietary changes can induce a change in the microbiota of the caecum leading to all of the above abnormalities.
  • Malabsorptive diarrhea: absorption is controlled by the surface epithelium cells.
  • Secretory diarrhea: the secretory role of the colon is controlled by the crypt epithelium. In normal circumstances the secretion is outweighed by the absorptive capacity of the colon:
    • Controlled by four paths: hormonal, enteric nervous system, bacterial enterotoxins and the immune system.
    • Bacterial enterotoxins increase cAMP or cGMP production leading to an increase in secretion.
    • Inflammatory mediators (histamine, prostaglandins, cytokines, etc) can directly stimulate enterocytes or activate the enteric nervous system leading to secretory responses.
  • Inflammatory cell mediators released from mast cells, monocytic or granulocytic phagocytes can induce chloride and water secretion and reduce sodium and chloride absorption.
  • Phagocyte derived reactive oxygen metabolites are released in response to inflammation, trauma or ischemia and these lead to chloride secretion.
  • Ileus can often occur as a sequalae to diarrhea and therefore gut motility should be monitored closely by auscultation and/or ultrasonography.
  • Free fecal water has no definitive cause but could be associated with diet or stress. Changes in the diet to an improved fiber quality can help.


  • No specific timecourse as the etiology is unknown.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Shaw S D & Stämpfli H (2018) Diagnosis and treatment of undifferentiated and infectious acute diarrhea in the adult horse. Vet Clin Equine Pract 34 (1), 39-53 PubMed.
  • Matthews J B (2014) Anthelmintic resistance in equine nematodes. Int J Parasitol Drugs Drug Resistance 4 (3), 310-315 PubMed.
  • Chapman A (2009) Acute diarrhea in hospitalized horses. Vet Clin North Am Equine Pract 25 (2), 363-380 PubMed.
  • Hassel D M, Smith P A, Nieto J E, Beldomenico P & Spier S J (2009) Di-tri-octahedral smectite for the prevention of post-operative diarrhea in equids with surgical disease of the large intestine: results of a randomized clinical trial. Vet J 182, 210-214 PubMed.
  • McGorum B C & Pirie R S (2009) Antimicrobial associated diarrhea in the horse. Part 1: Overview, pathogenesis and risk factors. Equine Vet Educ 21 (11), 610-616 VetMedResource.
  • Hillyer M (2004) A practical approach to diarrhea in the adult horse. In Pract 26 (1), 2-11 VetMedResource.

Other sources of information

  • Reed S, Bayly W & Sellon D (2010) Equine Internal Medicine. 3rd edn. Saunders Elsevier, USA.