A definitive cause of diarrhea can be determined in <50% of cases. Yet, treatment of most horses and foals with diarrhea is similar and thus allows supportive therapeutic management despite the lack of a definitive diagnosis.
Diarrhea in adult horses can be acute or chronic. Infectious agents that have been cited as potential causes of acute diarrhea in adult horses include numerous Salmonella serovars, Neorickettsia risticii, Clostridium difficile, C perfringens, Aeromonas spp, coronavirus, and cyathostomiasis. Other differential diagnoses for acute diarrhea in horses include ingestion of a toxicant(s), antimicrobial-induced colitis, NSAID toxicity, and sand enterocolopathy. An acute, fatal diarrheal disease of unknown etiology is known as colitis-X. Diarrhea that persists >1 mo is considered chronic and is often a diagnostic challenge. Chronic diarrhea can be caused by inflammatory or neoplastic conditions involving the intestine or by disruption of the normal physiologic process in the bowel. Differential diagnoses include sand enterocolopathy and infiltrative lesions, such as those associated with inflammatory bowel disease or intestinal lymphosarcoma. The body’s response to certain components of feed may play a role in chronic diarrhea of horses due to bowel inflammation but has not frequently been established as a cause.
Noninflammatory conditions of the colon can also result in diarrhea. These include altered fermentation in the large colon, which is potentially the result of altered intestinal flora or milieu secondary to antimicrobial treatment, alteration in diet, or unknown etiologies. Nonintestinal causes of chronic diarrhea include congestive heart failure and chronic liver disease. The diagnostic approach to these cases is aimed at differentiation of infiltrative diseases of the intestine from physiologic causes of diarrhea.
Because of the large volume of the colon and cecum of horses, massive fluid losses can occur in a short time. Thus, diarrhea in adult horses can be an explosive event with morbidity and mortality exceeding that associated with diarrheal diseases in other animals and people.