Water uptake in the equine gut takes place primarily in the large colon. If this function is lost for any reason, it results in diarrhea. Horses that are stressed by management or diet changes may have short-term diarrhea or soft stool. A single pile of soft or watery manure ("cow pie") followed by normal feces may be considered normal, especially for an anxious horse. However, multiple watery movements should always prompt a call to your vet.

Adult horses with severe diarrhea can become rapidly dehydrated and go into shock. The greatest concern with adult horse diarrhea is that the horse has infectious colitis, a condition caused mostly by bacterial organisms like Salmonella. Bacterial colitis can be fatal if not treated rapidly and correctly. Delayed treatment also results in a higher incidence of potentially fatal complications, like laminitis.

Less frequently, adult horses can have chronic diarrhea. The causes and diagnostics for longer term diarrhea are somewhat different. In chronic diarrhea, horses must adjust their water and electrolyte intake and excretion in order to maintain hydration in the face of excessive water loss from the colon.