Diarrhea is fecal material that is abnormally loose, watery, or more frequent than normal, indicating that the large intestine is not absorbing fluid at its usual rate. In horses, diarrhea is a symptom rather than a diagnosis; it signals gastrointestinal disturbance, and the cause determines severity and treatment. Normal equine normal defecation baseline produces well-formed fecal balls; any shift toward liquid or paste consistency is a departure from normal.
Common causes in adult horses include dietary changes (sudden introduction of lush grass, grain overload), parasite burden (cyathostomins emerging from the intestinal wall in late winter), antibiotic-associated disruption of gut flora, and infectious agents such as Salmonella or Potomac Horse Fever. In foals, foal heat diarrhea is a common benign occurrence in the first week of life, coinciding with the mare’s first post-foaling estrus; it typically resolves without treatment. Persistent or bloody diarrhea, or diarrhea accompanied by fever and systemic signs, is a veterinary emergency.
Any horse with significant diarrhea is at risk of rapid rapid fluid loss and dehydration risk, because large volumes of fluid are lost with each loose bowel movement. Assessment includes skin pinch testing and gum evaluation alongside monitoring of heart rate and temperature. Infectious diarrhea carries biosecurity implications; horses with suspected Salmonella should be isolated, and handlers should use barrier precautions to prevent spread. Treatment ranges from supportive care (fluid replacement, electrolytes, probiotics) to targeted antimicrobials depending on cause and severity.
Further Reading
Diarrhea as a clinical sign is described in the diarrhea article on Wikipedia. Causes, assessment, and treatment of diarrhea in horses are covered in the Merck Veterinary Manual section on Digestive Disorders of Horses.