Because of the large volume of the intestines of horses, massive fluid losses can occur in a short time. Thus, diarrhea in adult horses can be a serious event, and rates of illness and death may exceed those in other animals and humans. Any case of diarrhea in your horse that lasts more than a day or two, or that occurs repeatedly, is a cause for concern. Your veterinarian should be contacted for evaluation of the horse. Although a definitive cause can be determined in less than 50% of cases of diarrhea, treatment of most horses and foals with diarrhea is similar and can be started despite the lack of a precise diagnosis.