Some horses develop semiformed feces when first introduced to lush pastures, alfalfa hay, or a temporarily stressful situation (eg, trailer ride, racing, showing, visit to a veterinary hospital). This change in fecal consistency is not of medical importance as long as the horse is healthy in all other regards, but owners may be concerned. It is important that horses with diarrhea have a physical examination and appropriate laboratory tests to exclude infectious causes and to determine whether treatment is required. Usually, the fecal consistency returns to normal when the horse adapts to its new diet or the stressful situation resolves.
Other causes of diarrhea or semiformed to watery feces in horses include grain overload, thromboembolic disease of the colon, peritonitis, antimicrobial treatment, renal failure, numerous toxicoses (eg, blister beetles [cantharidin], salt poisoning, slaframine, amitraz, propylene glycol, phosphorus, selenium, nicotine, reserpine, arsenic, mercury, monensin, organophosphates, oleander, Japanese yew, castor bean, avocado, thorn apple, potatoes, heath, algae, acorn or oak, Hypericum, corn cockle, mycotoxicoses, horse tail [scouring rush]), and hyperlipidosis.