Got this on-line hope it helps....
Equine diarrhea has many causes and you must be a part time investigator to rule out each cause. Your first consideration should be whether the stools are simply loose, or true diarrhea. Diarrhea is loose, watery, unformed manure, with frequent bowel movements. Diarrhea in horses is not a disease in itself, it's a symptom. Sudden onset of diarrhea means that something is amiss and contacting your veterinarian is advised. Being unsure as to the cause, you may wish to isolate your horse from other animals and take prophylactic precautions until the nature of his upset is found.
Common causes of equine diarrhea are:
SAND INGESTION
If you live in an area with sandy soil or the stall's footing is sand, there's likelihood that sand ingestion may be the culprit. As your horse grazes or eats hay from the ground, small amounts of sand can be ingested and will accumulate after some time. Fecal samples are taken for diagnosis and your vet may suggest adding psyllium to his diet on a regular basis as well as feeding his hay in a bunker, rather than ground feeding.
INTESTINAL PARASITES
Strongyles have been implicated at times as a cause of diarrhea. A good deworming program is an easy and inexpensive way for prevention. Your veterinarian will be happy to suggest an excellent rotational worming protocol.
BACTERIA
Such bacterial diseases as Salmonella and Potomac Horse fever can produce profuse and watery stools. Special precautions should be taken when handling horses during these episodes. Veterinary intervention and isolating your horse is required. Salmonella in particular is quite contagious and can be passed to other horses and sometimes, to humans.
TUMORS
Tumors in the digestive tract can produce diarrhea. The most common are the cancer growths of lymphosarcoma and squamous cell carcinoma. An equine hospital is equipped with the latest technology for diagnosis and removal of these tumors may be indicated.
UNKNOWN CAUSES
Having exhausted all known causes of diarrhea your horse may still be having problems. After consulting your vet, he may
suggest that you try a different type of hay or grain. I can bear witness to the "unknown cause" of diarrhea.
Spinner was my coming two year old colt and suffered with diarrhea for a good three months before we found the cause. He had been poked, prodded, and x-rayed with still no root cause. He never lost weight nor seemed to suffer any ill effects from his symptom and he began to like both Probiotics and Kaopectate. My vet suggested that we try a different type of hay. Voila! The simple change of a hay source did the trick. As it turned out, this colt was particularly sensitive to nitrogen (found in fertilizer) in his hay. The following year we used less nitrogen in our fertilizer mix and he's never had a problem since.
I started him back on Sand Clear last night and we also have electrolytes that he hates! What fun
I started this regimine immediately and she had normal stools within 48 hours.
Merry Christmas to ya.