In rare cases, adult or growing horses develop chronic diarrhea without an obvious cause, and without showing other obvious signs of illness. They maintain a good appetite and attitude. Their physical exam and basic laboratory work is usually quite normal.

Diagnostics are performed, and all of the common causes are ruled out, but no diagnosis is made. Treatments are performed, but nothing seems to work.

Horses that are stressed may have watery manure. But this is usually intermittent, and corresponds to period of stress, separation from the herd or pasture mates, trailering and travel, etc...

Soft or watery manure may be normal for some horses. So if this is the only sign, it may be acceptable not to have a definitive diagnosis. However, usually there is an underlying cause that needs to be treated.

Anything that decreases colon function can cause inability to absorb water. Chronic inflammation of the colon wall, sand accumulation, and cancer of the colon wall can all result in chronic diarrhea and appear similar with basic diagnostics.

Unlike cases of acute colitis wherein horses become severely dehydrated, these horses are able to maintain their hydration in most cases.

I usually try to determine whether this is a result of sand accumulation. I have seen several cases in which there was no sand on fecal exam, but x-ray revealed sand in the colon. In these cases, I recommend psyllium to help move the sand out and management changes to prevent sand intake.

I may also perform a dental exam. Horses that cannot grind their feed properly will often have chronic loose manure or diarrhea. This may relate to larger than normal feed fibers reaching the colon. Water balance function is lost. Dental work is done to ensure the best grinding of feed possible.

In geriatric horses, there may not be anything that can be done. However, by switching to a pelleted feed, some of these horses regain normal manure.

In some cases, we give metronidazole (an antibiotic) to horses with chronic diarrhea and their manure firms up. I do not know why this happens. It is thought that this drug changes the flora in the intestine and improves colon function.

Feed allergies might also cause chronic diarrhea, but this is poorly understood in horses. Changing feeds, and possibly using steroids, might help. However, in my experience, steroids have not been particularly helpful.

Probiotics of different kinds are also used in these cases. There are many commercial probiotics available. I have had the best results with transfaunation - manure from a healthy horse given by stomach tube.