Thanks for getting back to me Caroline:
As Maggie is at 30 years of age, and along with the Cushing's, she's a tough cookie and lucky to have you caring for her. Obviously I can't examine her and give you a definitive diagnosis (although I would love to visit France again...) but there are two possible conditions that are a strong possibility that have me concerned. Based on your history of her care, I think we can safely rule out internal parasites, although it wouldn't hurt to check. With older horses that I have seen that have chronic diarrhea, it is usually associated with inflammatory bowel disease (IBD) or a type of cancer (often lymphoma). As you are probably aware, with Cushing's disease, the body produces too much cortisol, and one of the effects of this is a weakened immune system. This is why the life span can be shortened, as the horse is more prone to disease/infections, and has less of an ability to fight them off. If either of these is the case with Maggie, neither of them would be good news, as the treatment for IBD is steroids (which she already has too much of), and an intestinal cancer is terminal. That said, however, it is relatively easy to diagnose by your Veterinarian obtaining a rectal mucosal biopsy (the lining of the rectum). I'm sure your Vet probably knows this procedure, but it involves mild sedation, then performing a rectal exam along with an open blood collection tube, and pinching off a piece of the rectal lining between the thumb and the edge of the tube. This poses no danger to the horse when done correctly. The sample is then sent off to a pathologist for analysis. You might also consider having some normal bloodwork performed, to rule out any type of organ failure, which can contribute to diarrhea as well.
I took you at your word in saying that you were prepared for an honest answer, and I must say that at her age and medical condition, the prognosis is not good. That said, I still recommend looking into a diagnosis, as I have been pleasantly surprised more than once!
I will certainly keep good thoughts for Maggie, and if you have the time, please keep me posted as to what you decide to do. It gets frustrating being in this position, and not knowing how my "on-line patients" are doing...
Thanks again for getting back to me, and please let me know if you have any further questions.
All the best, ***** ***** Happy New Year to you too, Caroline.