I have had horses over the years demonstrate most of the symptoms you have, except for the circling left and neurological stuff. But two or three times over the last fourteen odd years of owning horses, I have had a horse stop eating, stop drinking, get very depressed, and run a very low grade fever. The vet treated it like a colic, gave the horse some fluids, etc. but I wouldn't describe it as a colic since the horse wasn't in any obvious pain, just not feeling well. Blood tests showed nothing unusual. Obviously the main concern is that it might develop into a colic since the horse isn't eating or drinking. In any case, it only lasted a few days -- probably within five days the horse was fine. Vet figured it was a virus of some sort.
Not sure if that helps you, especially as your horse was showing signs of neurological impairment, which is a lot scarier.