A 15-year-old Quarter Horse gelding presented to the Cornell University Hospital for Animals for the primary complaint of acute ataxia. Earlier that day he was noted to appear off during work and was then seen to be dribbling urine. The referring veterinarian was called to the barn and felt a dramatically distended bladder on rectal palpation. The referring veterinarian then passed a urinary catheter, drained the bladder, and recommended immediate referral to Cornell. On presentation the gelding was bright, alert, and responsive and his vital parameters were within normal limits. He was immediately admitted to isolation, due to the possibility of an equine herpesvirus infection. A neurologic exam was performed and he was found to be a grade 2/5 ataxic in both hind limbs, with placement deficits in the left hind. He had full body muscle fasciculations. All point-of-care bloodwork values were normal. Soon after admission he began dribbling urine and was found to have an enlarged bladder on rectal palpation. This seminar will explore the differentials and diagnosis of acute ataxia in the horse. An in-depth summary of equine protozoal myelitis, one of the primary differentials in the case, will be included, highlighting the difficulties in diagnosing and treating this disease. The outcome and management of the above case will be discussed.