What is your diagnosis? Cerebrospinal fluid from a dog

A 6-year-old spayed female Boxer dog was presented to the Louisiana State University Veterinary Teaching Hospital with a 3-week history of ataxia, walking sideways, crossing over of limbs, and dragging her hind feet. She had a repair of her left cranial cruciate ligament approximately 4 months previously with uneventful recovery. There was no history of diarrhea or vomiting. Results of a CBC and urinalysis were unremarkable. The only abnormality noted on serum biochemical profile was mild hypercholesterolemia (296 mg/dL, reference interval [RI] 150–240 mg/dL). Cutaneous and mucosal lesions were not present. Age-related changes were seen on thoracic radiographs. Neurologic examination revealed severe ataxia, worse on the left side, and a left head tilt. She had delayed hopping in the forelimbs and fell when hopped in the rear. No abnormalities were observed on computed tomographic scan of the brain. Magnetic resonance imaging of the brain revealed a lesion involving approximately 50% of the left side of the pons and medulla and extending dorsally to the 4th ventricle. Cerebrospinal fluid (CSF) collected from the atlanto-occipital cistern revealed a total nucleated cell count of 2000/mL (RI o 5/mL) and a total protein of 1898 mg/dL (RI o 30 mg/dL). A cytocentrifuged preparation of CSF was stained with Wright–Giemsa and examined (Figure 1).