A 3-year-old intact male domestic shorthair presents with progressive neurological signs including ataxia, vertical nystagmus, and behavioral changes over 6 weeks. CSF analysis reveals a protein of 420 mg/dL with a mixed pleocytosis (predominantly neutrophils and macrophages). Coronavirus antibody titer in CSF is 1:3200. Abdominal ultrasound shows no effusion. Which histopathological finding in the brain would be MOST pathognomonic for the underlying diagnosis?

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