The gross findings in viral encephalitis may be underwhelming. The intact brain may show hyperemia (as seen here) due to vascular congestion and edema. Fixed and cut specimens may show punctate hemorrhages in the white matter.

Acute and subacute encephalitis are characterized by meningeal and perivascular inflammatory infiltrates that are predominantly T cells. In addition, “microglial nodules” composed of small, tight clusters of activated microglia may be seen around dying neurons.

Cerebrum
Slide 76 (cerebrum, luxol blue/cresyl violet) [DigitalScope]
Slide 76b (toluidine blue & eosin) [DigitalScope]
  
The cerebral cortex is loosely stratified into layers containing scattered nuclei of both neurons and glial cells.  Examine the layered organization of the cerebral cortex using slide 76 stained with luxol blue/cresyl violet [ORIENTATION] (which stains white matter tracts and cell bodies) or toluidine blue and eosin [ORIENTATION] (TB&E, toluidine blue stains the nuclei and RER of cells whereas eosin stains membranes and axon tracts).  Typically one or more sulci (infoldings) will extend inward from one edge of the section.  Examine the gray matter on each side of the sulcus using first low and then high power.  Neurons of the cerebral cortex are of varying shapes and sizes, but the most obvious are pyramidal cells.  As the name implies, the cell body is shaped somewhat like a pyramid, with a large, branching dendrite extending from the apex of the pyramid toward the cortical surface, and with an axon extending downward from the base of the pyramid.  In addition to pyramidal cells, other nuclei seen in these sections may belong to other neurons or to glial cells also present in the cortex.  You may be able to see subtle differences in the distribution of cell types in rather loosely demarcated layers. There are 6 classically recognized layers of the cortex:

1. Outer plexiform (molecular) layer: sparse neurons and glia
2. Outer granular layer: small pyramidal and stellate neurons
3. Outer pyramidal layer: moderate sized pyramidal neurons (should be able to see these in either luxol blue or TB&E-stained sections)
4. Inner granular layer: densely packed stellate neurons (usually the numerous processes aren’t visible, but there are lots of nuclei reflecting the cell density)
5. Ganglionic or inner pyramidal layer: large pyramidal neurons (should be able to see these in either luxol blue or TB&E-stained sections)
6. Multiform cell layer: mixture of small pyramidal and stellate neurons

Pyramidal cells in layers III and V tend to be larger because their axons contribute to efferent projections that extend to other regions of the CNS –pyramidal neurons in layer V of motor cortices send projections all the way down to motor neurons in the spinal cord!

Deep to the gray matter of the cerebral cortex is the white matter that conveys myelinated fibers between different parts of the cortex and other regions of the CNS. Be sure you identify the white matter in both luxol blue and TB&E-stained sections, as it will appear differently in these two stains. Review the organization of gray and white matter in cerebral cortex vs. spinal cord.

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An immunohistochemical stain for West Nile Virus is positive.