The lymphocytic reactive population comprised an approximate equal proportion of T and B lymphocytes. In both cases, the meningeal inflammatory component was relatively minor. There were focal areas of tissue necrosis associated with the most intense areas of chronic inflammation. This can lead to clinical and radiologic confusion with herpes simplex encephalitis, particularly when temporal lobe involvement is present. The accumulation of viral particles has been well demonstrated in neurons and, to a lesser degree, in glial cells.
Neuronal accumulation is largely cytoplasmic.
27 Mechanisms of neuronal or cellular entry, including any putative membrane viral receptors, are currently unknown. The pathologic changes described here are also similar to those seen with other arboviruses, such as with the equine encephalidities and St. Louis viral encephalitis. Typically, viral inclusions are not found, although previous human neuropathologic examinations are limited to the initial case published in 1959. Ultrastructural examination failed to show viral particles in this case, and this was not unexpected given the marked sampling limitations and postmortem artifactual changes. However, in keeping with the marked grey matter involvement at higher levels of the neural axis, including the lower brain stem, it is quite reasonable to assume that myelitic inflammatory changes involving the spinal grey matter were present.