Of the arbovirus infections in the United States, eastern equine encephalitis (EEE) is among the most serious, as a large proportion of those infected develop encephalitis; approximately one-third die and a similar number, more often children, are left with disabling abnormalities— mental retardation, emotional disorders, recurrent seizures, blindness, deafness, hemiplegia, extrapyramidal motor abnormalities, and speech disorders. While only a small proportion of those exposed become infected, the poliomyelitis and parkinsonian syndromes of the Flaviviruses may be permanent residua as mentioned earlier (Solomon). On the other extreme, La Crosse encephalitis, which affects mostly children, has an almost uniformly benign outcome. The rate of progression from a nondescript febrile viral syndrome to encephalitis is similarly low in the arbovirus infections and mortality rate varies from 2 to 12 percent in different outbreaks.
Diagnosis The CSF findings are much the same as in aseptic meningitis (lymphocytic pleocytosis, mild protein elevation, normal glucose values). Occasionally, early sampling of CSF may show few or no cells and subsequent tests may be more typical of an inflammatory disorder. Recovery of virus from blood or CSF is usually not possible and PCR testing is routinely only applied during local epidemics and for the detection of herpes viruses. Recently, it has been possible to use next generation sequencing to identify obscure causes of encephalitis. However, antiviral immunoglobulin (Ig) M antibody is present in the serum and CSF within the first days of symptomatic disease and can be detected and quantified by means of ELISA, making it preferable to other testing serologic for specific diagnosis. Some patients have not developed antibodies by the time of admission to the hospital and the test may have to be repeated in several days. The MRI may be normal or show signal changes and edema in the cortex, basal ganglia, or thalamus (the latter is described particularly in the
Japanese B virus group, West Nile, EEE, and rabies).
Pathology Perivascular cuffing by lymphocytes and other mononuclear leukocytes and plasma cells, as well as a patchy infiltration of the meninges with similar cells, are the usual histopathologic hallmarks of viral encephalitis. There is widespread degeneration of single nerve cells, with neuronophagia as well as scattered foci of inflammatory necrosis involving both the gray and white matter. The brainstem is relatively spared. In some cases of eastern equine encephalitis, the destructive lesions may be massive, involving the major part of a lobe or hemisphere and are readily displayed by MRI, but in the other arbovirus infections the foci are microscopic in size (Deresiewicz et al). West Nile virus may produce a regional pattern of neuronal damage that affects the anterior horn cells of the spinal cord, a poliomyelitis, as mentioned earlier. Pathologic descriptions of this process have been provided by our colleagues and by others (see Asnis et al).