Approach Considerations
The diagnostic workup of St. Louis encephalitis virus (SLEV) infection is based on clinical features, history of exposure, and epidemiologic history. According to the US Centers for Disease Control and Prevention (CDC), guidelines for the diagnosis of arboviral encephalitis include febrile illness or mild aseptic meningitis or encephalitis and 1 of the following:
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A 4-fold increase in the antivirus antibody titer between the acute and the convalescent periods
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Isolation of SLEV via culture or detection via nucleic acid amplification from tissue, blood, or cerebrospinal fluid (CSF) [12]
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Specific immunoglobulin M (IgM) antibody to SLEV
The white blood cell (WBC) count is usually not elevated. Pyuria or proteinuria may occur. More than one third of patients develop hyponatremia due to SIADH.
Antibody evaluation
Antibody titers are considered to be significant if in excess of 1:320 by hemoagglutination inhibition, 1:128 by complement fixation, 1:256 by immunofluorescence, or 1:160 by the plaque reduction neutralization test.
CSF examination
CSF examination reveals pressure that ranges from normal to mildly elevated, normal glucose levels, and protein levels that range from normal to mildly elevated. Initially, polymorphonuclear leukocytic pleocytosis occurs, followed by lymphocytic or monocytic leukocytosis. In most cases, the CSF WBC count is less than 200 cells/µL.
Serologic testing
Initial serologic testing consists of IgM capture enzyme-linked immunoassay (ELISA), microsphere-based immunoassay (MIA), and IgG enzyme-linked immunoabsorbent assay (ELISA). If the initial results are positive, further confirmatory testing may delay the reporting of final results. It is also helpful to test CSF IgM antibody for the presence of CNS infection and local antibody production.
Evaluation of fatal cases
In fatal cases, diagnosis can be confirmed via nucleic acid amplification, histopathology with immunohistochemistry, and virus culture. The specimens require specialized laboratories, including those at the CDC and a few state laboratories.
CT scanning and MRI
Neuroimaging using conventional computed tomography (CT) scanning and magnetic resonance imaging (MRI) is not helpful in establishing a diagnosis of SLEV infection.
Histologic Findings
Microscopically, as in all viral encephalitides, widespread degeneration of single nerve cells occurs with neuronophagia and scattered foci of inflammatory necrosis involving the gray and white matter. The brain stem is relatively spared. Perivascular cuffing with lymphocytes and plasma cells occurs. Patchy infiltration of the meninges with microglial nodules also develops. Notably, no axonal or demyelinating lesions occur.
Pathologic features of St. Louis encephalitis are evident only in the CNS, although St. Louis encephalitis virus (SLEV) has been isolated from vitreous humor, lung, liver, spleen, and kidney.
Grossly, the brain and the meninges appear swollen, with widely distributed changes in the brain, mostly in the substantia nigra, thalamus, hypothalamus, cerebellum, cerebral cortex, basal ganglia, and cervical spinal cord, with more involvement of gray matter than white matter.
