Arboviruses tend to be limited to certain geographic areas, reflecting the reservoir and mosquito vector. St. Louis encephalitis virus is present throughout the United Statesin birds. California encephalitis virus, common in the Midwest, is carried by rodents and spread by mosquitoes. Easternequine encephalitis virus is limited to the East Coast in birds.Western equine encephalitis virus is present throughout theMidwest and West in birds. West Nile virus infection occurs worldwide and causes outbreaks of summer encephalitis inNorth America. The principal vector for West Nile virus isthe Culex pipiens mosquito, but the organism can be isolatedin nature from a wide variety of Culex and Aedes species.A broad range of birds serves as the major reservoir for WestNile virus.
Acute infectious encephalitis usually is preceded by a prodrome of several days of nonspecific symptoms such as sore
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West Nile encephalitis produces a broad spectrum of illness from asymptomatic infection to death. Severity of disease increases with advancing age. Most children are asymptomatic. Typical symptoms include mild, nonspecific extraneurologic illness characterized by fever, rash, arthralgias, lymphadenopathy, gastrointestinal complaints, and conjunctivitis.
The diagnosis is established presumptively by the presenceof characteristic neurologic signs, typical epidemiology findings, and evidence of infection by cerebrospinal fluid (CSF)analysis, electroencephalogram (EEG), and brain imagingtechniques. Encephalitis may result from infections withbacteria, Mycoplasma, Rickettsia, fungi, and parasites andfrom many noninfectious diseases, including metabolic diseases (encephalopathy), such as Reye syndrome, hypoglycemia, collagen vascular disorders, drugs, hypertension, andmalignancies.
The diagnosis of viral encephalitis is supported by examination of the CSF, which typically shows a lymphocytic pleocytosis, slight elevation in protein content, and normal glucose level. The CSF occasionally may be normal. Extreme elevations of protein and reductions of glucose suggest tuberculosis, cryptococcal infection, or meningeal carcinomatosis. The EEG is the definitive test and shows diffuse, slow wave activity, although focal changes may be present. Neuroimaging studies may be normal or may show diffuse cerebral swelling of the parenchyma or focal abnormalities. A temporal lobe focus on EEG or brain imaging is the characteristic feature of herpes simplex virus (HSV) infection.
Serologic studies should be obtained for arboviruses (including West Nile virus, if indicated by the patient risk factors), Epstein-Barr virus, Mycoplasma pneumoniae, cat-scratch disease, and Lyme disease. Additional serologic testing for less common pathogens should be performed as indicated by the travel, social, or medical history. In most cases of viral encephalitis, the virus is difficult to isolate from the CSF. Polymerase chain reaction tests for HSV, enteroviruses, West Nile virus, and other viruses are available and should be sent. Viral cultures of stool and a nasopharyngeal swab may also be obtained. Even with extensive testing and the use of polymerase chain reaction assays, the cause of encephalitis remains undetermined in one third of cases.