In North America, viruses that cause encephalitis include the arboviruses, HSV-1, herpes zoster, Epstein-Barr virus (EBV), and rabies. Entry portals are highly virus specific for the encephalitis-producing viruses. The arboviruses ( arbo meaning "arthropod borne") are transmitted by mosquitoes and ticks, and rabies is transferred by the bite of an infected animal. Impaired immune status may play a role in herpes zoster and cytomegalovirus (CMV) encephalitis. Common to all is preliminary viral invasion of the host at a site where replication takes place that is outside the central nervous system. Most viruses then reach the nervous system hematogenously during viremia. However, at least three important viruses—rabies, herpes simplex virus (HSV), and herpes zoster—reach the spinal cord and eventually the brain by traveling backward within axons from a distal site where they have gained access to nerve endings.

Once in the brain, the virus enters neural cells. Neurologic dysfunction and damage are caused by the disruption of neural cell functions by the virus and by the effects of the host's inflammatory responses. Gray matter is predominantly affected, resulting in cognitive and psychiatric signs, lethargy, and seizures. Multifocal white matter damage occurs predominantly in postinfectious encephalomyelitis and rarely during acute encephalitis. Sensorimotor deficits referable to one hemisphere or to the spinal cord are more typical of this immune-mediated pathologic process, which may follow viral encephalitis and meningitis of any type. Clinical Features

Encephalitis should be considered in patients presenting with the following clinical features singly or in combination: new psychiatric symptoms, cognitive deficits (e.g., aphasia, amnestic syndrome, or acute confusional state), seizures, and movement disorders. Features of meningeal involvement, such as headache and photophobia, are usually, but not invariably, present. The same is true for fever.

Patients with herpes zoster (shingles or chickenpox), EBV, or CMV encephalitis (lymphadenopathy and hepatosplenomegaly) may have a history and signs typical of clinical syndromes caused outside the central nervous system by these viruses. Other circumstances of the case may suggest both the broad diagnosis of encephalitis and a specific viral cause (Table.,,22.7.-.3). For example, a late-summer encephalopathy suggests the possibility of an arbovirus encephalitis, and an animal bite for which no antirabies treatment was obtained has obvious relevance.

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