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- Inflammation of the meninges and brain associated with clinical evidence of neurologic dysfunction
- System(s) affected: nervous
- Synonym(s): meningoencephalitis
3.5 to 7.4/100,000 per year
- Seasonal variation (e.g., arboviruses, enteroviruses)
- Nonseasonal (e.g., herpes simplex virus [HSV], varicella zoster virus [VZV])
- Vaccines have altered prevalence and seasonality (e.g., mumps and measles were common in winter before routine immunization).
Etiology and Pathophysiology
- Most enter through respiratory or GI tract with subsequent hematogenous spread.
- Other modes of entry include:
- Neurotropic spread (e.g., rabies)
- Mosquito vectors (e.g., Japanese encephalitis, dengue virus encephalitis)
- Tick vectors (e.g., tick-borne encephalitis virus, Powassan virus, and Colorado tick fever virus)
- Specific symptom complexes:
- Neuronal involvement: seizures
- Oligodendroglia: may cause demyelination, cortical infection, or reactive parenchymal swelling; altered levels of consciousness
- Brainstem neurons: coma, respiratory failure
- Microglia, macrophages: neurologic dysfunction
- Perivascular mononuclear infiltrates seen in postinfectious encephalomyelitis
- Most common etiologies in the United States are HSV, West Nile virus (WNV), and enteroviruses. Zika virus has gained attention (associated with congenital microcephaly); most U.S. cases reported in travelers to Caribbean, Central and South America
- Despite extensive evaluations, the etiologic agent is often not identified in viral encephalitis.
- Increased incidence in infants and elderly
- Impaired immunity
- Occupation (e.g., lab or animal care workers)
- Recreational activities (e.g., camping, hunting—mosquito; arthropod exposure)
- Transfusion and transplantation
- Travel to endemic areas
- Recent vaccinations/unvaccinated status
- Use of mosquito and tick repellents (DEET, picaridin) and appropriate clothing to prevent arthropod bites
- Avoid travel to endemic areas. Promptly remove ticks.
- Eliminate mosquito-breeding sources.
- Vaccination (e.g., mumps, measles, polio, rabies, Japanese encephalitis)
Commonly Associated Conditions
- Increased intracranial pressure (ICP)
- Inappropriate antidiuretic hormone (ADH) secretion