Description

• Inflammation of the meninges and brain associated with clinical evidence of neurologic dysfunction
• System(s) affected: nervous
• Synonym(s): meningoencephalitis

Epidemiology

Incidence
3.5 to 7.4/100,000 per year

Prevalence

• 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.

Risk Factors

• 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

General Prevention

• 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

• Seizures
• Hyperthermia
• Increased intracranial pressure (ICP)
• Inappropriate antidiuretic hormone (ADH) secretion