West Nile Virus (and Other Arbovirus Encephalitis)



  • Viruses transmitted by an arthropod vector that can cause CNS infections, undifferentiated febrile illness, acute polyarthropathy, and hemorrhagic fevers
  • Most arboviral infections are asymptomatic.
  • West Nile virus (WNV) is an arbovirus in the flavivirus family.
  • WNV was first recognized in the United States in 1999 during an outbreak of encephalitis in New York City and is now the most common epidemic viral encephalitis in the United States.
  • >150 arboviruses are known to cause human disease.
  • Other arboviruses can produce similar syndromes or acute hemorrhagic fevers.


  • Arboviruses are spread by mosquitoes, ticks, and sand flies. The major vector for WNV in the United States is the Culex mosquito. WNV has been spread through blood transfusions, transplanted organs, and, rarely, intrauterine.
  • Arboviruses are maintained in nature through cycles of transmission among birds, horses, and small animals. Humans and domestic animals are infected incidentally as “dead-end” hosts.
  • Disease among birds has been a hallmark of WNV in the United States and has served as a sensitive surveillance indicator of WNV activity.
  • Each North American arbovirus has specific geographic distributions and is associated with a different ratio of asymptomatic to clinical infections. These agents cause disease of variable severity and have distinct age-dependent effects. WNV has now been identified throughout the United States and is also found in Europe, Africa, and Asia.


  • The peak incidence of arboviral encephalitis occurs during the late summer and early fall. Seasonality depends on the breeding and feeding seasons of the arthropod host.
  • WNV is the leading cause of arboviral CNS disease. Encephalitis is most commonly seen in older adults, generally aged >60 years, and severe WNV disease is more common in immunocompromised individuals. Symptomatic cases of WNV in children are unusual.
  • Fewer than 10 cases each of Eastern equine encephalitis and Western equine encephalitis are reported nationally each year. Eastern equine encephalitis tends to produce a more fulminant illness than LaCrosse or Western equine encephalitis.

General Prevention

  • Public health department efforts focus on surveillance of viral activity to predict and prevent outbreaks:
    • Active bird surveillance to detect the presence of WNV activity
    • Active mosquito surveillance to detect viral activity in mosquito populations
    • Passive surveillance by veterinarians and human health care professionals to detect neurologic illnesses consistent with encephalitis
    • Screening of blood and organ donors
  • Personal precautions to avoid mosquito bites including use of repellents, protective clothing, and screens; avoiding peak feeding times (dawn and dusk); and installation of air conditioners
  • Coordination of mosquito control programs in endemic infection areas
  • Vaccines for prevention of most arbovirus infections are not available. A vaccine is available for Japanese encephalitis, yellow fever (YF), and tick-borne encephalitis for travelers to endemic areas who are planning prolonged stays.
  • Infection control measures
    • Standard precautions are recommended for the hospitalized patient.
    • Respiratory precautions are recommended when vector mosquitoes are present.
    • Patients with dengue and YF can be viremic and should be protected against vector mosquitoes to avoid potential transmission.


  • The incubation period for WNV and other arboviral encephalitis agents is 2 to 14 days (up to 21 days in immunocompromised hosts).
  • The incubation period reflects the time necessary for viral replication, viremia, and subsequent invasion of the CNS.
  • Virus replication begins locally at the site of the insect bite; transient viremia leads to spread of virus to liver, spleen, and lymph nodes. With continued viral replication and viremia, seeding of other organs including the CNS occurs.
  • Virus can rarely be recovered from blood within the 1st week of onset of illness but not after neurologic symptoms have developed.


  • Arboviruses can be divided into two groups based on the predominant clinical syndrome.
    • In the United States, seven arboviruses are important causes of encephalitis: WNV, California encephalitis virus (LaCrosse strain), Eastern equine encephalitis, Western equine encephalitis, St. Louis encephalitis, Powassan encephalitis virus, and Venezuelan equine encephalitis virus.
    • Arboviruses such as YF, dengue fever, chikungunya, and Colorado tick fever typically cause acute febrile diseases and hemorrhagic fevers and are not characterized by encephalitis.
  • Clinical manifestations of WNV
    • Asymptomatic: most common
    • Self-limited febrile illness: 67% of symptomatic cases
    • Neuroinvasive disease: aseptic meningitis, encephalitis, or flaccid paralysis—1% cases

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