West Nile virus

Background

• Single-stranded positive-sense enveloped virus in the family Flaviviridae and genus Flavivirus
• Essentially worldwide distribution
• Vector is mosquitoes
• Main reservoir is birds
• As well as being vector-borne, can also be transmitted by blood transfusion, solid organ transplantation, and vertically

Clinical Manifestations

• Incubation period of 3 to 14 days
• Up to 80% of infections are asymptomatic; of those with symptoms, fever is most common
• Some form of CNS disease develops in 1:150 people
  • Meningoencephalitis
  • Acute flaccid paralysis
  • 10% mortality, with about half of survivors having neuropsychiatric sequelae

Differential Diagnosis

• Other causes of meningitis, encephalitis, and acute flaccid paralysis

Diagnosis

• Serology
  • Serum or CSF for IgM antibodies is the standard test; usually positive by 3 to 8 days, and persist for 2 to 3 months
    • If submitted CSF, must also submit serum
  • May cross-react with other flaviviruses
  • Can be seronegative if immunosuppressed (e.g. rituximab), therefore may need to do PCR
• PCR
  • RNA virus, therefore needs reverse transcriptase step (RT-PCR)
  • Can be done on serum, CSF, and tissue
• On MRI, may have a slight preference for thalamus

Management

• Supportive care