West Nile virus

Background

Virology

• Single-stranded positive-sense enveloped virus in the family Flaviviridae and genus Flavivirus, within the Japanese encephalitis serocomplex
• Five lineages, of which only lineage 1 and 2 cause outbreaks in humans

Epidemiology

• Essentially worldwide distribution
• Vector is mosquitoes, particularly Culex mosquitoes
• Main reservoir is birds, particularly crows, jays, magpies, and ravens; also horses
  • Humans are an incidental host and not generally involved in transmission
• Disease in humans is seasonal, with most cases July through September
• 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, with range of 2 to 21 days
• Up to 80% of infections are asymptomatic
• Of the 20% with symptoms, fever is most common
  • Other common symptoms include headaches, generalized weakness, morbilliform or maculopapular rash (often occuring at defervescence), and myalgias
  • Less common symptoms include arthralgias, chills, eye pain, vomiting, diarrhea, and lymphadenopathy
  • Generally self-resolves within days to weeks with complete recovery, though fatigue may linger

West Nile Neuroinvasive Disease

• Some form of CNS disease develops in 1:150 people
• Meningoencephalitis
  • Meningitis with usual presentation (nuchal rigidity, headache, photophobia, phonophobia, fever)
  • Encephalitis with altered level of consciousness, lethargy, personality changes
• Acute flaccid paralysis with acute onset and rapidly-progressive limb weakness
  • Usually asymmetry, areflexic/hyporeflexic, with preserved sensation
  • Usually but not always in the context of meningoencephalitis
• Also may have nausea/vomiting, myalgias, arthralgias, chills, rash, ataxia, visual disturbance, tremors, myoclonus, bulbar dysfunction
• Usually self-resolves over weeks to months, but half have neuropsychiatric sequelae (including function/cognitive dysfunction and paralysis)
  • 10% mortality

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

Further Reading

• West Nile Virus: Review of the Literature. JAMA. 2013;310(3):308–315. doi:10.1001/jama.2013.8042
• West Nile virus neuroinvasive disease. Ann Neurol. 2006;60:286-300. doi:10.1002/ana.20959