West Nile virus

Background

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

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

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

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

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