West Nile virus

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West Nile virus (WNV) is an arbovirus and one of the Flavivirus genus known to cause neuroinvasive disease, including Flavivirus encephalitis. According to the CDC, the majority of mosquito-borne disease in the continental United States is attributed to West Nile virus infection .

Epidemiology

Epidemics have occurred along the migratory routes of birds in the US, Europe, Africa, the Middle East and West Asia . In recent history, frequency and severity of West Nile virus outbreaks are increasing . Of note, West Nile virus is implicated in the majority of viral encephalitis cases in the US .

Clinical presentation

Presentation varies with the severity of response to West Nile virus infection. The majority will remain asymptomatic. Approximately 20% will develop classical symptoms of a viral fever - or West Nile fever in this case - including fever, malaise, fatigue, myalgia, arthralgia, retro-orbital pain, maculopapular rash and lymphadenopathy .

Less than 1% of those infected will present with neuroinvasive disease, often in the context of advanced age or immunodeficiency :

  • neuromuscular involvement
    • most commonly a poliomyelitis-like syndrome, with acute onset, asymmetric flaccid paralysis of one or more limbs
    • encephalitis, bowel and bladder dysfunction often occur in conjunction
    • myalgia and somatosensory disturbance are less common
    • may be mistaken for Guillain-Barré syndrome
  • encephalitis, meningitis or encephalomeningitis
    • presenting symptoms including headache, neck stiffness, somnolence, coma, movement disorder, confusion and seizures

CSF sampling typically demonstrates an abnormally high lymphocyte count, or pleocytosis, with high total protein and WNV-specific IgM antibodies .

Pathology 

West Nile virus is a single-stranded RNA of the genus Flavirirus and family Flaviviridae. Birds are the host organism and mosquitos, via their salivary glands, the vector of disease transmission.

Radiographic features

MRI
  • T2/FLAIR: 
    • hyperintensity within deep brain structures, including the thalami, basal ganglia and midbrain structures ; the latter includes the red nucleus, cerebral peduncle and substantia nigra
    • increased signal intensity in the mesial temporal structures has been described in a number of cases
  • DWI: 
    • ​high diffusion signal has been reported within the basal ganglia and disseminated throughout the white matter
    • interestingly, DWI may detect inflammation prior to signal intensity changes detected using T2/FLAIR

History and etymology

The first isolation of West Nile virus dates to 1937, from the West Nile district of Northern Uganda .

Differential diagnosis

Infection by one of the pathogenic members of the Flavivirus genus may present similarly, including:

In addition, basal ganglia T2 hyperintensity has numerous differential diagnoses, notably including:

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