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West Nile Virus Encephalitis


KEY FACTS

Terminology

  • West Nile virus (WNV), West Nile fever, West Nile neuroinvasive disease

  • Mosquito-transmitted acute meningoencephalitis

Imaging

  • Head CT usually normal

  • MR with DWI, T1 C+

    • Classic: Bilateral basal ganglia, thalamic hyperintensity

    • Patchy, poorly demarcated hyperintense foci in cerebral white matter on T2WI/FLAIR

    • Enhancement usually absent (has been reported)

    • DWI may show restricted diffusion

  • Other sites of involvement

    • Brainstem

    • Corpus callosum splenium

    • Mesial temporal lobes

    • Cerebellum

    • Spinal cord, cauda equina

Pathology

  • WNV: Flavivirus (like Japanese encephalitis)

  • Arthropod-transmitted (mosquito)

Clinical Issues

  • ~ 1/140 patients infected with WNV develop CNS symptoms

    • Incubation period: 3-14 days

  • ~ 80% of infected individuals asymptomatic

    • Mild febrile syndrome (West Nile fever) in 20%

    • Meningoencephalitis in < 1%

    • Rare: Anterior myelitis

  • CNS symptoms more common in diabetics, immunocompromised

  • Treatment is supportive; hydration, antipyretics, airway and seizure management as indicated

  • No human vaccine

    • Best way to prevent infection? Fight the bite

  • Mortality rate is ~ 10%

Axial T2WI MR in a 24-year-old man with West Nile encephalitis demonstrates asymmetric T2 hyperintensity in the thalami
. Posterior limb of left internal capsule is also involved
.

Axial T1 C+ MR in the same patient shows low signal in the thalami
, more pronounced on the left with no abnormal enhancement. Anatomic areas commonly affected by West Nile encephalitis are basal ganglia, thalami, brainstem, mesial temporal structures, and cerebellum.

Axial FLAIR MR in a patient with West Nile virus meningoencephalitis shows bilaterally symmetric high signal intensity in the thalami
. There is subtle hyperintensity in the basal ganglia and right internal capsule
.

Axial NECT in the same patient performed 6 hours prior to the MR exam is normal. No edema is seen in the thalami or white matter. Indeed, early in the disease course, CT scans are frequently unremarkable.

TERMINOLOGY

Abbreviations

  • West Nile virus (WNV), West Nile fever (WNF), West Nile neuroinvasive disease (WNND)

Definitions

  • Mosquito-transmitted acute meningoencephalitis caused by West Nile arbovirus

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