Microbiology · Other viruses and prions
West Nile virus !
- Virus
- Enveloped
- For completeness
A bird flavivirus spread by Culex mosquitoes; most infected people have no symptoms, but a few develop meningitis or encephalitis.
Updated 2026-11-01
At a glance
- Shape
- Spherical, icosahedral capsid
- Genome
- Single-stranded, positive-sense RNA
- Envelope
- Enveloped
- Family
- Flaviviridae, genus Orthoflavivirus (Japanese encephalitis complex)
- Size
- About 50 nm
Morphology
- Envelope with the E and M proteins
- Related to dengue, yellow fever and Japanese encephalitis viruses
- Positive-sense RNA genome translated as a single polyprotein
Transmission
- Bite of Culex mosquitoes infected by feeding on birds
- Rarely transfusion, organ transplant, breastfeeding or mother to fetus
- No casual person-to-person spread
Reservoir: Wild birds; humans and horses are dead-end hosts
Vector: Mosquitoes of the genus Culex.
Isolation precautions
What the care team adds to standard precautions.
- Standard
Virulence factors
- Multiplies in skin and lymph nodes and reaches the blood
- Crosses the blood-brain barrier mainly in older and immunocompromised people
- Damages neurons of the brainstem and anterior horn of the spinal cord: flaccid paralysis
Diseases it causes
- Neuroinvasive disease: meningitis, encephalitis and acute flaccid paralysis Viral encephalitis (herpes simplex encephalitis) →
- West Nile fever (febrile illness with headache, aches and rash)
Clinical picture
- About 80% of infections cause no symptoms
- About 1 in 5 develop fever with headache, muscle aches and rash
- About 1 in 150 develop neuroinvasive disease, more often over age 50 and in transplant recipients
- Asymmetric flaccid paralysis resembling poliomyelitis
Diagnosis
- IgM in cerebrospinal fluid or serum (capture ELISA)
- Neutralisation tests to confirm, because of cross-reaction with other flaviviruses such as dengue
- RT-PCR on blood or cerebrospinal fluid, most useful in the immunocompromised
Treatment
No specific antiviral: supportive treatment, with hospitalisation, intravenous fluids and respiratory support in neuroinvasive disease.
Drug class only; doses and regimens are set by the clinician from the current guideline.
Prevention
- Avoid mosquito bites and remove breeding sites
- Blood donor screening where transmission occurs
- No human vaccine; there is one for horses
Test yourself
Think of the answer, then tap the question to see it.
What are the reservoir and the vector of West Nile virus?
Birds are the reservoir and Culex mosquitoes the vector.
What share of West Nile virus infections is neuroinvasive?
About 1 in every 150 infections.
Which polio-like neurological picture can it cause?
Acute asymmetric flaccid paralysis from damage to anterior horn motor neurons.
Which test is used to diagnose West Nile neuroinvasive disease?
Detection of specific IgM in cerebrospinal fluid.
Related
Related conditions
← Back to the group: Other viruses and prions
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: West Nile virus (WHO, 2025); About West Nile | West Nile Virus | CDC (CDC, 2025).
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Representative model, for educational purposes only.