[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Other viruses and prions](https://durgical.com/en/microbiology/groups/other-viruses-and-prions/) › West Nile virus

[Microbiology](https://durgical.com/en/microbiology/) · [Other viruses and prions](https://durgical.com/en/microbiology/groups/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

## Key points

- Flavivirus; reservoir in birds, Culex vector; humans and horses are dead-end hosts.
- Mostly asymptomatic; neuroinvasive disease is rare and hits older and immunocompromised people.
- Acute asymmetric flaccid paralysis (polio-like) in a febrile adult in summer.
- Diagnosis: IgM in CSF; beware of cross-reaction with dengue.

## 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) →](https://durgical.com/en/conditions/viral-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

## Memory aid

"Bird to Culex, Culex to neuron": West Nile travels in that order and the human is the last stop.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/microbiology/west-nile-virus/)

## Related

- [Equine encephalitis viruses and Mayaro virus](https://durgical.com/en/microbiology/equine-encephalitis-viruses-and-mayaro-virus/)
- [Dengue virus](https://durgical.com/en/microbiology/dengue-virus/)
- [Yellow fever virus](https://durgical.com/en/microbiology/yellow-fever-virus/)
- [Zika virus](https://durgical.com/en/microbiology/zika-virus/)

## Related conditions

- [Viral encephalitis (herpes simplex encephalitis)](https://durgical.com/en/conditions/viral-encephalitis/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Other viruses and prions](https://durgical.com/en/microbiology/groups/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](https://www.who.int/news-room/fact-sheets/detail/west-nile-virus) (WHO, 2025); [About West Nile | West Nile Virus | CDC](https://www.cdc.gov/west-nile-virus/about/index.html) (CDC, 2025).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/microbiology/west-nile-virus/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/west-nile-virus/