West Nile virus

West Nile virus (WNV) is a single-stranded RNA virus belonging to the genus Orthoflavivirus within the family Flaviviridae. It is genetically related to Japanese encephalitis, dengue, Zika, and yellow fever viruses. The virus exhibits an enveloped, icosahedral structure with a virion diameter of 45–50 nm, composed of structural proteins E and M embedded in a host-derived lipid envelope. Its genome is approximately 11,000 nucleotides long and encodes three structural and seven nonstructural proteins, with replication dependent on host and viral proteases for polyprotein cleavage. WNV maintains a natural cycle between birds and Culex mosquitoes, with humans and horses serving as incidental, dead-end hosts.

Infection with WNV is usually asymptomatic—estimated at ~80% of cases. Among symptomatic individuals, ~20% develop West Nile fever, characterized by fever, headache, fatigue, myalgia, nausea, vomiting, occasional skin rash (typically on the trunk), and lymphadenopathy. A small proportion (<1%) experience neuroinvasive disease, including encephalitis, meningitis, or poliomyelitis, presenting with high fever, neck stiffness, altered mental status, coma, tremors, convulsions, muscle weakness, and paralysis. Neuroinvasive disease carries significant morbidity and mortality, particularly among older adults and immunocompromised individuals. Diagnosis relies on detection of IgM antibodies in serum or cerebrospinal fluid, which may persist beyond one year. No specific antiviral therapy or vaccine is available for humans.

WNV is endemic across sub-Saharan Africa, parts of southern Europe, the Middle East, Central and South Asia, and Australia. Since its introduction into the United States in 1999 via importation from Israel and Tunisia, it has spread widely across North America—from Canada to Venezuela—and established persistent transmission cycles. Major outbreaks have occurred in Greece, Israel, Romania, Russia, and the USA, often coinciding with bird migration corridors and peak mosquito activity seasons. The virus’s emergence outside its original range underscores the risk of climate-driven vector expansion and global travel-related importation. Surveillance data indicate that human cases are underreported due to asymptomatic infections and limited diagnostic capacity in many regions.

The primary route of human infection is through the bite of infected female Culex species mosquitoes, which acquire the virus after feeding on infected birds. The virus circulates in a bird–mosquito–bird cycle, with birds acting as amplifying hosts and mosquitoes as vectors. Human infection occurs when mosquitoes feed on viremic birds and subsequently bite humans or other vertebrates. Rarely, transmission can occur via blood transfusion, organ transplantation, or breast milk; one case of transplacental transmission has been reported. There is no evidence of sustained human-to-human transmission through casual contact or respiratory droplets.

Older adults (>60 years) and immunocompromised individuals are at highest risk for developing severe neuroinvasive disease. While all age groups can be infected, children and young adults more commonly experience mild or asymptomatic illness. Individuals with underlying conditions such as diabetes, hypertension, or chronic kidney disease may also face increased risk of complications. Occupational exposure (e.g., laboratory workers, field epidemiologists, mosquito control personnel) and outdoor recreational activities during peak mosquito season elevate exposure risk. No specific ethnic or geographic susceptibility has been identified beyond ecological and climatic drivers of vector abundance.

Prevention focuses on reducing exposure to mosquito bites through personal protective measures (e.g., use of insect repellent, wearing long-sleeved clothing, eliminating standing water) and community-level vector control (e.g., larviciding, adulticiding). Public health surveillance includes monitoring avian mortality events and mosquito pools for WNV presence, especially during summer months in temperate zones. No human vaccine is currently licensed, though several candidates have undergone clinical evaluation. Blood and organ donor screening for WNV is not routinely implemented globally, though it is recommended in high-risk areas during active transmission seasons.

Surveillance of WNV should prioritize reporting of neuroinvasive cases, avian die-offs, and mosquito pool testing, particularly in regions with known endemicity or recent introductions. Case definitions should distinguish between West Nile fever and neuroinvasive disease, given differing clinical severity and resource needs. Because most infections are asymptomatic, passive surveillance alone may underestimate true incidence; active serosurveillance and environmental sampling improve detection sensitivity. In settings where WNV is newly introduced or re-emerging, coordinated intersectoral collaboration between public health, veterinary, and environmental agencies is essential to mitigate spillover risk to humans and animals.

1. 1 WHO Fact Sheet. West Nile virus.
2. 2 WHO Q&A. News.
3. 3 Wikidata. West Nile virus.
4. 4 Wikipedia. West Nile virus - Wikipedia.

ICD-10

ICD-11

Total cases

267

Peak month

2021-10

Coverage

3 reporting countries · 2000-01-01 → 2022-10-22