Key facts: severity and risk
West Nile virus (WNV) is a mosquito-borne flavivirus first identified in Uganda in 1937 and now found on all continents except Antarctica. Most human infections are asymptomatic; among symptomatic cases, most have mild fever and self-limited illness (West Nile fever). A small proportion develop severe neuroinvasive disease such as meningitis, encephalitis, or flaccid paralysis. Overall case-fatality ratios are low but rise with age and in people with certain comorbidities. Outcomes depend on prompt recognition and supportive care, not on antiviral drugs, because no licensed human vaccine or specific antiviral exists for WNV.
Magnitude of risk: how common severe disease and death are
Approximately 80% of infections are asymptomatic. About 20% of infected people develop West Nile fever with fever, headache, body aches, nausea, vomiting, sometimes a rash, and fatigue. Roughly 1 in 150 infected people develop severe neuroinvasive disease, with higher risk among older adults and people with weakened immune systems or chronic conditions such as diabetes, hypertension, or kidney disease. Case-fatality rates for neuroinvasive disease vary by outbreak and population but are commonly in the range of 5–15%, influenced by age, comorbidities, and timing of care. Many who survive severe WNV infection continue to experience fatigue, weakness, or cognitive symptoms for months or years.
Disease profile and progression
Incubation and initial illness
After an infected mosquito bite, the incubation period is typically 2 to 6 days but can range from 2 to 14 days. Initial illness (West Nile fever) resembles other viral fevers and usually resolves on its own within days to weeks. Clinical diagnosis is based on symptoms, season, and geography, supported by serology or PCR in the appropriate public health context.
Neuroinvasive disease and complications
In rare cases, the virus can invade the central nervous system, causing meningitis, encephalitis, or acute flaccid paralysis. Neuroinvasive disease can require hospitalization, intensive care, and prolonged rehabilitation. Long-term deficits may include muscle weakness, pain, cognitive changes, and mood symptoms. People with suppressed immune systems are at increased risk of prolonged or disseminated infection.
Who is at higher risk of severe outcomes
Risk of severe disease and death increases with age, particularly among people older than 65 years. Organ transplant recipients and others with immunosuppression due to medical conditions or medications are at elevated risk. Underlying conditions such as diabetes, high blood pressure, chronic kidney disease, and alcohol use disorder further increase the likelihood of severe outcomes. These factors are well-established from decades of outbreak investigations and surveillance data.
Prevention and public health measures
Because there is no human vaccine for West Nile virus, prevention focuses on reducing mosquito bites and controlling mosquito populations. Key strategies include using insect repellent, wearing long sleeves and pants during peak biting times, screening windows and repairing tears, eliminating standing water where mosquitoes breed, and supporting local mosquito-control efforts during peak season. These measures are especially important for people at higher risk and in areas with known WNV activity.
Diagnosis, care, and monitoring
Healthcare providers diagnose West Nile virus infection using blood or cerebrospinal fluid tests, often coordinated with public health laboratories. Care is primarily supportive, including hospitalization, respiratory support, and management of complications such as increased intracranial pressure or weakness. People who recover from severe disease should follow up for rehabilitation and monitor for persistent symptoms, as recovery can be slow and incomplete. Reporting to public health authorities helps track circulation and intensity of WNV in a given season.
Global distribution and seasonal pattern
West Nile virus has spread from its original region in Africa to Europe, the Middle East, North America, West Asia, and parts of Oceania. In temperate climates, transmission typically peaks in late summer and early autumn when mosquito density and environmental conditions favor viral amplification in birds and mosquitoes. In some regions, distinct seasonal outbreaks recur annually, while in other areas WNV is more consistently present at lower levels year-round. Local public health authorities provide updates on risk and testing recommendations.