Epidemiology

West Nile virus (WNV) has caused human infection in Africa, Europe, the Middle East, west and central Asia, Oceania, and North America.[3][4] It first appeared in the western hemisphere as an outbreak of encephalitis in New York City in 1999. Between 1999 and 2004, the disease spread across the US, into Canada, and to the Caribbean islands and Latin America.[5][6] Even in areas with abundant WNV-carrying mosquitoes, the overall prevalence of infection has not exceeded 5%.[1]

In the US, 2566 human cases were reported in 2023, with 1738 cases (68%) classified as neuroinvasive disease, and 182 deaths (a 7% case fatality rate, mostly from neuroinvasive disease).[7]​ The national incidence of neuroinvasive disease in 2022 was 0.25 per 100,000. The incidence increased with age (0.01 per 100,000 in people ages <10 years to 0.78 per 100,000 in people ages ≥70 years), and was higher in males (0.32 per 100,000) compared to females (0.19 per 100,000).[8]​ West Nile virus is the most common cause of neuroinvasive arboviral disease in the US, accounting for 92% of reported cases.[9] ​The largest outbreak in a US county occurred during May to December 2021 in Maricopa county, Arizona (1487 cases and 101 deaths, a 6.8% case fatality rate). The majority of cases occurred in older adults ≥60 years of age, and 64% of cases had neuroinvasive disease.[10]

​​​ CDC: ​​​data and maps for West Nile Opens in new window

A total of 709 locally acquired human cases were reported in the European Union and its neighboring countries in 2023, with 67 deaths (a 9.5% case fatality rate). The reported case count is lower than that of 2022.[11] The geographical range of human cases of WNV has also been expanding to more northern and western parts of Europe, including detection of the virus in a bird and Culex mosquitoes in the Netherlands in August and September 2020, and subsequently a human case in October 2020.[12]

Virus activity correlates with mosquito activity: in temperate regions, virus activity is highest from July to October, and in warmer climates such as the southern US, virus activity is high year-round. Age and immune status do not seem to affect susceptibility to infection. However, older or immunocompromised individuals are at higher risk of neuroinvasive disease.[13] There is no clear evidence of a predilection for either sex.

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