The Spread of the Mosquito-Transmitted West Nile Virus in North America and Europe

Abstract

West Nile virus (WNV) disease, a mosquito-transmitted Flavivirus infection, represents a substantial public health research interest. This virus was unknown in the Western hemisphere until it was introduced in 1999 into an immunologically naïve population. WNV caused an epizootic and epidemic in New York City. The infection then swept over North America, causing mass mortality in birds and cumulatively 60,000 human cases, half of whom were hospitalised, mostly with neurological symptoms. The virus closely resembled a goose virus isolated in Israel in 1998. Mosquitoes of the genus Culex were identified as the insect viral vectors. WNV can infect more than 300 bird species, but in the US, the American robin (Turdus migratorius) represented the ecologically most important bird viral reservoir. Mosquito-to-mosquito viral transmission might amplify the viral spread, and iatrogenic WNV transmission was also observed, leading to the screening of blood products. Compared with African WNV isolates, the New York WNV isolate NY99 showed a mutation in the nonstructural protein NS3 that increased its virulence in birds and was also observed in WNV outbreaks from Romania in 1996 and from Russia in 1999. During its spread across the US, NY99 acquired a mutation in the envelope gene E that favoured viral infection in the insect vector. Europe reported 1200 annual WNV cases in 2024, with a focus in Mediterranean countries, but a northward spread of the infection to Germany and The Netherlands was also noted. Global warming is likely to affect the geographical distribution of vector-borne infections such that people living in temperate climate areas might be increasingly exposed to these infections. Therefore, research on temperature effects on WNV transmission by Culex mosquitoes has become a recent focus of research. Pertinent climate aspects of WNV infections are retraced in the present review.