United Kingdom: West Nile virus detected for the first time in native mosquitoes

A retrospective study conducted in the United Kingdom reveals the first detection of the West Nile virus (WNV) in local mosquitoes in 2023. Although the health risk remains low, this virological finding reinforces calls for increased surveillance in the context of climate change.   West Nile virus (WNV), which has been present on the European continent since the 1990s, had never before been identified in the United Kingdom. For the first time, British scientists have confirmed the presence of the virus’s RNA in two pools of Aedes vexans mosquitoes trapped in July 2023 in Nottinghamshire, England. The findings, published in the journal Eurosurveillance, are part of the national Vector-borne real-time arbovirus detection and response (VB-RADAR) surveillance program. Molecular analyses identified West Nile virus (WNV) lineage 1a, a strain already circulating in continental Europe, the Middle East, and the United States. The positive samples were detected by PCR and confirmed by Sanger sequencing, although it proved impossible to isolate the live virus in cell culture.  

Proactive surveillance within the “One Health” framework

Since the introduction of the Usutu virus into the United Kingdom in 2020, health authorities have strengthened their arbovirus surveillance strategy. The VB-RADAR project, part of a “One Health” approach, combines mosquito trapping, bird surveillance, and analysis of environmental factors across 26 sites throughout the United Kingdom. Between March 2023 and March 2025, more than 31,000 mosquitoes from 22 different species were collected, primarily Culex pipiens and Culex modestus , two species known for their role in transmitting the West Nile virus. However, the two positive samples came from Aedes vexans mosquitoes, a species that is less common in the United Kingdom but occurs locally in high densities in summer wetlands.  

An Opportunistic Vector Under Surveillance

Aedes vexans is not considered a primary vector of WNV in Europe, unlike mosquitoes of the genus Culex . However, this species is opportunistic and generalist, feeding on birds as well as horses and humans. Its ability to reach high densities and act as an intermediate vector (between animals and humans) makes it a link in the virus’s epidemiological chain that should not be overlooked. For several years, complaints related to mosquito bites had been reported at the trapping site in northern Nottinghamshire. This pressure led to the implementation of active environmental management of the wetland aimed at reducing local populations of Aedes vexans .  

An Isolated Detection, but Conditions Conducive to the Emergence of the Virus

To date, no active circulation of WNV has been detected in other regions of the United Kingdom, either in mosquitoes or birds. However, the summer of 2025 was marked by intense heat waves, similar to those of 2023. These weather conditions can shorten the virus’s incubation period in mosquitoes, promoting its replication and increasing the risk of transmission. The researchers note that while current climate models suggest the United Kingdom remains generally unfavorable for the sustained establishment of WNV, extreme weather events could lead to sporadic transmissions or even localized outbreaks.  

Call for Enhanced Entomological and Veterinary Surveillance

The study highlights the importance of an integrated surveillance system covering vectors (mosquitoes), wildlife, domestic animals, and human populations. Early detection of a zoonotic virus such as WNV is essential to prevent transmission to humans, which can lead to severe neurological complications in some individuals. Although the outbreak detected in 2023 remains limited at this stage and has had no documented clinical impact, it illustrates the increasing vulnerability of the United Kingdom to emerging arboviruses, against a backdrop of global warming and increased biological mobility (through bird migration or the passive transport of mosquitoes).  

A Threat to Human Health

WNV, transmitted primarily by Culex mosquitoes, causes dozens of severe human cases each year in Southeast Europe. The confirmation of its genetic presence in British mosquitoes therefore constitutes an epidemiological warning sign, even in the absence of active infection in humans or animals. The North American experience serves as a reminder that the West Nile virus was introduced into the United States in 1999 by means that remain unclear, before spreading across the entire continent. The United Kingdom cannot afford to ignore this precedent.