Research and analysis

HPR volume 19 issue 5: news (21 May 2025)

Updated 21 May 2025

First detection of West Nile virus in UK mosquitoes

UK findings

The Animal and Plant Health Agency (APHA) and UK Health Security Agency (UKHSA) have been conducting research looking for evidence of mosquito borne viruses entering the country by testing birds and mosquitoes. In May 2025, APHA reported that fragments of West Nile virus (WNV) genome had been detected by PCR testing in 2 pools of Aedes vexans mosquitoes (each of 10 mosquitoes) collected by UKHSA in Nottinghamshire during 2023. This is the first evidence of WNV in mosquitoes in the UK. A total of 200 pools of mosquitoes from the same site were PCR tested, with 198 pools testing negative. A further 30,000 mosquitoes and 300 birds have been PCR tested from other areas in England most suitable for WNV, with all samples from 2023 and 2024 testing negative.

This isolated finding was limited to one small site in Gamston (Retford), Nottinghamshire, England, and there is currently no evidence of ongoing epizootic activity anywhere in the UK. Aedes vexans are native to the UK and although they can be found widely at very low densities, in a few locations in England they can occur at high densities associated with summer flooded river landscapes. While uncommon, they are known to cause nuisance biting in a small number of areas in the country, including in parts of Nottinghamshire. Land use changes in the affected area have significantly reduced mosquito numbers.

While Aedes vexans is potentially a competent vector for WNV (in other words, able to transmit virus between animals), Culex modestus is still considered the primary bridge vector (see below) for WNV and has not been found as far north as Nottinghamshire.  All Culex modestus mosquitoes tested to date in England were negative for WNV.

The risk of a human outbreak is considered to be very low. To date, no locally acquired human cases of WNV have been reported in the UK, although a small number of travel-associated cases have occurred in the past. The main risk of WNV for UK residents continues to be travel to endemic areas overseas.

Background

WNV is a vector borne disease belonging to the Flaviviridae family, which also includes other viruses such as dengue and yellow fever. The natural reservoir of WNV is birds, and the virus typically circulates between birds and mosquitoes through bird-biting mosquitoes, particularly Culex pipiens. Other mosquitoes may bite both birds and humans (known as bridge vectors) and have the potential to transmit WNV to humans, with Culex modestus mosquitoes implicated as the principal vector in outbreaks of WNV in humans and horses in Europe. Other potential bridge vectors may include Aedes vexans mosquitoes.

Most people infected with WNV are asymptomatic or present with mild symptoms such as influenza-like illness (fever, headache or myalgia), swollen lymph nodes and a rash. While most mild infections resolve within 3 to 6 days, a small number of cases (under 1%) are severe and may experience neuroinvasive disease, typically presenting as meningitis, encephalitis or acute flaccid paralysis (sudden weakness), with high fever, neck stiffness, disorientation/confusion, severe muscle weakness, tremors, convulsions, paralysis and coma. Other neurological symptoms may include unsteadiness, weakness of facial muscles, visual disturbance or eye pain (see WNV symptoms).

Diagnosis is made by sending a blood sample, in the first instance, to the UKHSA Rare and Imported Pathogen’s Laboratory (RIPL). Cases of unexplained encephalitis or suspected WNV encephalitis should be discussed at multi-disciplinary encephalitis meetings, including infectious diseases, microbiology and virology doctors, neurologists and specialist radiologists (see WNV diagnosis). There is no available treatment, and no human vaccination.

The first large outbreak of WNV in Europe occurred in Romania in 1996 and, since then, human cases have been identified in several European countries including France, Italy, Portugal, Spain, Poland and Germany.

Factors including climate change and changes in land use have likely increased the geographical distribution of vectors capable of spreading diseases such as WNV, which may be introduced to a non-endemic country when an infected, migratory bird arrives having been bitten by a mosquito in an endemic area.

Further information

Key points and sources of further information are as follows:

• recently updated clinical advice on West Nile virus including symptoms, diagnosis and epidemiology, is available on the UKHSA guidance page at: West Nile virus: epidemiology, diagnosis and prevention
• West Nile virus is a notifiable organism and should be reported to UKHSA if detected
• acute encephalitis (including suspected WNV encephalitis) is a notifiable disease and should be reported to UKHSA.
• the European Centre for Disease Prevention and Control (ECDC) publishes surveillance data and updates for West Nile virus infection.