West Nile virus: epidemiology, diagnosis and prevention
Source: gov.uk (2025-05-21)

Clinical advice on West Nile virus including symptoms, diagnosis and epidemiology.

Background

West Nile virus (WNV) belongs to a family of viruses called Flaviviridae, along with other viruses such as dengue (https://www.gov.uk/government/collections/dengue-fever-guidance-data-and-analysis) and yellow fever (https://www.gov.uk/guidance/yellow-fever-guidance-data-and-analysis).

Although the reservoir of WNV is birds, the virus can be transmitted to humans via mosquitoes. WNV was first detected in the West Nile district of Uganda in 1937. The first large outbreak in Europe occurred in Romania in 1996. In Europe, cases have been identified in several countries including France, Italy and Portugal.

See surveillance and updates for cases in Europe (https://www.ecdc.europa.eu/en/infectious-disease-topics/west-nile-virus-infection/surveillance-and-updates-west-nile-virus).

In 1999, WNV spread to North America and has subsequently been detected in every state in mainland USA. See CDC maps and data (https://www.cdc.gov/west-nile-virus/).

To date, there are no known cases of locally acquired WNV in the UK, although there have been a small number of travel-associated cases.

Natural reservoir

The natural reservoir animals for WNV are birds. This means WNV circulates from birds to mosquitoes through bird-biting mosquitoes, known as enzootic vectors, notably Culex pipiens.

Some mosquitoes readily bite birds and mammals, including humans, and can transmit the virus, known as bridge vectors. Culex modestus (https://www.gov.uk/guidance/surveillance-for-culex-modestus#:~:text=Culex%20modestus%20is%20considered%20an,commonly%20found%20in%20drainage%20ditches.) is a principal bridge vector for WNV and has been implicated in outbreaks in humans and horses in Europe. It is associated with wetland habitats and commonly found in drainage ditches. Other mosquitoes that bite birds and humans could potentially act as bridge vectors, including for example: Culex modestus, Aedes vexans and Coquilletidia richiardii.

Mosquito surveillance (https://www.gov.uk/guidance/nationwide-mosquito-survey) occurs across England, and UKHSA also runs a nuisance biting mosquitoes reporting scheme (https://www.gov.uk/guidance/mosquitoes-how-to-report). Culex modestus was detected in the UK in 2010, for the first time since 1944. Culex modestus is now established in wetlands in Essex and Kent either side of the Thames estuary and as far north as coastal Suffolk. It has also been detected in parts of the Cambridgeshire Fens and in few localities along the south coast in Sussex and Hampshire (Figure 1).

Figure 1: Distribution of Culex modestus mosquitoes detected in south-east England.

In March 2025, fragments of WNV RNA were detected in 2 pools of 10 female Aedes vexans mosquitoes in Gamston (Retford) in Nottinghamshire, England. These were detected as part of a research study in samples collected in July 2023. This is the first evidence of WNV detected from any mosquito in the UK. Mosquitoes feeding on infected birds may become infected, but this does not necessarily mean they are involved with ongoing transmission.

Aedes vexans are native to the UK and although they can be found quite widely at very low densities. In a few locations in England they can occur at high densities associated with summer flooded river landscapes. They are rarely detected elsewhere. While uncommon, they are known to cause nuisance biting in a small number of areas in the country, including in parts of Nottinghamshire, where summer flooding and poor drainage have contributed to the problem.

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 for WNV and has not been found as far north as Nottinghamshire.

Risk in the UK

The probability of a human outbreak of WNV in the UK is currently considered, at most, very low. Whilst there are ecological conditions in the UK suitable for the vector and wildlife host species are abundant, there has been only one historical detection of WNV in a single site of mosquitoes (as above). There is no evidence to suggest ongoing epizootic activity in any areas of the UK at the current time.

To date, no locally acquired human cases of WNV have been reported in the UK. However, there have been a small number of travel-associated cases in the past. The main risk of WNV for UK residents is still travel to endemic areas overseas.

Transmission

Transmission occurs primarily through the bite of an infected mosquito. Humans are considered dead-end-hosts. This means an infected human cannot transmit the virus back to mosquitoes. Therefore imported human cases do not lead to local transmission. In Europe, the highest levels of transmission occur in summer and early autumn, when mosquito activity is high.

Other, very rare routes of transmission include via blood transfusion, organ donation or vertical transmission from mother to child during pregnancy, delivery or breastfeeding.

Symptoms

The incubation period of WNV ranges from 2 to 14 days, and up to 21 days in immunocompromised people. Most infections are asymptomatic. The majority of clinical cases who do develop symptoms have a mild disease which includes a sudden onset of influenza-like illness (fever, headache, myalgia). Patients may also develop lymphadenopathy and a maculopapular or morbilliform rash on the neck, trunk, arms or leg. Most uncomplicated infections resolve in 3 to 6 days.

Fewer than 1% (about 1 in 150) of infected individuals develop disease in the brain and nervous system. This typically presents as meningitis, encephalitis or acute flaccid paralysis (sudden weakness) with high fever, neck stiffness, disorientation or confusion, severe muscle weakness, tremors, convulsions, paralysis and coma. Other neurological symptoms may include unsteadiness, weakness of facial muscles, visual disturbance or eye pain. WNV acute flaccid paralysis presents as isolated limb weakness or paralysis and may progress to respiratory failure. WNV may also be associated with Guillain-Barré syndrome or inflammation of the spinal cord (radiculopathy).

Those aged over 50 years old, or with underlying medical conditions (such as cancer, diabetes, hypertension and kidney disease) are at greatest risk of severe disease. The case fatality rate is highest in patients aged over 70 years old.

Diagnosis

WNV infection should be considered as a differential diagnosis in any person with a febrile or acute neurological illness, particularly with a travel history to areas where WNV is known to circulate. WNV is know to circulate in Europe, the African continent, North America, Asia and other parts of the world.

WNV infection should be considered in anyone in the UK with unexplained neurological symptoms associated with recent or concurrent fever where no other diagnosis has been identified, particularly in patients who live near, or who have visited, areas where Culex modestus mosquitoes have been found (see Figure 1 above).

Testing for WNV is by antibody testing and PCR at the UKHSA Rare and Imported Pathogens Laboratory (https://www.gov.uk/government/collections/rare-and-imported-pathogens-laboratory-ripl) (RIPL). If WNV is suspected, or if there is uncertainty around whether testing is indicated, the referring clinician should contact a clinician at RIPL to discuss the case.

Email xxx@xxx.xx (mailto:xxx@xxx.xx) Telephone xx-xxxx-xxxx (available 9am to 5pm, Monday to Friday)

WNV is a notifiable organism and should be reported to UKHSA (https://www.gov.uk/guidance/notifiable-diseases-and-causative-organisms-how-to-report#laboratories-report-notifiable-organisms-causative-agents) if detected.

Acute encephalitis (which would include suspected WNV encephalitis) is a notifiable disease and should be reported to UKHSA (https://www.gov.uk/guidance/notifiable-diseases-and-causative-organisms-how-to-report#laboratories-report-notifiable-organisms-causative-agents).

Treatment

No vaccine or specific antiviral treatment is available.

Over the counter pain relievers can be used to reduce fever and mild symptoms. In severe cases, patients often need to be hospitalised to receive supportive treatment such as IV fluids, pain medication and nursing care.

Prevention

There is currently no vaccine available for use in humans.

To reduce the risk of WNV infection, travellers visiting countries considered risk areas should practice mosquito bite avoidance measures (https://www.gov.uk/government/publications/mosquito-bite-avoidance-for-travellers).

Further advice for travellers is available from the National Travel Health Network and Centre (NaTHNaC) website (https://travelhealthpro.org.uk/).

WNV in animals

See information about WNV disease in animals (https://www.gov.uk/guidance/west-nile-fever).

Mosquito surveillance

See national mosquito surveillance (https://www.gov.uk/government/publications/mosquito-surveillance/mosquito-nationwide-surveillance).