Research and analysis

Qualitative assessment of the risk that West Nile virus presents to the UK human health population

Updated 25 June 2025

Summary of risk assessment for West Nile virus in the UK population

Overview

West Nile virus (WNV) infections have long been recognised in Europe where there is routine surveillance for human and equine disease. There is no evidence of locally-acquired human cases of WNV in the UK to date, and as of June 2025 only 7 confirmed travel-associated cases have been reported since 2000.

Populations of competent mosquitoes (for example, Culex modestus) in the UK, that may act as bridge vectors, have been detected in Essex and Kent. More recently, they’ve also been detected in Cambridgeshire, Hampshire and West Sussex. Culex modestus larvae were identified in Dorset in 2014, but not since, despite ongoing surveillance.

Furthermore, in 2025, 2 samples of pooled Aedes vexans mosquitoes (collected in July 2023 from Nottinghamshire, England) tested positive for WNV on polymerase chain reaction (PCR). Sequencing of fragments confirmed this as lineage 1. Although uncommon in the UK, established populations of Aedes vexans have been identified near Norwich (in July 2017), parts of Nottinghamshire (since 2018) and more recently in the Cambridgeshire Fens (2024), with very low abundance at a number of other sites (1).

Several conditions are required for WNV transmission and establishment in the UK. These include:

• temperatures high enough to sustain transmission cycles
• pathogen introduction coinciding with a period of sufficiently high vector activity
• a vector biting season long enough to allow sufficient amplification of the pathogen in the vector
• host populations to facilitate spillover into humans

Modelling suggests that current UK temperatures are generally too low for inter-annual WNV transmission cycles to be established, but could permit some local short term transmission where there are high numbers of infected mosquitoes if they bite infected birds. Projected increases in UK temperatures are likely to increase the risk of WNV outbreaks, with epidemics possible by the second half of the century.

Assessment of the risk of infection in the UK

The probability of human infection with West Nile virus in the general UK population is considered, at most, very low.

For higher risk groups including individuals living, working or visiting areas with active human-biting mosquito vectors co-located with infected birds, the probability of infection would be considered low.

The impact of West Nile virus on human health in the UK would be considered low for the general UK population, but moderate for higher risk groups including the elderly or immunosuppressed individuals.

Level of confidence in assessment of risk

Good

Current evidence gaps

There is uncertainty about the:

• route for WNV infected mosquitoes (adults or larvae) to be introduced directly to the UK (while the pathways are known, data on how many times it could happen, or how long infected mosquitoes would survive and mosquito biting rates at different times of the year for humans, horses, and other wild birds are not)
• persistence of infected mosquitos and whether the detection in Aedes vexans mosquitos collected in 2023 represents a single time point or the whole mosquito population prevalence during the year
• route for WNV infected wild birds to be introduced to the UK
• serology of individuals identified at the site of WNV detection, and the possibility of asymptomatic or low virulence infection in humans or horses

Actions and recommendations

For animal health and veterinary professionals

The guidance is to:

• continue to monitor vector and host populations, including enhanced surveillance of mosquitoes and birds for arboviruses through Defra/UKRI funded programmes such as Vector Borne Real time Arbovirus Detection And Response (VB RADAR)
• continue surveillance for autochthonous WNV animal infections
• raise awareness amongst veterinary professionals and encourage testing of equidae and wild bird species in relevant areas of the UK with known vector populations

For public health professionals

The guidance is to:

• raise awareness amongst medical professionals and encourage testing of humans with compatible illness living in or visiting relevant areas of the UK with known vector populations
• continue surveillance for autochthonous human infections in those with a compatible illness (fever and neurological symptoms) throughout the UK
• raise awareness of the Wetland Mosquito Survey Handbook with local and national government, authorities and resilience forums

For both public and animal health professionals

The guidance is to:

• develop and disseminate a one health control strategy for infection with WNV combining actions for monitoring, surveillance, pro-active communications, and control measures pertaining to vectors and hosts

Step 1. Assessment of the probability of infection in the UK human population

This section of the assessment examines the likelihood of an infectious threat causing infection in the UK human population. Where a new agent is identified there may be insufficient information to carry out a risk assessment and this should be clearly documented. Read this in conjunction with the Probability Algorithm found at Annexe A.

Is this is a recognised human disease?

Outcome

Yes

Quality of evidence

Good

West Nile virus (WNV) is a viral infection of birds transmitted by mosquitoes, although horses and humans can also be infected but are dead-end hosts and not involved in the transmission cycle of WNV. The virus was first isolated in 1937 from a woman with fever in the West Nile district of Uganda and it was later recognised as a cause of meningo-encephalitis (2). WNV strains are characterised into several lineages, of which lineage 1 is globally widespread, including in Europe. Lineage 2 strains, endemic in southern Africa, appeared in Europe for the first time in Hungary in 2004. Over subsequent years, lineage 2 viruses have become widespread over south and central Europe and the eastern Mediterranean region, spreading westwards (3 to 6).

Europe has experienced sporadic cases and outbreaks of WNV in humans and horses since the 1960s (2). After the first large outbreak in Romania in 1996, WNV was recognised as a public health concern in Europe. While lineage 2 viruses remain predominant in Europe, a re-emergence of lineage 1 has been observed in parts of Southern Europe (7, 8). WNV infection is considered endemo-epidemic in parts of Europe, affecting countries in southern, eastern and western Europe (see Figure 1) and is a re-emerging public health challenge, with annual seasonal outbreaks during the summer months and early autumn (the main transmission season is between April and November). WNV infection in animals such as horses and birds is reportable to the World Organisation for Animal Health (WOAH)(9) and is a notifiable pathogen of horses in the UK. The virus is a notifiable organism in the UK (10), and acute encephalitis (including suspected WNV encephalitis) is a notifiable disease (10). Monitoring of cases in Europe continues annually throughout the transmission season (11).

Figure 1. Geographical distribution of West Nile virus human cases reported to the European Centre for Disease Prevention and Control (ECDC) between 2013 and 2023 (12).

Figure 2. Geographical distribution of West Nile virus human cases, and equid and bird infections reported to ECDC in 2024, up to 4 December 2024. Further information can be found on ECDC’s West Nile virus webpages (13).

Figures 1 and 2 show the spread of WNV in Europe on a map. The map shows that cases are generally more concentrated in Southern Europe.

In recent years, there has been a gradual expansion of the geographical range of reported human cases of WNV to northern and western points in Europe, with case numbers peaking in 2018 (1,503 cases) (Figure 3). This is the largest number of WNV infection cases ever recorded in the continent, exceeding the cumulative number of all cases recorded in the previous 7 years (Figure 3). The notification rate for locally acquired WNV infections was almost 8 times higher in 2018 compared with 2017, and 4 times higher than reported in 2019. Almost all affected countries reported their highest annual incidence in 2018 and a longer than normal transmission season was observed (14). Subsequently, 2024 saw the second highest reported number of locally acquired cases (1,171 cases) since 2018, followed by 2022 (1,108 cases) (15).

In 2019, both Germany and Slovakia reported their first autochthonous human WNV cases. The 2022 season saw an exceptionally early locally acquired case reported in Italy, with disease onset on 16 April 2022 (16). In 2024, Poland reported its first locally-acquired case, which was classified as a probable case. The 2024 season saw cases reported from 5 countries in the European Region (Albania, Czechia, Kosovo, Slovenia and Turkey) that had not reported cases in the preceding 4 or 5 years (13). Albania reported its largest ever number of WNV cases on record during 2024 (106 cases) (13).

Figure 3. Annual number of human cases of locally acquired WNV with a known place of infection in the EU and EEA between 2011 and 4 December 2024. Source: ECDC (15)

Is the disease endemic in humans within the UK?

Outcome

No

Quality of evidence

Good

Mosquito-borne diseases and transmission of mosquito-borne viral infections in humans are currently extremely rare events in the UK, unlike many parts of Europe. Between 2002 and 2012, enhanced surveillance for human cases of WNV in the UK took place annually between 1 June and 31 October, the critical period for WNV transmission in Europe, and only travel-related cases were identified (17). Between 2000 and June 2025, a total of 7 confirmed cases of WNV infection in humans were recorded in UK residents, all acquired through travel (18). All cases of undiagnosed encephalitis referred to UKHSA’s Rare and Imported Pathogens Laboratory (approximately 1 to 2 cases per week) are tested for WNV Immunoglobulin M. No locally-acquired cases of WNV have been reported in the UK to date (18).

Is the disease endemic in animals within the UK?

Outcome

No

Quality of evidence

Good

A 2001 to 2002 study looking for evidence of infection with WNV amongst both migratory and non-migratory birds suggested that virus was already present in resident (non-migratory) birds in the UK (19). However, these serological findings were not supported by subsequent research and WNV was not isolated.

There have been no detections of WNV through the annual testing of wild birds found dead in the UK (approximately 300 to 400 per year) by the Animal and Plant Health Agency (APHA) (20). Avian hosts are, however, not likely to be a limiting factor for WNV transmission in the UK: birds ubiquitous in the UK, such as carrion crows (Corvus corone) (21) and house sparrows (Passer domesticus) (22), have been found to be susceptible and sufficiently viraemic to support transmission of WNV. A recent enhanced surveillance programme for flavivirus in UK birds has detected presence of Usutu virus, but all have so far been negative for WNV (23). While vectors such as Culex modestus are present in the UK, an enhanced surveillance programme in the North Kent marshes found no mosquito collected to be positive for WNV (24).

In March 2025, as part of a joint investigation with UKHSA, the British Trust for Ornithology, and the Institute of Zoology, the APHA reported provisional findings of fragments of WNV RNA in 2 pools of female Aedes vexans mosquitoes from Gamston (Retford) in Nottinghamshire, England. These were identified from samples collected by UKHSA in July 2023, which were sent to APHA for arbovirus testing as part of a research programme. This is the first evidence of WNV detected in any mosquito in the UK, with 200 pools of 10 mosquitoes being tested, returning 2 PCR positive pools for WNV. Vector control measures have been implemented at Gamston (Retford) to control the population of Aedes vexans. In 2024, mosquito densities were low at the site and there were no reports of nuisance biting, therefore no mosquitoes were tested from this locality. More than 25,000 mosquitoes of other species collected from a range of other locations in southern England, collected in 2023 and 2024, tested negative for WNV.

Aedes vexans are native to the UK and can be found in large densities at a very small number of locations associated with summer flooded river landscapes in England (1, 25). While uncommon, they are known to cause nuisance biting in a small number of areas of the country, including in Gamston (Retford), where summer flooding and poor drainage has previously been experienced. Importantly, reshaping of parts of this wetland has been undertaken in 2025 to reduce the risk of flooding and drying events, therefore minimising suitable habitats for this mosquito species.

Are there routes of introduction into the UK?

Outcome

Yes

Quality of evidence

Good

Defra regularly perform qualitative assessments of the potential risk factors and likelihood for introduction of WNV into the UK (26, 27, 28). Routes of induction include:

• the importation of infected animal germplasm
• the legal trade of live poultry and captive birds
• infected mosquitoes being blown across to the UK from affected countries
• seasonal migrating birds from WNV endemic countries
• movements of wild birds, in any month, crossing the English channel from affected European countries
• mosquitoes imported via plants or by means of transport vehicles

All of these routes are currently deemed to present overall a very low risk of introduction to the UK and is dependent on the pathway and the time of year (26, 27). It is expected that horses may arrive in the UK having been infected in Europe. However, there are increasing numbers of vaccinated horses and either way, this is not a route for establishment of infection in the UK.

For WNV to be imported into the UK in viraemic migratory birds, the birds have to acquire virus prior to their arrival in the UK and remain viraemic long enough for them to be of concern when reaching the UK. When viraemic on arrival, they need to be in a location with active local mosquitoes for transmission to take place. The putative enzootic vector Culex pipiens overwinters as females, so there is a possibility of some biting in spring, but most C. pipiens activity occurs from July onwards, as does the adult activity of the putative bridge vector, C. modestus. Therefore, it is likely there would need to be viraemic birds in the UK during July and August for transmission by these mosquitoes to occur. However, any mosquito species feeding on an infected migratory bird could subsequently test positive for WNV.

There is uncertainty about the route for infected mosquitoes to be introduced directly to the UK. While the pathways are known, data on how many times it could happen, or how long infected mosquitoes would survive and biting rates at different times of the year are not.

If the virus was endemic in UK birds and mosquitoes, there would be an increased risk of transmission to humans. If there was a requirement for the virus to be re-introduced each year by migratory birds, then there is a likely disconnect between the arrival time of long-haul viraemic birds from Africa (March to May) and the activity of the UK enzootic and bridge vectors (July to September). Given the recent expansion of the WNV range in Europe, short-range migration of birds from affected countries in Europe, which can occur at any time of the year, may allow for the introduction of viraemic birds into the UK at peak vector activity times. However, there are potential limitations by weather on the extrinsic incubation of the virus, in contrast to southern Europe.

Any changes in mosquito seasonality, patterns of bird movements, the timing of viraemia and endemic WNV transmission in more of western Europe could affect the potential for incursion and local transmission in the UK.

Are effective control measures in place to mitigate against these routes of introduction?

Outcome

No

Quality of evidence

Good

The implementation of mitigation strategies to prevent the introduction of WNV by bird migration or mosquitoes is impractical. Proposed or currently applied measures to minimise the risk of WNV to the UK population concentrate on reducing the impact of WNV by the early detection of infections in human or animal hosts and vectors, awareness of WNV, education on how to minimise mosquito exposure, and vector control.

The 2004 UK WNV contingency plan (29), currently under review, sets out measures to raise awareness about potential infections. These measures aim to:

• enhance surveillance for the virus
• alert clinicians to the symptoms of West Nile fever
• communicate with veterinarians about the clinical signs of WNV in horses
• to control mosquito populations

It is acknowledged if WNV becomes endemic in indigenous competent vectors, an outbreak could be difficult to manage.

Currently, the primary strategy to minimise the risk of WNV to the UK population is surveillance to inform vector control, and raising public awareness of measures to minimise mosquito bites. Monitoring and testing of potential cases of WNV in humans, horses, birds and enhanced mosquito surveillance (and arbovirus testing) is undertaken in the UK during the critical period for WNV transmission in Europe. No cases of bird, horse or human UK-acquired WNV have been detected thus far as of June 2025, although similar bird associated mosquito-borne flaviviruses have been detected, for example, Usutu virus (18).

To monitor the distributions of mosquito vector populations in the UK, UKHSA runs a passive mosquito surveillance scheme with a dedicated database to record the incidences of mosquitoes nationally. The scheme can be contacted by email on (mosquito@ukhsa.gov.uk)

UKHSA also runs a network of mosquito traps nationwide (30) targeting up to 30 locations across England each year. In recent years this has been supplemented by the VB RADAR project (31).

Do environmental conditions in the UK support the natural vectors of disease?

Outcome

Yes

Quality of evidence

Good

Of the 36 recorded species of mosquito in Britain, at least 9 species could potentially transmit WNV and 13 could act as bridge vectors as they bite both birds and humans (32, 33). Should WNV be introduced to the UK, the most likely vectors would be mosquitoes belonging to genus Culex. Up until 2010, the most likely enzootic (transmission between birds) and bridge vectors (transmission from birds to humans) were the widely distributed Culex pipiens complex. Until recently, except for localised urban infestation of Cx. pipiens molestus, there appeared to be few situations in the UK where humans and livestock were exposed to sustained risks of exposure to mammal biting Culex and hence the primary WNV vectors (34).

However, an established population of Cx. modestus was discovered in the North Kent Marshes in 2010 (35). This was the first time this species, a recognised bridge vector for WNV, had been detected in the UK since 1944. In 2012, evidence of low numbers of Cx. modestus were found in the Cambridgeshire Fens (36). Since then, UKHSA with academic colleagues have reported the presence of Cx. modestus across a number of wetland sites in North Kent (from Swanscombe to Sandwich) and in coastal Essex (from Rainham to Fingringhoe and Horsey) (Figure 4) (37, 38). New and existing wetlands in this area may provide new habitats for Cx. modestus (39), and it appears that, in fact, it is expanding to new wetland sites. Recently, there has been new evidence of this mosquito in a few wetlands in Suffolk (Orford Ness), West Sussex and Hampshire (around Pagham and Portsmouth) and more widely in the Cambridgeshire Fens. UKHSA continues to monitor populations in these parts of England.

These findings highlight that Cx. modestus may be more widespread in the UK than previously realised, and in some locations, they are abundant. The WNV risk to humans and horses may be higher in these locations; however, further research on biting rates, host preference and dispersal, in addition to nationwide and targeted surveillance is ongoing (24, 37). However, where this mosquito occurs there is little to no evidence on nuisance biting reported to local authorities.

Figure 4. Distribution of Culex modestus mosquitoes detected in south-east England. Source: UKHSA (40)

Other potential bridge vectors include Culex molestus, which can cause nuisance biting in some urban areas, and is usually subjected to ongoing and rapid mosquito control. Recent research has shown that this species may be more common, even in rural areas, with evidence of the presence of hybrids with Culex pipiens (41).

In March 2025, the APHA reported provisional findings of fragments of WNV RNA in 2 pools of female Aedes vexans mosquitoes collected by UKHSA from wetlands on the River Idle (near Gamston, Retford) in Nottinghamshire. These were identified from historical samples (collected in July 2023) sent to APHA for arbovirus testing as part of a research programme. Aedes vexans are native to the UK and can be found at high densities in a small number of locations in summer flooded river landscapes in England. While uncommon, they are known to cause nuisance biting in a small number of areas of the country, including in villages along the River Idle near Retford, which experience summer flooding and poor drainage. Importantly, reshaping of parts of this wetland has been undertaken in 2025 to reduce the risk of flooding and drying events and thus minimise suitable habitats for these vectors. Over-wintering of WNV is thought to be unlikely as far north as Nottinghamshire; however, large areas of South and Eastern England are considered climatically suitable for WNV transmission. While Aedes vexans is a competent vector for WNV, Cx. modestus, which is considered the primary bridge vector for WNV, as only been primarily detected in North Kent and Essex along the Thames estuary in the UK, and more recently in parts of the Cambridgeshire Fens, on the south Sussex and Hampshire coast and in coastal Suffolk.

As both susceptible wild bird species and known bridge vectors are present in the UK, it remains an open question why WNV outbreaks have so far been limited to southern and central Europe. Although, there is a likely disconnect between the arrival time of long-haul viraemic migratory birds from Africa (March to May) and the activity of the UK enzootic and bridge vectors (July to September). Given the recent expansion of the WNV range in Europe, short-range migration of birds from affected countries in Europe, which can occur at any time of the year, may allow for the introduction of viraemic birds into the UK at peak vector activity times. However, there are potential limitations by weather on the extrinsic incubation of the virus, in contrast to southern Europe.

While various vector competence studies using Cx. pipiens mosquitoes from northern Europe resulted in lower transmission rates than studies performed with southern European mosquitoes, a direct comparison did not find any difference in competence (42, 43). However, temperature has been shown to increase vector competence of European mosquitoes for WNV (44, 45, 46), and it is believed that cooler summer temperatures and large diurnal variations in have so far limited the spread of WNV to northern European countries (42, 44). Modelling suggests that current UK temperatures are generally too low for inter-annual WNV transmission cycles to be established. Projected increases to UK temperatures in the coming years may increase the risk of WNV outbreaks, with epidemics possible by the second half of the century (47).

There would need to be viraemic birds in the UK during July and August for transmission by native mosquitoes to occur but with endemic WNV areas in Northern and Western Europe the possibility of viraemic birds entering the UK in these months increases.

Will there be human exposure?

Outcome

Yes, for the high risk groups including individuals living, working or visiting areas with active human-biting mosquito vectors co-located with infected birds

No for the general population

Quality of evidence

Good

WNV is maintained in a mosquito-bird-mosquito cycle. However, in favourable environmental conditions mosquitoes can proliferate and the risk of transmission to humans increases. The vast majority of infections are acquired through the bite of an infected mosquito. People can protect themselves by taking appropriate anti-mosquito measures (2).

Human exposure in the eastern US was linked to transmission by a hybrid form of Cx. pipiens which exhibited both bird and human biting tendencies. Cx. pipiens pipiens in the UK is believed to be predominantly bird biting and is unlikely to act as a bridge vector (34). Cx. pipiens molestus does bite humans, but evidence of bird biting in the UK is unknown. Recent studies have shown molecularly that Cx. pipiens/molestus hybrids do occur (48). However, there is no evidence so far to suggest that they are common, although they do appear to be widespread in southern England. Therefore, human exposure to the transmission of WNV by Cx. pipiens in the UK is likely to be different (much less) than has occurred in the US (34), even though vector competence for European variants of Cx. pipiens has been demonstrated (44).

Cx. modestus is a recognised bridge vector in Europe. However, as the human population is low in the marshes immediately adjacent to the Thames estuary and around other wetlands sites where they occur and little is known about the dispersal ranges of Cx. modestus in the UK, it is difficult to quantify the significance of this vector to human exposures (35). However, UKHSA follow up all concerns about nuisance biting across England including the region where Cx. modestus occurs, to monitor which species are causing human biting (49). Some initial data is now available on host preference, confirming that Cx. modestus do seek blood meals from birds (including migratory) as well as humans (48, 50, 51).

Although human-biting Culex are likely to be the main vectors of WNV to humans in the UK, other mosquito species may play a role in transmission. For example, Coquillettidia richiardii bites both birds and humans and is implicated in WNV transmission elsewhere in Europe (34). Additionally, laboratory studies of vector competence of other British mosquitoes for WNV, such as Aedes detritus and Aedes vexans, have shown the potential for experimental infection in the laboratory (52, 53). However, there is no evidence to suggest they are significantly involved in transmission in the field.

Research into biting preferences (48, 50, 51, 54) and factors affecting the competence of relevant mosquito populations is ongoing (42, 43).

Are humans highly susceptible?

Outcome

No

Quality of evidence

Good

While humans can become infected with WNV, approximately 80% of human cases will have no symptoms. Approximately 20% of cases will develop mild influenza-like illness which generally lasts 3 to 6 days. A small proportion (less than 1%) can develop more severe disease such as aseptic encephalitis, meningitis or meningo-encephalitis. Increasing age, particularly in those over 50 years (55), underlying health conditions, including cardiovascular or renal diseases, and immunosuppression, are the greatest risk factors for the development of serious disease and death (55, 56, 57).

Outcome of probability assessment

The probability of human infection with West Nile virus in the general UK population is considered at most very low. For higher risk groups including individuals living, working or visiting areas with active human-biting mosquito vectors co-located with infected birds, the probability of infection would be considered low.

Step 2: Assessment of the impact on human health

The scale of harm caused by the infectious threat in terms of morbidity and mortality: this depends on spread, severity, availability of interventions and context. Please read in conjunction with the Impact Algorithm found at Annexe B.

Is there human-to-human spread of this pathogen?

Outcome

No

Quality of evidence

Good

There is no direct person-to-person spread. WNV transmission through blood transfusions and organ transplants from infected donors has been reported, but these methods of transmission contribute very small numbers of cases to the overall burden of the disease. It is also possible for WNV to be transmitted from mother to unborn child or through breast milk (2, 58), but such cases are extremely rare.

Is there zoonotic or vector borne spread of this pathogen?

Outcome

Yes

Quality of evidence

Good

WNV is maintained in an enzootic cycle between ornithophagic mosquitoes and birds. Bridge-vector mosquitoes (those that feed on both birds and mammals) can spread the virus to humans, horses and other incidental hosts. Rare cases of zoonotic transmission have been described during horse or bird autopsy (56, 59, 60). Mosquitoes are responsible for the vast majority of human transmissions.

For zoonoses or vector-borne disease, is the animal host or vector present in the UK?

Outcome

Yes, in some areas

Quality of evidence

Good

The main reservoir host of WNV are birds and the biological vectors are mosquitoes, principally Culex species. Both hosts and vectors are present in the UK and therefore could be part of effective transmission cycle at certain times of the year (26, 34).

Is the UK human population susceptible?

Outcome

Yes

Quality of evidence

Good

There is no evidence to suggest that the UK population would not be susceptible to WNV, if they were exposed to infected mosquitos. Only a sub-population of individuals would be considered highly susceptible because of their more frequent exposure. There is currently no vaccine licensed to protect humans and no individual level natural immunity or cross protection from other infections. WNV infections acquired abroad have been recorded in a very limited number of individuals in the last ten years. Since 2000, and as of June 2025, a total of 7 confirmed cases of WNV infection in humans had been recorded in UK residents, all acquired through travel.

Does it cause severe disease in humans?

Outcome

Yes

Quality of evidence

Good

The majority of people infected with WNV (around 80%) are asymptomatic. Around 20% may develop a mild flu-like illness. Less than 1% of cases can develop more severe disease such as aseptic encephalitis, meningitis or meningo-encephalitis. The case fatality rate in patients with neuro-invasive illness is up to 17% (2). Both WNV lineages 1 and 2 are associated with clinical disease in humans (3, 4).

Severe disease has the potential to occur in all ages; however, older individuals, particularly those aged over 50 years, and those who are immunosuppressed are at greater risk of experiencing severe disease. The risk of severe disease also increases in individuals with genetic risk factors (particular single nucleotide polymorphisms in several genes) (57), as well as those with comorbidities and alcohol abuse (2, 55).

Would a significant number of people be affected?

Outcome

No

Quality of evidence

Good

The majority of those affected would be individuals who are exposed to and bitten by infected mosquitoes. Although there is limited information on the incidence of mosquito biting in the UK, sustained human biting is currently considered to be a localised event (61). Under these circumstances it is unlikely that a large proportion of blood, tissue, or organ donors would be affected.

Are effective interventions available?

Outcome

Yes and no

Quality of evidence

Good

The risk of contracting WNV infection can be reduced but not entirely mitigated, by preventing exposure to mosquitoes (use of repellent, long sleeves, avoiding being outside at dusk and dawn in areas where mosquito vectors are most active, and reducing habitats around homes).

If there is an outbreak of WNV infection at a time when mosquitoes are active, measures to control mosquito populations, by either targeting their breeding sites or, more rarely, killing adult mosquitoes, will be considered based on local and national risk assessments. A handbook is available for wetlands managers on assessing suitability of wetlands for mosquitoes (62).

Screening of blood, tissue, and organ donors could be initiated and extended to donors who do not have history of travel as a means of surveillance, as is done in Southern European countries which experience outbreaks or periods of increased risk.

There is currently no human vaccine available and there is no specific antiviral therapy, only supportive care (2).

Outcome of impact assessment

The impact of West Nile virus on human health in the UK would be considered low for the general population, but moderate for higher risk groups including the elderly or immunosuppressed individuals.

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Annexe A. Assessment of the probability of infection in the UK population algorithm

Annexe B. Accessible text version of assessment of the probability of infection in the UK population algorithm

Outcomes are specified by a tick (☑) beside the appropriate answer. Where the evidence may be insufficient to give a definitive answer to a question, the alternative is also considered with the most likely outcome shown with 2 ticks (☑☑) and the alternative outcomes with a tick (☑).

Outcomes are specified by a ☑ (tick) beside the appropriate answer.

Question 1: Is this a recognised human disease?

Yes: go to question 3. ☑ (tick)
No: go to question 2.

Question 2: Is this a zoonosis or is there a zoonotic potential?

Yes: go to question 4.
No: the probability of infection in the UK population is considered very low.

Question 3: Is this disease endemic in humans within the UK?

Yes: go to question 5. (see note 1, below)
No: go to question 4. ☑ (tick)

Note 1: this pathway considers reverse-zoonosis of a pathogen already in circulation in the human population.

Question 4: Is this disease endemic in animals in the UK?

Yes: go to question 8.
No: go to question 5. ☑ (tick)

Question 5: Are there routes of introduction into animals in the UK?

Yes: go to question 6. ☑ (tick)
No: the probability of infection in the UK population is considered very low.

Question 6: Are effective measures in place to mitigate against these?

Yes: the probability of infection in the UK population is considered very low.
No: go to question 7. ☑ (tick)

Question 7: Do environmental conditions in the UK support the natural vectors of disease?

Yes: go to question 8. ☑ (tick)
No: the probability of infection in the UK population is considered very low.

Question 8: Will there be human exposure?

Yes, individuals interacting with infected non-avian wildlife: go to question 9. ☑ (tick)
No: the probability of infection in the general UK population is considered very low. ☑ (tick)

Question 9: Are humans highly susceptible? (see note 2, below)

Yes: go to question 10.
No: the probability of infection in the UK population is considered low. ☑ (tick)

Note 2: includes susceptibility to animal-derived variants.

Question 10: Is the disease highly infectious in humans?

Yes: the probability of infection in the UK population is considered high.
No. the probability of infection in the UK population is considered moderate.

Annexe C. Assessment of the impact on human health algorithm

Annexe D. Accessible text version of assessment of the impact on human health algorithm

Outcomes are specified by a ☑ (tick) beside the appropriate answer.

Question 1: Is there human-to-human spread?

Yes: go to question 4.
No. go to question 2. ☑ (tick)

Question 2: Is there zoonotic or vector-borne spread?

Yes: go to question 3. ☑ (tick)
No: the impact of infection in the UK population is considered very low.

Question 3: For zoonoses or vector-borne disease, is the animal host or vector present in the UK?

Yes: go to question 4. ☑ (tick)
No: the impact of infection in the UK population is considered very low.

Question 4: Is the human population susceptible?

Yes: go to question 5. ☑ (tick)
No: the impact of infection in the UK population is considered very low.

Question 5: Does it cause severe disease in humans?

Yes, high risk groups: go to question 8. ☑ (tick)
No: go to question 6.

Question 6: Is it highly infectious to humans?

Yes: go to question 9.
No: Go to question 7.

Question 7: Are effective interventions available?

Yes: the impact of infection in the UK population is considered very low.
No: the impact of infection in the UK population is considered low.

Question 8: Would a significant number of people be affected? (see note 1 , below)

Yes: go to question 10.
No: go to question 9. ☑ (tick)

Note 1: this question has been added to differentiate between those infections causing severe disease in a handful of people and those causing severe disease in larger numbers of people. ‘Significant’ is not quantified in the algorithm but has been left open for discussion and definition within the context of the risk being assessed.

Question 9: Are effective interventions available?

Yes: the impact of infection in the UK population is considered low. ☑ (tick)
No: the impact of infection in the UK population is considered moderate. ☑ (tick)

Question 10: Is it highly infectious to humans?

Yes: go to question 12.
No: go to question 11.

Question 11: Are effective interventions available?

Yes: the impact of infection in the UK population is considered moderate.
No: the impact of infection in the UK population is considered high.

Question 12: Are effective interventions available?

Yes: the impact of infection in the UK population is considered high.
No: the impact of infection in the UK population is considered very high.

Annexe E: About the Human Animal Infections and Risk Surveillance group

This document was prepared by the UK Health Security Agency (UKHSA) on behalf of the joint Human Animal Infections and Risk Surveillance (HAIRS) group.

HAIRS is a multi-agency cross-government horizon scanning and risk assessment group, which acts as a forum to identify and discuss infections with potential for interspecies transfer (particularly zoonotic infections). Its work cuts across several organisations, including:

• UKHSA
• Department for Environment, Food and Rural Affairs (Defra)
• Department for Health and Social Care (DHSC)
• Animal and Plant Health Agency (APHA)
• Food Standards Agency (FSA)
• Food Standards Scotland (FSS)
• Public Health Wales (PHW)
• Public Health Scotland (PHS)
• Department of Agriculture, Environment and Rural Affairs for Northern Ireland (DAERA)
• Welsh Government
• Scottish Government
• Public Health Agency of Northern Ireland
• Department of Agriculture, Food and the Marine, Republic of Ireland
• Health Service Executive, Republic of Ireland
• Infrastructure, Housing and Environment, Government of Jersey
• Isle of Man Government
• States Veterinary Officer, Bailiwick of Guernsey

Information on the risk assessment processes used by the HAIRS group can be found at HAIRS risk assessment process.

Version control

Date of this assessment: June 2025

Version: 4.0

Reason for the assessment: Update to epidemiology and detection of West Nile virus in 2 Aedes vexans mosquito pools collected from Nottinghamshire, England, in July 2023.

Completed by:

-Jolyon Medlock (UKHSA) - Michael Reynolds (UKHSA) - Helen Roberts (DEFRA) - Catherine Houlihan (UKHSA) - Amira Yunusa (UKHSA)

The update was reviewed by HAIRS members.

Non-HAIRS group experts consulted:

• Kyle Adair (UKHSA)
• Alexander Vaux (UKHSA)
• Arran Folly (APHA)
• Rob Paton (UKHSA)
• Su Brailsford (UKHSA)

Date of previous risk assessments: July 2006, October 2012, November 2017, December 2020

Information on the risk assessment processes used by the HAIRS group can be found online.