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Scientists comment on the first detection of West Nile virus in UK mosquitoes, announced by the UK Health Security Agency.
Professor Paul Long, microbiologist at King’s College London, said:
“Although virus-infected mosquitos have been found in the UK there’s no evidence yet that these mosquitoes will survive in the UK climate, although prolonged warm weather does increase the chances.”
Prof Tom Solomon, CBE director of the NIHR Health Protection Research Unit on Emerging Infections and of the Pandemic Institute in Liverpool said:
“The detection of West Nile virus in UK mosquitoes is a significant finding, especially as our climate changes. The virus is transmitted among birds by mosquitoes, and humans become infected when they are coincidentally bitten by an infected mosquito. It is most likely the virus arrived in the UK via an infected migrating bird. Climate change impacts on numbers and types of mosquitoes and allows emerging viruses to become established, and we are likely to see more of this, and similar viruses in the future.
“West Nile virus mostly causes just a febrile illness in humans, and can infect people with no symptoms at all. However, in a small proportion of patients the virus can cause neurological disease, which may lead to paralysis or even death. Unfortunately, there is no specific treatment. There have been no human cases so far, caused by virus transmitted in the UK, as far as we are aware. (We have had a handful of cases of West Nile virus among returning travellers). However, scientists, including those in our teams, are looking for evidence of unknown infection in humans. It is also important that doctors are aware that the virus is here in the UK, so that they can send samples from patients with brain infections of unknown cause.
“Given this is just a few infected mosquitos, the risk to the public is low. However, it underscores the need for surveillance for these kinds of pathogens, and funding for research to develop the diagnostic tests, treatments and vaccines that may ultimately be needed, the kind of work we are doing at the NIHR Health Protection Research Unit on Emerging and Zoonotic Infections and at The Pandemic Institute in Liverpool.”
Felipe Colón, Technology Lead in Data for Science and Health at Wellcome, said:
“Climate change isn’t just a future threat; it’s already reshaping disease patterns. As the climate continues to warm and rainfall patterns shift, the risk of diseases such as West Nile Virus spreading to Europe has significantly increased. This is not a new phenomenon, Europe has become increasingly more hospitable for West Nile Virus since the 1980s, with a period of rapid warming creating the perfect conditions for the virus to spread. Countries like the UK that were once considered low risk are now more vulnerable to outbreaks – a concern only amplified by the fact that there are currently no approved treatments for West Nile Virus.
“This case demonstrates the importance of good disease surveillance systems to monitor the risk and incidence of infectious diseases, especially those impacted by climate change. Thanks to the UK’s surveillance network and this research programme, the UK’s healthcare network will have a heightened awareness for potential human cases, increasing understanding of how this might have come about and enhancing our ability to treat any cases earlier and fully determine the risk to the UK population.”
Prof Lance Turtle, Chair in Immunity and Infectious Diseases, University of Liverpool, said:
What is West Nile virus and how dangerous is it to humans?
“West Nile virus is a mosquito transmitted virus that was first discovered in West Nile district of Uganda. The natural host is birds and it is spread between them by mosquitoes. Humans and horses can also be infected by the virus. If the virus infects humans, in most cases nothing will happen (in 8 out of 10 cases roughly). A small number of people, about 2 out of 10, get sick with a fever. A very small number, probably about between 1 in 100 and 1 in 200 get a more severe illness where the virus gets into the brain. This is a serious disease which can be fatal or do permanent brain damage in some cases. There are no effective treatments known, and no vaccine. There have been no cases of West Nile virus infection in humans acquired in the UK to my knowledge. There have been some cases that have occurred in travelers who became infected overseas and brought the illness back to the UK.”
Does this mean that the West Nile virus in these mosquitoes can transmit the disease to humans?
“This finding suggests that West Nile virus might be in the UK. It isn’t conclusive. The press release doesn’t give a lot of detail on exactly what the findings are, but it suggests that part of the genetic information of the virus has been found. This isn’t the same as finding the whole virus, which would give us more confidence that the virus is in the UK. But it is suggestive. There are other possibilities, like parts of another related virus have been misidentified. There’s another related virus that we know is in the UK called Usutu virus, but the title of the press release suggests the team are confident that this is indeed West Nile virus – there’s no reason to think they might be wrong.
“I don’t know much about this particular mosquito species, Aedes vexans. It’s not the typical mosquito that we would associate with West Nile virus transmission, at least not to people. The risk to humans is hard to quantify exactly but will be very low at the present time. The risk from West Nile due to travel is much greater. It’s found in many places – parts of Europe, especially Romania, East Africa, Israel, the USA, India etc.”
“There is no evidence to my knowledge that there is any transmission in the UK. This finding does not suggest that.”
How could the virus have got here inside the mosquito? The mosquito itself? Or via the infected horse?
“Most likely in one of two ways – it came inside a mosquito, or in a bird. There’s no reason to think it is in a horse.”
How worrying is this development for the public?
“This is certainly a bit worrying, that’s why UK HSA have a surveillance program to look for it. We have been expecting something like this for a while – there have been cases of West Nile in nearby countries like the Netherlands, for example, in 2020. However at the moment I think the risk is low and we do not need to be unduly worried – just aware. We can detect humans cases by laboratory testing, either of people who have the right kind of illness, or by looking at large scale for antibodies in blood. Some of this work is already being done in the UK. We are also on the lookout for cases of disease like this through the NIHR Health Protection Research Unit in Emerging and Zoonotic Infections, which works in partnership with UK-HSA.”
Does this necessarily mean that we’ll start more mosquitoes in the UK with West Nile Virus?
“That’s hard to say. This could be a one-off, or it could be the start of a more sustained presence of West Nile in the UK. With warming temperatures, and given that West Nile is in other countries close by, we may end with the virus establishing itself in the UK. In similar countries like the Netherlands the number of cases in humans has been very small, so the risk to humans remains low.”
Prof Steven Sinkins, Professor at Centre for Virus Research, university of Glasgow, said:
What is West Nile virus and how dangerous is it to humans?
“West Nile virus is a member of the flavivirus group related to Japanese Encephalitis virus, and also dengue, Zika and yellow fever viruses, all of which are mosquito-borne. Most human cases have no symptoms or cause a mild fever, but especially in older people it can sometimes progress to infection of the brain and surrounding tissues or spinal cord (encephalitis / meningitis), and this can be fatal or lead to paralysis. Treatment is supportive only. There have been no known UK-acquired cases to date.”
Does this mean that the West Nile virus in these mosquitoes can transmit the disease to humans?
“Not necessarily – the virus needs to be able to make it to the mosquito saliva to be transmitted, and this has not been shown here.”
How could the virus have got here inside the mosquito? The mosquito itself? Or via the infected horse?
“West Nile virus most commonly circulates among birds, so migratory birds are the most likely source.”
How worrying is this development for the public? Should we be concerned about this? How easily can we detect potential future cases?
“It is definitely not a cause for alarm, but does warrant increased surveillance and potentially planning for increased mosquito control.”
Does this necessarily mean that we’ll start more mosquitoes in the UK with West Nile Virus?
“It is too early to say. West Nile virus did spread quickly across the USA after it was first introduced in 1999, but their climate is warmer and there are far higher numbers of susceptible mosquitoes.”
Prof James Wood, Infectious Disease Epidemiologist at the University of Cambridge and Co-Director of Cambridge Infectious Diseases, University of Cambridge, said:
“It is very interesting that UKHSA and APHA have published evidence of WNV infection in mosquitoes in England, although it is not especially surprising as it has been known for some time that previously infected migratory bird species have been travelling to and in some cases then spending summers in the UK. WNV has become more prominent across Europe over the last 10-20 years. Risk mapping has identified parts of England, in particular in the South, as sites where WNV transmission might be expected. It is not clear whether this finding is simply a new observation of something that has been occurring for years, or a new occurrence. APHA has monitored dead birds for WNV for some time, but not detected it and while this surveillance is to be applauded, surveillance in healthy wild birds has never been undertaken. Flaviruses such as WNV, more generally often do not cause disease in their avian hosts. The natural cycle for transmission for many of these does not involve mammalian hosts, but spillover into horses and humans, with clinical disease consequences is known as a concern for WNV when it does happen. Encephalitis is seen occasionally in both humans and horses.
“Veterinary and public health authorities have known for some time about the risks of WNV transmitting to humans and horses and this new finding does emphasise the importance of medical and veterinary surveillance in cases of unexplained encephalitis. When WNV is known to be circulating locally, then enhanced mosquito avoidance approaches (stabling of horses, especially at dawn and dusk in high risk areas) may be warranted.”
Dr Robert Jones, Assistant Professor at the London School of Hygiene & Tropical Medicine, said:
“There is currently no evidence that these fragments of genetic material are linked to any human cases of West Nile virus infection.
“However, this remains a significant finding by the Vector-Borne RADAR team and highlights the importance of continued mosquito surveillance across the country. If a mosquito acquires the virus and is a competent vector, it may transmit the virus to other individuals, some of whom may go on to develop the disease. Most West Nile virus infections are asymptomatic.
“The mosquito could have acquired the virus by feeding on an infected bird. It is possible that a bird became infected in another part of the world and then returned to the UK carrying the virus. While some birds show signs of infection, many species can carry West Nile virus without appearing to have an illness.
“Similarly, humans occasionally return to the UK from abroad carrying arboviral infections. However, this rarely leads to local transmission, either because our local mosquito species are not competent vectors, or because the chances of a mosquito acquiring the virus and passing it on are very low.
“As the UKHSA has outlined, the risk of West Nile virus to the UK population remains very low. However, due to climate change and the movement of people and goods, we are seeing a general expansion of mosquito-borne diseases into regions not previously associated with these infections—and that trend is expected to continue. Ongoing surveillance is essential to ensure that any future infections are detected early.”
Prof Heather Ferguson, Professor of Infectious Disease Ecology at the University of Glasgow, who leads the Mosquito Scotland project, said:
“This finding highlights the critical importance of having robust surveillance programmes in place to enable detection of the arrival, and later establishment, of new pathogens in the UK. We have known for some time that climate and associated environmental changes are driving the expansion of mosquito vector species, and enhancing the probability of pathogen transmission. While the UK is still considered low risk for transmission of most mosquito-borne diseases of humans, this first detection of West Nile Virus in British mosquitoes is in line with predictions of how climate is increasing the likelihood of transmission.
“West Nile a zoonotic virus that mainly spreads between bird species, but can occasionally spillover into humans and horses if they are bitten by an infected mosquito. The majority of infections in humans are asymptomatic (80%) or cause only very mild, flu-like symptoms that pass quickly. However, in a very small proportion of cases (less than 1%) – usually in older people or those with compromised immune systems – symptoms can be more severe including encephalitis or meningitis, and in very extreme cases can cause death. However, even with the finding of West Nile Virus fragments in mosquitoes in the UK, the risk of any infection in humans is considered extremely low at present, and even lower for severe infections.
“While many people consider the risk from mosquito-borne diseases arising only from the invasion of ‘tropical’ mosquito species into the UK, this is not the case. Although the mosquito species in which West Nile Virus has been found in the UK is not currently widespread, several mosquito species native to the UK are also known to be capable for transmitting the virus, but do not do so at present because the environmental conditions, including temperature, are not favourable for transmission. This could change in the future, which is all the more reason why we need surveillance, such as the invaluable programme that is being carried out by UKHSA which has led to this discovery. Without these programmes, there would be no way to track and respond to these potential risks before they take hold.”
Prof Martin Hibberd, Professor of Emerging Infectious Disease at the London School of Hygiene & Tropical Medicine, said:
“I’m pleased to see that a UKHSA research surveillance is in operation and is able to give us this early warning of a potential health threat from West Nile Virus.
“This is a timely reminder of the importance of funding health and disease research, and I hope that these findings will lead to more extensive surveillance, to give a clearer picture of the scale of the problem.
“With the likely arrival of West Nile Virus into the UK, we also need to be prepared for the potential arrival of other viruses from the Flaviviridae family, such as the viruses that cause dengue.
“However, detecting these viruses in mosquitos does not mean that they are able to effectively transmit the virus to cause outbreaks. More work needs to be done to assess the likely chances of these mosquitos passing the virus to humans, which is known to be reduced in cooler climates, such as the UK, compared to the tropical environments these viruses usually circulate in.
“This research and surveillance should become part of a heightened preparedness to combat the effects of climate change.”
Dr Ilaria Dorigatti, Senior Lecturer, Imperial College London, and Dr Rebecca Christofferson, Associate Professor in the Department of Pathobiological Sciences, LSU School of Veterinary Medicine, both said:
“West Nile Virus is a zoonotic flavivirus that circulates in a bird-to-mosquito and mosquito-to-bird cycle and occasionally spills over into the human population, with sporadic cases in horses. Both humans and horses are considered “dead end” hosts, which means humans and horses can acquire the virus when bitten by an infectious mosquito but cannot transmit the virus back to mosquitoes. Most human infections may develop a self-limiting febrile illness that resolves without any treatment, but a small proportion of infections develops severe neurological conditions that require hospitalisation. Older age is associated with a higher risk of developing and reporting severe disease. The same pattern is seen in horses where severe neurological symptoms can occur. In horses, unlike people, there is a licensed vaccine (Equilis West Nile).
“The virus circulates endemically across several countries in Europe, where transmission is seasonal and lasts between April and October. In Europe, cases of West Nile are reported through the European Surveillance System TESSy with over 1400 human cases documented in 2024. Across Europe, nearly 500 cases of equid infection were reported in 2024. To date, no locally acquired human cases of West Nile virus has been detected in the UK, all cases reported in the UK were acquired when travelling abroad.
“Importantly, there are mosquitoes in the UK that are capable of transmitting WNV. Aedes vexans (the mosquito that has been detected positive) bite a wide array of animals, including birds and mammals, primarily at dusk. Culex pipiens also bite preferentially in the evening hours and twilight. Another mosquito that has been transiently detected in the UK, Aedes albopictus, can also carry WNV and bites during the day, most often at dawn and dusk.
“The risk to the human population remains low, and early detection in mosquito pools points to the success of UK vector surveillance programmes. However, the detection of WNV in UK mosquito populations does call for heightened awareness. This also demonstrates a need for strengthening surveillance as changing climate make conditions more suitable for the mosquitoes to live longer, be in contact with people more, and transmit the viruses they carry.”
Prof Matthew Baylis, Oxenhale Chair of Veterinary Epidemiology, University of Liverpool, said:
“West Nile virus is related to dengue virus, yellow fever virus, Zika virus and tick-borne encephalitis virus. West Nile virus is transmitted by mosquitoes between birds. It can cause disease, and sometimes severe disease in people and horses, with occasional fatality. Generally, about eighty percent of infected people are asymptomatic; twenty percent suffer fever, achiness, tiredness and other fairly mild symptoms; but one percent can develop encephalitis, which is sometimes fatal.
“Antibodies to West Nile virus have previously been reported in birds in the UK; and now the virus has been detected in mosquitoes. West Nile has never been transmitted to people in the UK.
“It is possible that the virus entered the UK in mosquitoes, perhaps carried to the UK on winds, or hitching a ride in a plane, train or automobile. But it is much more likely that it was brought in by a migratory bird, which was then fed upon by mosquitoes. Humans and horses do not develop sufficient viraemia (i.e. concentration of virus in blood) to infect mosquitoes so they are unlikely to be the source.
“This should not be a major cause of concern: the spread of West Nile from southern Europe to northern Europe in the last few years has not led to a significant public health issue, although there have been cases in both humans and horses. I will not be surprised if we see a few cases in the UK too. There is a need to heighten surveillance, in humans, birds and mosquitoes. This is especially important given that severe cases that report or are reported to medical or veterinary authorities tend to be the tip of the iceberg, with many infected people or horses carrying the virus but not showing symptoms or signs.”
Dr Steven White, Theoretical Ecologist, UK Centre for Ecology & Hydrology (UKCEH), said:
“West Nile virus is a virus that is usually transmitted by Culex mosquitoes. The primary hosts are birds which can pass on the virus to feeding female mosquitoes that need the bird blood for their reproduction cycle.
“Humans and horses can also become infected if they are fed on by an infected mosquito, but once a human or horse becomes infected the virus cannot be passed on, unlike viruses such as dengue and chikungunya. Thus, humans and horses are known as dead-end hosts. However, the virus can cause West Nile fever in humans, which is mostly mild, but can cause a fever, vomiting, a headache or rash, with only a small fraction of people developing more severe symptoms.
“Not only are birds important in the transmission cycle of West Nile virus, but they are also important for its spread. Birds pose the most likely route of the virus’s introduction into the UK, whereby a migratory bird is infected somewhere in continental Europe where the virus has established (e.g. France, Italy, Spain, Germany) and flown to the UK, then mosquitoes feed on the bird.
“The risk to the human population remains low, however, as this is the first time that West Nile virus has been detected in the UK ecosystem, we must remain vigilant. Our research shows that climate change is likely to increase the risk in the UK as suitability for our native mosquitoes and viral replication increases*. Thus, it is possible that in the near future that West Nile virus could establish in our bird population which then increases the risk to humans and horses.”
* Paper – https://doi.org/10.1098/rsif.2021.0049
Prof Grant Hughes, Department of Vector Biology at Liverpool School of Tropical Medicine, said:
What is West Nile virus and how dangerous is it to humans?
“West Nile virus is a flaviviruses transmitted mainly between birds and mosquitoes, but occasionally mosquitoes will feed on other vertebrates including humans which can lead to virus infections. Symptoms in humans include fever, head and body aches, flu-like symptoms and in rare occasions the virus can infect the nervous system which leads to further complications. In horses there is a vaccine. This is the first detection of WNV in the UK, but other similar viruses such as Usutu virus have been identified in birds.”
Does this mean that the West Nile virus in these mosquitoes can transmit the disease to humans?
“It appears PCR has been used to find the virus in these mosquito pools, but it is not known if these mosquitoes can transmit. Other studies have shown that this mosquito species is a competent vector for this virus. Culex mosquitoes in the UK will also likely be able to transmit this virus to humans.”
How could the virus have got here inside the mosquito? The mosquito itself? Or via the infected horse?
“The virus potentially entered the UK via migratory birds.”
How worrying is this development for the public?
“If we look at Europe we can see West Nile virus and Usutu viruses’ cases expanding to new territories as mosquitoes expand their geographic range. Dengue transmission has recently been seen in Paris. Given that there are mosquitoes in the UK that can transmit these viruses it will be likely that additional transmission events will occur in the future. Further surveillance of mosquito and bird populations will be important for monitoring the virus in the UK and evaluating the risk of virus transmission to humans over time.”
Does this necessarily mean that we’ll start more mosquitoes in the UK with West Nile Virus?
“The arrival of West Nile Virus into the UK was more of a “when” than an “if”. It will be likely that there will be future transmission event in the future.”
Any other useful information relevant to this situation.
“We are actively investigating Wolbachia [a native bacterium that lives within insects]-based control strategies that could be used to reduce mosquito numbers and/or stop mosquitoes transmitting viruses.”
Prof James Logan, Professor of Medical Entomology and Director of Arctech Innovation, London School of Hygiene & Tropical Medicine (LSHTM), said:
What is West Nile virus and how dangerous is it to humans?
“West Nile virus (WNV) is a mosquito-borne virus that affects birds, animals and humans. In most people it causes mild, flu-like symptoms or no symptoms at all — but in rare cases, it can lead to serious neurological illness, including encephalitis or meningitis. There is no specific treatment or vaccine for humans. While WNV has not previously circulated in the UK, its presence in parts of Europe has been growing for years, and its eventual appearance here was expected.”
Does this mean that the West Nile virus in these mosquitoes can transmit the disease to humans? Is there any evidence it has done already?
“If the mosquito is infected, then yes — it can transmit the virus to humans. But it’s important to stress: transmission requires a full local cycle involving infected birds and suitable mosquito species. As of now, there is no evidence of human infection acquired in the UK. However, the detection of the virus in mosquitoes marks a significant step in that direction.”
How could the virus have gotten here?
“The virus most likely arrived via an infected bird or mosquito — both can travel considerable distances, especially with seasonal migration. The infected horse cannot have passed the virus on; horses, like humans, are “dead-end” hosts, meaning they don’t contribute to the onward spread. The horse is a warning sign, not the source.”
How worrying is this development for the public?
“This development is serious, but it is not unexpected. It does not require public alarm, but it does call for vigilance and investment in long-term preparedness. Public health agencies and veterinary networks have surveillance systems in place, but as conditions change — with warmer weather, increased mosquito activity and shifting bird migration — we need to stay one step ahead. This is a moment to recognise that the UK is no longer immune to some diseases once considered ‘tropical’.”
Does this necessarily mean that we’ll start seeing more mosquitoes in the UK with West Nile Virus?
“Not necessarily — but the conditions that make it possible are here, and becoming more favourable. The detection of WNV in even a small number of local mosquitoes is a signal that the door is now open. Whether the virus takes hold will depend on a combination of environmental, biological and human factors — including how well we prepare.”
Any other useful information relevant to this situation.
“This is part of a broader trend we can no longer ignore. Climate change, global trade and travel, and changing ecosystems are bringing vector-borne diseases — like West Nile, dengue and others — closer to home. We are entering an era where we must learn to live smarter in a bug’s world. That means stronger surveillance, better public communication, and a public health system ready for the challenges ahead. This is not the beginning of a crisis, but it is a signpost to the future. How we respond now matters.”
Declared interests
Prof Grant Hughes: “I have an industry partnership with Google to develop Wolbachia based strategies for mosquito control suitable for use in the UK. This work is funded by the BBSRC.”
Prof James Logan: CEO of Arctech Innovation (declaration – we are an innovation companies making products that detect and protect against vector borne diseases).
Dr Steven White: No interests to declare.
Prof Matthew Baylis: I do not have any conflicts of interest relevant to this story.
Prof James Wood: Employed at the University of Cambridge and an officer of the British Equine Veterinary Association (BEVA).
Dr Ilaria Dorigatti and Dr Rebecca Christofferson: We have no competing interests to declare.
Prof Martin Hibberd: My research into disease prevention has been funded by a wide variety of sources over the years including pharmaceutical companies such as Roche, Novartis, and Johnson & Johnson.
Dr Robert Jones: I am employed by both LSHTM and Arctech Innovation.
I am a professor of immunity and infectious diseases at the University of Liverpool. I am a clinician as well as a researcher. I study mosquito borne virus infections, such as Japanese encephalitis which is a similar virus to West Nile virus. I work on a vaccine for Zika virus (another mosquito transmitted virus) and I am a joint patent holder on this vaccine. I am the lead investigator of the ARBO-UK study which is recruiting patients with illnesses like West Nile. I declare consulting fees from MHRA and Bavarian Nordic, and speakers’ fees from Eisai Ltd, Medscape, and the Primary Care Cardiovascular society. I declare consulting fees from Astrazeneca and Synairgen, paid to the University of Liverpool. I received travel and accommodation support for conference attendance (ECCMID) from AstraZeneca.
Professor Tom Solomon: Director of the NIHR Health Protection Research Unit on Emerging and Zoonotic Infections and of The Pandemic Institute in Liverpool. He was involved in the West Nile Virus outbreak in the USA in the late 1990s, and has been studying mosquito-borne viruses for 30 years.
Professor Paul Long: No conflicts.
For all other experts, no response to our request for DOIs was received.