UK surveillance identifies traces of West Nile virus in mosquitoes

Fragments of West Nile Virus have been identified in mosquitoes collected in the UK for the first time, according to a UK Health Security Agency (UKHSA) monitoring programme designed to understand the emergence and transmission of vector-borne disease in the UK. 

West Nile Virus (WNV) is a vector-borne disease belonging to the Flaviviridae family, and is typically found in birds and bird-biting mosquitoes. In rare cases, mosquitoes can transmit the virus to humans or horses. Other viruses belonging to the Flaviviridae family include those that cause dengue, Zika and yellow fever. 

West Nile virus (WNV) is endemic in various regions across the globe, including Europe, Africa, the Middle East, West and Central Asia, and North America, as well as South America, USA and Australia. In most people, it causes mild, flu-like symptoms or no symptoms at all but in rare cases, around 1 in 150, illness can progress to serious neurological illness, including encephalitis or meningitis. There is no specific treatment or vaccine for humans. 

The virus fragments were detected in two samples of Aedes vexans mosquitoes collected by the UKHSA from wetlands on the River Idle near Gamston, Nottinghamshire in the UK. 

Roger Hewson, Professor of Virology at the London School of Hygiene & Tropical Medicine (LSHTM) said: "The recent detection of West Nile virus (WNV) genomic material in UK mosquito populations is a significant finding, although not wholly unexpected given the virus's steady northward expansion across Europe. 

"Climate change, migratory bird movements, and increasingly favourable conditions for mosquito vectors all contribute to the changing landscape of vector-borne disease risk in the UK. 

"Our teams at the London School of Hygiene & Tropical Medicine are driving forward research on Aedes vexans mosquitoes, which are native to the UK. The school's expertise in vector biology and the ecology of viruses transmitted to humans by mosquitoes is helping us understand the risk of viruses like West Nile and dengue establishing in the region, and how we may be able to detect them as quickly as possible." 

Robert Jones, Assistant Professor at LSHTM, 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. However, this remains a significant finding by the Vector-Borne RADAR team and highlights the importance of continued mosquito surveillance across the country. 

"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." 

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 West Nile Virus detected in the UK, we also need to be prepared for the potential arrival of other viruses from the Flaviviridae family, such as those that cause dengue. 

More work needs to be done to assess the likely chances of these mosquitoes 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." 

Martin Hibberd, Professor of Emerging Infectious Disease at LSHTM

James Logan, Professor of Medical Entomology at LSHTM and Director of Arctech Innovation, said: "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. 

"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."