West Nile virus detected in mosquitoes in the UK for the first time

West Nile virus, a mosquito-borne infection that can be fatal to humans, has been detected in the UK for the first time.

The UK Health Security Agency (UKHSA) said today that a research project it runs with the Animal and Plant Health Agency (APHA) identified “fragments” of the virus from Aedes vexans mosquitoes collected from wetlands on the River Idle in Nottinghamshire in July 2023, which tells scientists the insects were carrying the pathogen.

Arran Folly at APHA says the virus was detected due to “pot luck” when the samples were tested by his team. Additional screening will be deployed around the Idle this year, he says, to try to catch any further instances. Nottinghamshire isn’t typically included in APHA’s screening programme, which is concentrated on the south and east of England, where infections like West Nile are considered more likely to emerge, due partly to the warmer weather typical of these areas.

Humans and horses can be infected by West Nile virus from a mosquito bite. In people, around 20 per cent of those infected will experience symptoms, which include fever, headache, body aches, vomiting and diarrhoea. In rare cases, the virus can cause serious inflammation of the brain and spinal cord, which can be fatal. There is no approved human vaccine.

UKHSA said that the virus has only been detected in two mosquito samples and there is no evidence that any humans or horses have yet been infected in the UK. The organisation said the risk to the public is very low, but added that it is issuing advice to healthcare workers to test people with brain inflammation – or encephalitis – of unknown cause.

“While this is the first detection of West Nile Virus in mosquitoes in the UK so far, it is not unexpected as the virus is already widespread in Europe,” Meera Chand at UKHSA said in a statement.

West Nile virus is already widespread across Africa, Europe and North America. In Europe, the virus has been expanding northward in recent years as the climate has warmed, reaching the Netherlands in 2020.

The spread of West Nile virus is linked to high temperatures in spring and summer, droughts in summer, and warm winters, all of which are becoming more common in northern Europe. Although APHA hasn’t detected any West Nile virus in mosquitoes or birds in 2024, it could re-emerge this year due to the recent spell of warm weather in the UK. “We’ve had a very warm May,” says Folly. “If we were going to get any cases or any detections, it would be in the next couple of weeks. This is the first detection. There are likely to be more in the future.”

Scientists in the UK have been on high alert for the arrival of West Nile virus since 2020, when a similar mosquito-borne virus – Usutu – was first detected in London. West Nile virus spreads in the same way as Usutu and requires the same environmental conditions.

“I’m not surprised in the slightest,” says Matthew Baylis at the University of Liverpool, UK. “I’ve been waiting year on year really, expecting West Nile to be detected in the UK.”

He says it is likely the virus was brought to the UK by a bird and is probably circulating among more than one mosquito species in the country. But he stresses there is no reason for public alarm. “There will be a small risk of some people; some equines getting West Nile disease, but it will be rare,” he says, pointing to the limited number of cases of the infection in other northern European nations.