The West Nile virus has been found in mosquitoes collected in Britain for the first time, the UK Health Security Agency has said.
The virus, which mostly spreads to people through mosquito bites, can cause severe, life-threatening illness in about 1 in 150 people who are infected, according to the World Health Organisation.
It is part of the flavivirus family, and while it can cause neurological disease and paralysis – most people will make a full recovery.
It is usually found in birds, and typically circulates through bird-biting mosquitoes. In rare cases mosquitoes can transmit the virus to humans or horses.
The UKHSA has assessed the risk to the general public as very low, but is issuing advice to healthcare professionals.
Its arrival is due to climate change, with experts saying warmer weather has made it possible for mosquitoes to spread further north than they have in previous years.
Dr Meera Chand, UKHSA Deputy Director for travel health, zoonoses, emerging infections, respiratory and tuberculosis, said: “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.
“The risk to the general public is currently assessed as very low. Vector research of this kind is designed to give us early warning of potential threats so that we can enhance our disease surveillance and control activities and ensure patients receive appropriate testing.”
The flavivirus family also includes the viruses that cause dengue and yellow fever.
Seven cases of the illness imported by travellers in the past 25 years, with no cases ever reported locally.
The UKHSA said there is "no evidence to suggest ongoing circulation of the virus in birds or mosquitoes in the UK".
Professor Matthew Baylis, from the 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 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.
"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.”
