Deadly West Nile virus found in UK mosquitoes for first time - what is it?

West Nile virus has been detected in collected in Britain for the first time, as diseases borne by the insects spread further afield due to . Fragments of genetic material linked to the virus were found in two samples of Aedes vexans mosquitoes collected in July 2023 by UK Health Security Agency (UKHSA) from wetlands on the River Idle near Gamston in Nottinghamshire.

Usually found in birds, the virus is endemic in many parts of the world, including regions of Europe, Africa, the Middle East, West and Central Asia, North America, South America, the USA and Australia. Its range had expanded in recent years to northern and western regions of mainland Europe. In rare cases, mosquitoes can transmit the virus to humans or horses. There is currently no vaccine for humans.

The UKHSA said the risk to the public was very low, but it is issuing advice to healthcare professionals so patients with encephalitis of unknown cause can be tested as a precaution.

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.

"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."

There have been no cases of locally acquired West Nile Virus detected in humans or horses in the UK to date, although seven cases linked to travel have occurred since 2000.

Most cases of West Nile virus in humans do not cause any symptoms. Sufferers who do show signs typically have mild -like illness, including fever, headache and muscle pain.

Some patients may also experience swelling of their lymph nodes and a rash on the neck, trunk, arms or leg.

Most infections will go away in three to six days, but in around one in 150 cases, sufferers develop disease in the brain and nervous system.

This includes meningitis, encephalitis or acute flaccid paralysis (sudden weakness) with high fever, neck stiffness, disorientation or confusion, severe muscle weakness, tremors, convulsions, paralysis and coma.

People aged 50 and over or with underlying medical conditions, such as , or , are at highest risk of severe disease. Death rates are highest in people aged over 70.

Dr Jolyon Medlock, head of medical entomology and zoonoses ecology at UKHSA, said mosquitoes were common in wetlands and and can breed in habitats such as water butts and buckets.

He added: "Mosquitoes in the UK are also commonly found in wet woodland areas and various aquatic habitats, including ponds, ditches, marshes, and even garden water butts or cisterns.

"To avoid being bitten by a mosquito when in these types of areas, wear long-sleeved clothing and trousers to cover your arms and legs, use insect repellent on your skin (ideally one that contains the ingredient DEET), close windows and doors whenever possible, or use blinds or screens."