For the first time, traces of the West Nile virus have been found in mosquitoes in the UK, according to a report published this week by the UK Health Security Agency.
Author
- Paul Hunter
Professor of Medicine, University of East Anglia
Here's what you need to know about the virus and the disease it causes.
What is West Nile virus?
West Nile virus is a mosquito-borne virus first identified in Uganda in 1937 . It belongs to the same viral family as dengue and yellow fever. The virus is most commonly transmitted by Culex mosquitoes , particularly the species Culex pipiens, which mainly feeds on birds .
Birds are the primary host for West Nile virus, and the virus spreads in a cycle from infected birds to mosquitoes and then back to birds. Occasionally, mosquitoes can transmit the virus to humans or other animals.
Most human infections - around 80% - cause no symptoms. When symptoms do occur, they are usually mild: fever, fatigue, headaches, body aches and sometimes nausea. But in rare cases, around one in 150 infections, the virus can cause severe illness, including encephalitis (inflammation of the brain) or meningitis. Older adults, especially those over 50, are most at risk of serious complications.
The virus cannot normally be spread from person to person, though rare cases of transmission have occurred through blood transfusions or from mother to baby during pregnancy.
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How did it get to the UK?
Although the exact route isn't known, experts believe the virus may have arrived in the UK via migratory birds infected elsewhere. The mosquitoes probably picked up the virus after feeding on these birds during their northward journey.
The detection was made as part of a routine mosquito surveillance programme run by the Animal and Plant Health Agency. Mosquitoes collected from marshlands in south-east England tested positive in PCR (polymerase chain reaction) tests, which detect fragments of the virus's genetic material.
It's important to note that a positive PCR test doesn't necessarily mean the virus is infectious. After a mosquito becomes infected, the virus needs time - several days - to multiply inside the mosquito before it can be transmitted. And this process is highly temperature dependent.
Can the virus spread in the UK?
The UK's relatively cool climate has, until now, helped keep mosquito-borne diseases at bay. At summer temperatures of around 15°C, it can take up to 100 days for the virus to develop inside a mosquito - longer than the insect's lifespan. In contrast, in hotter climates (above 30°C), this process can take just a few days.
For a local outbreak to occur, there would need to be a critical mass of infected birds and mosquitoes, with enough warm weather to sustain multiple cycles of transmission. So far, that hasn't happened in the UK.
But climate change could alter the equation. With rising global temperatures and longer, hotter summers, the conditions that allow viruses such as West Nile to spread may become more common in the UK.
What's happening elsewhere?
West Nile virus was once limited to Africa and the Middle East but has spread significantly in recent decades. Large outbreaks have been recorded in countries including Greece, Romania, Israel , Russia and the US.
The US outbreak began in New York City in 1999 when an unusual number of birds were found dead in a city zoo. A veterinary pathologist at the Bronx Zoo, Tracey McNamara, helped link the bird deaths to the human illnesses being reported.
Since then, the virus has spread across most of the US , Canada and parts of South America, resulting in over 60,000 reported human cases, 28,000 hospitalisations and more than 3,000 deaths.
In 2024, 19 European countries reported a total of 1,436 local cases , most in men over 65, with 125 deaths. Most were in Italy, Greece and Spain - countries with hot, mosquito-friendly summers.
Outbreaks were also reported in birds and horses, which are both susceptible to the virus.
Should UK residents be concerned?
While the detection of West Nile virus in UK mosquitoes is noteworthy, experts emphasise that the public health risk remains very low. No human cases have been reported in the UK to date, and current summer temperatures are not yet conducive to sustained transmission.
The greater risk for most British people probably comes from travel - particularly to southern Europe, where cases are rising.
Travellers are advised to take standard mosquito precautions : wear light-coloured clothing, long sleeves and trousers, and use insect repellent, especially in the evening when mosquitoes are most active.
For now, the virus is unlikely to spread widely in the UK. But as climate patterns shift, continued surveillance and public awareness will be key to staying ahead of the risk.
Paul Hunter consults for the World Health Organization. He receives funding from National Institute for Health Research and has received funding from the World Health Organization and the European Regional Development Fund.
