Blackbird deaths point to looming West Nile Virus threat in the UK

A deadly virus is killing blackbirds across the UK. Beyond the risk to the birds, its spread indicates that mosquito-borne viruses now pose a growing threat to humans and animals in the country, in part as a result of climate change.

The virus in question, Usutu, originated in South Africa in 1959 but is now widespread in Europe. It causes deadly disease in certain bird species, particularly blackbirds, and was first detected in the UK in 2020. In some parts of the country, most notably London, blackbird populations have dropped by more than 40 per cent since 2018. “We first noticed the decline at the same time as Usutu popped up,” says Hugh Hanmer at the British Trust for Ornithology. 

Although devastating for bird life, Usutu poses a low risk to humans and mammals. Infections in people are rare and generally only cause a mild fever, but the arrival of the virus in the UK marked the first time a mosquito-borne viral zoonosis – a disease that can be transmitted from an animal to a human – had emerged in animal hosts in the country. Virus experts are keeping a close watch on how far and fast the disease is spreading because it could be a template for the future spread of other mosquito-borne diseases.  

For example, the West Nile virus spreads in the same way as Usutu and requires the same environmental conditions. “The same mosquitoes that can transmit Usutu typically can transmit West Nile, and the same birds which act as hosts [for Usutu] can also act as hosts of West Nile,” says Arran Folly at the UK’s Animal and Plant Health Agency (APHA). 

Humans can also be infected by West Nile virus from a mosquito bite, but its symptoms can be more severe than those of Usutu. 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 known human vaccine.

Climate change has helped accelerate the spread of West Nile virus through northern and eastern Europe, research shows, as the virus thrives in warm summer temperatures. In the Netherlands, Usutu was first detected in 2016 and West Nile virus followed in 2020. UK officials fear a similar pattern will play out in their country, with studies demonstrating that the climate there is becoming increasingly hospitable to mosquito-borne viruses. “The idea is that, if we have Usutu here, West Nile is probably going to come at some point and is likely to persist, given the right conditions,” says Folly.

In response to the threat, APHA launched a project in 2023 to track the emergence and transmission pathways of Usutu and other mosquito-borne viruses in wild birds. This virus-tracing infrastructure will be vital if the country is to respond quickly to West Nile’s arrival, says Folly. “Our real goal, or drive from a governmental point of view, is to be able to detect these [new viruses] circulating in animal populations before we get transmission to humans.”

Reina Sikkema at Erasmus University Medical Center in Rotterdam has been studying the emergence of Usutu and West Nile virus in the Netherlands. Although West Nile hasn’t been detected since 2022, she believes the virus is circulating at a low level, kept in check currently by the country’s relatively cool climate. “I believe it is present, but it needs the right circumstances to flare up,” she says. A UK detection of West Nile is now all but inevitable, says Sikkema, but she believes similar climatic factors could prevent the virus spreading too widely for now.

But rising summer temperatures, including the increasing frequency of tropical nights – which the UK’s Met Office weather agency defines as when minimum temperatures fail to fall below 20°C – could change the picture in the UK, the Netherlands and other northern European nations in coming years, warns Sikkema. “Mosquito-borne disease is not [just] on your Spanish holiday or when you go to the South Americas,” says Folly.

As well as the potential risk of West Nile virus to people, Folly says we shouldn’t forget what Usutu is doing to the UK’s blackbirds: “If 40 per cent of humans dropped dead in Greater London, you’d know about it quite quickly.”