A mosquito-borne disease freshly arrived in Britain has spread large distances, with scientists racing to understand the risks to wild birds.

Infected insects can spread the deadly Usutu virus to blackbirds, raising fears for the famous songsters.

New data shows Usutu has spread across much of southern England in five years, and has been linked to declines in some blackbird populations.

Scientists are monitoring its spread amid warnings that mosquitoes and the diseases they carry may expand their range under climate change.

“We’ve seen that the virus has spread further than we thought it might do, and it’s persisted,” Dr Arran Folly of the Animal and Plant Health Agency (APHA) told the BBC.

Scientists at the APHA in Weybridge, Surrey, have been tracking mosquito-borne diseases in wild birds for decades, amid warnings that climate change is turning Europe into a potential breeding ground for the insects.

Longer summers, hotter temperatures and heavy rainfall are creating conditions for the nuisance insects to move into areas that were previously inhospitable to them.

Until 2020, all results came back clear. Then, after the summer heatwave of that year, Usutu was detected in several blackbirds in Greater London.

“Blackbirds specifically are quite susceptible to the virus and since 2020 we’ve found a decline in blackbirds of approximately 40% in Greater London,” said Dr Folly.

“It gives an indication that in the future we might get other viruses that are transmitted by mosquitoes emerging in the UK.”

Diseases such as Usutu are a growing threat to wild birds, amid a host of other pressures, including habitat loss, climate change and pesticide use.

• Usutu virus was first detected more than half a century ago around southern Africa’s Usutu River

• It has since spread around the world, reaching Europe three decades ago, and was picked up for the first time in the UK in 2020

• Blackbirds are particularly susceptible to the virus, which can also infect horses and, from time to time, humans.

The latest data shows that Usutu has spread further than the scientists expected.

It has now been detected in wild birds across much of southern England, at least as far west as Dorset and as far north as Cambridgeshire.

How big a risk Usutu poses to wild birds is uncertain. The virus has been linked to mass die-offs of blackbirds elsewhere in Europe, though that doesn’t seem to be the case in Britain.

And the blackbird remains one of the commonest garden birds with numbers holding steady in many parts of the country, especially in rural areas, and in the north.

To untangle the puzzle – and gather more data on blackbird numbers – the scientists have joined forces with the British Trust for Ornithology (BTO).

They are calling for volunteers to count blackbirds in their gardens over the summer months to find out more about the comings and goings of the birds.

Around now blackbirds are breeding and raising their young, frequently seen hopping along the ground and singing from the branches of trees on summer evenings.

Lucy Love, a garden birdwatch ambassador for the BTO, knows the blackbirds in her gardens by sight and has grown fond of them.

“They’re beautiful birds – intelligent, friendly and they have the most beautiful song with a lovely melodic tone to it,” she explained.

“And we cannot lose them – they’re a vital part of our ecosystem.”