Hopes of saving the red squirrel from oblivion have been raised, with the discovery that some of the animals have developed immunity to a deadly virus.

Eight have been found to have naturally developed immunity to the squirrelpox virus, which threatens to kill off the species in Britain by the middle of the century. The discovery was described as “the first sign of hope” for the species since it began a steep decline at the end of the 19th century.

The red squirrel, Sciurus vulgaris, is native to Britain but has been driven inexorably from its home territories by the grey squirrel, Sciurus carolinensis, which is thought to have been introduced from North America in 1876.

Grey squirrels carry the squirrelpox virus but are immune to it, so when they come into contact with their cousins the red squirrels are killed by the disease. Research shows that the geographical spread of the virus and the grey squirrel are virtually identical.

Times Archive, 1959: Red squirrels on the ground

Observations from a Norfolk wood

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The eight red squirrels identified by researchers and reported in the journal EcoHealth are the first seen in the wild with immunity to the virus. This raises hopes that enough reds will develop immunity to assure the future of the species in Britain, or that a vaccine can be created.

“We were absolutely delighted to find signs of immunity in red squirrels after years of seeing the squirrelpox virus devastating populations throughout England and Wales,” said Anthony Sainsbury, of the Zoological Society of London. “This finding is the first sign of hope in the long struggle to save the species from extinction in the UK. It’s pretty positive news for the red squirrel.”

The red squirrels found with immunity had died from other causes and were clear of the disease. They were among 508 carcasses collected by members of the public and submitted to the Institute of Zoology or the Veterinary Laboratories Agency for analysis.

Now that red squirrels have been shown to be capable of having a natural resistance to the virus, scientists want to carry out research to find out how widespread the immunity is. They would hope that, just as some rabbits developed immunity to myxomatosis and avoided being wiped out, a pool of red squirrels will survive.

Alternatively, if resistance remains limited to a tiny proportion of the population, researchers expect to be able to develop a vaccine within a decade.

Dr Sainsbury added: “Immunity to the squirrelpox virus should give red squirrels a fighting chance against the grey invaders, without which red squirrels would undoubtedly be destined to lose the battle for survival in the UK.”

Red squirrels have disappeared from most of England and Wales, though they cling on in isolated pockets such as the Isle of Wight and Brownsea Island. There are estimated to be 140,000 left in Britain with most of them in Scotland and Northern Ireland – fewer than 15,000 are in England and 3,000 in Wales – compared with 2.5 million greys.

Scotland remains the stronghold of the reds, but with grey squirrels and the virus advancing they are in danger of all but dying out within 25 years.

The invader

— Head and body length: 23-30 cm
— Weight: 400-800g
— Lifespan: 8-9 years
— Population: 2.5 million
— Native to North America
— Predators: owls, hawks, cats, foxes, pine martens
— Diet includes acorns, flowers, nuts, bulbs and tree bark
— Seek food on ground

The native

— Head and body length: 20-22cm
— Weight: 280-350g
— Lifespan: 6 years
— Population: 140,000
— Native to Europe
— Predators: polecats, pine martens, wildcats, owls and goshawks
— Diet includes seeds, fruit, fungi and lichens
— Prefer the treetops

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