Researchers find signs of immunity in Formby population that was nearly killed off by the deadly disease transmitted by greys

The first case of a wild red squirrel surviving the poxvirus carried by greys has been recorded by researchers who have discovered encouraging signs of resistance to the deadly disease.

An isolated colony of red squirrels at Formby, Merseyside, were decimated by an outbreak of squirrelpox in 2008, which saw the population crash by 85% to less than 200 squirrels.

The disease, which is transmitted to reds by grey squirrels who remain unaffected by it, is thought to be a significant factor in the precipitous decline of the much-loved native mammal across Britain.

But scientists from the University of Liverpool monitoring the population at Formby have identified individual red squirrels which have contracted but survived the virus.

One animal was captured with ulcers around its eyes, mouth and nose – common signs of squirrelpox – and tested positive for the disease but after two months at a nearby RSPCA centre, was found to be negative again so was released back into the wild at Formby. The squirrel, nicknamed Clark because of its apparent super powers, was tracked with a radio collar and recaptured in good health on two further occasions.

Dr Julian Chantrey, from the University of Liverpool's Institute of Integrative Biology, said that while some individuals had survived by chance, blood tests of survivors from Formby found that a small number – less than 10% – had antibodies which would suggest they had recovered from infection in the past. But he warned that this was not yet proof that squirrels were developing immunity to the disease.

"Over time, a degree of resistance should become established in the population but this is not yet full immunity," he said. "The Formby red squirrel population will become infected by squirrelpox again and it will cause significant mortality but hopefully it won't be as high as 85% next time."

It is hoped that red squirrels' resistance to squirrelpox might prove similar to rabbits with myxomatosis, which initially killed around 98% of the rabbit population before the animal bounced back. But Chantrey said the outlook was less positive for red squirrels. The myxomatosis virus became less severe because it was, in effect, destroying itself by killing its rabbit host so effectively. With squirrelpox virus, there is no imperative for it to become less virulent to red squirrels because it lives within the grey squirrel population as well.

Andrew Brockbank of the National Trust, which part-funded the research, said: "Red squirrels would feature on many people's list of favourite British wildlife and it's been a huge relief to see the numbers recovering at Formby in the last five years after the 2007-08 crash. Whether red squirrels have any long term immunity or not remains to be seen. But the recovery of the population and the findings of the research give us hope that red squirrels have a better chance of survival at Formby than we had thought possible just a few years ago."

A red squirrel is being checked for the impact of poxvirus infection during a field study. Photograph: Dr Julian Chantrey/Institute of Integrative Biology/University of Liverpool

The red squirrel research at Formby, undertaken by PhD student and vet Tim Dale in collaboration with Lancashire Wildlife Trust and part-funded by the National Trust, will only continue if new funding can be found, with researchers still desperate to discover exactly how the disease is transmitted by greys so that reds might be better protected from it. Chantrey said it was crucial that the Formby population of red squirrels continued to be protected from incursions by disease-bearing greys, with a patrolled buffer zone around the red population, which has returned to 80% of its pre-pox level.

Red squirrels have disappeared from much of southern Britain in the last century with the march of the larger North American grey squirrel, which outnumbers reds by 2.5 million to 160,000.