Red squirrels have been discovered that have immunity to a lethal pox virus that has wiped out large numbers of the animals in England and Wales.
The finding is being seen as a sign of hope for the native species, which has seen dramatic declines across the country since the introduction of North American grey squirrels in the 19th century.
The majority of grey squirrels are naturally resistant to the squirrel pox virus, but carry it in their blood. Scientists believe they pass it on to red squirrels when they mark their territory with scent.
Anthony Sainsbury, a lecturer in wild animal health at the Zoological Society of London, led a team that asked people to send in dead red squirrels for autopsy. Tests identified eight red squirrels, one in Wales and seven from different colonies in northern England, with antibodies to the virus in the tissues. This suggests they had contracted the pox but had developed a resistance to it.
"It's good news because we've never detected this immunity before. It means it's feasible to protect them with a vaccine," said Sainsbury.
"The idea has been mooted before, but we haven't known for sure if the red squirrel would respond to a vaccine. What we'd like to do is develop a vaccine, which we would give to them either through bait or by trapping them," Sainsbury said.
The scientists are unsure whether a small population of immune squirrels has existed for a long time, or whether the virus has become less lethal, allowing some squirrels to recently build up resistance to it. The study appears in the journal EcoHealth.
The effects of the virus have been devastating on red squirrel numbers. Infected squirrels develop painful lesions on their faces and underbellies and usually die of the disease within a week.
A recent epidemic in an isolated squirrel colony in Sefton in Lancashire has wiped out 60% of the animals. Studies have shown that where grey squirrels tread, reds die out 17 to 25 times faster.
At the end of the 19th century, red squirrels were found throughout England and Wales in their millions, but are now restricted to Scotland, small parts of Cumbria, Northumberland and Lancashire and offshore islands.
According to ZSL there are now thought to be fewer than 140,000 red squirrels in the UK, with just 15,000 in England.
Sainsbury said: "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."
Currently, the only way to control the disease is to cull grey squirrels.