Red squirrel 'becoming immune' to killer virus

The embattled red squirrel – feared to be facing extinction in England and Wales within 25 years – may have developed a last-gasp trick to save it from a deadly virus being spread by its grey American cousin. Wild red squirrels have developed an apparent immunity to the squirrel pox virus, which was killing off the last their remaining communities in England and Wales, as well as threatening the much larger native squirrel population in Scotland.

A survey of the corpses of more than 500 red squirrels collected for laboratory analysis found that eight had been exposed to the squirrel pox virus but died of other causes. One captive red squirrel had also survived exposure to the virus, scientists found. They believe the presence of antibodies to the virus is strong circumstantial evidence that some wild red squirrels have developed a natural immunity to the disease – a fact which could lead to the development of a squirrel pox vaccine.

It is thought there are only about 140,000 red squirrels left in Britain, about 75 per cent of them north of the Scottish Border. This compares with about 2.5 million grey squirrels, which were introduced from America in 1876. Many red squirrel populations in England and Wales are declining rapidly as the grey rivals encroach on their territory and spread the squirrel pox virus, which does not appear to affect greys.

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Tony Sainsbury, of the Zoological Society of London, who led the investigation, said it was previously thought that the virus was 100 per cent lethal to red squirrels and there was little or no chance of them developing immunity. "We have found either that they can be exposed to the disease and still manage to survive, or that they have been exposed to the virus and have not developed the disease," he said. "It may represent a sign of co-evolution between host and virus, similar to the way the myxomatosis virus in rabbits co-evolved to become a less lethal form of the disease. The same may be occurring in red squirrels."

The discovery that some red squirrels are able to mount an immune attack on the squirrel pox virus is welcome because it suggests that the native species may be able to make a comeback, Dr Sainsbury added, saying: "Immunity to the squirrel pox 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."