The Emergence Of Squirrelpox As A Threat To Scotland's Red Squirrels

Published on 30 January 2009 in Ecosystems and biodiversity , Food, health and wellbeing

red squirrel (courtesy of Christopher Sutherland)


Red squirrel numbers in the UK have been declining for the last century. The reasons for this may be many-fold, but one factor is the concomitant increase in the number of grey squirrels. The grey squirrel is not native to the UK. It was introduced, from the USA and Canada, in small numbers to various sites across the UK in the second half of the 19th century. At the time it was thought that it would pose no threat to the native red squirrel. However by 1930 it had been recognised that red squirrels were disappearing and, more often than not, being replaced by grey squirrels. Competition for woodland resources, with the greys being much more successful than the reds, was proposed as the most likely reason for this replacement. However, in addition, epidemics of disease in the red squirrels were blamed for the local extinction of some red squirrel colonies. By the late1990s it was realised that pox disease, caused by a virus now known as squirrelpox virus (SQPV), was a major contributory factor in the deaths of red squirrels across England and Wales and that the virus appeared also to infect grey squirrels, but with no obvious disease in them.

Key Points

healthy red squirell (image supplied courtesy of Jim Wilson)Monitoring of blood samples taken from both grey and red squirrels has been performed for the last 10 years. A large proportion of grey squirrels in England and Wales have been infected with the virus with no obvious affects, whereas, the vast majority of red squirrels that were infected with the virus were found dead or dying of pox disease. We have evidence that only a very small number of red squirrels are capable of surviving the disease in the wild.

Disease in red squirrels was found only in areas of the country where it was known that the grey squirrels had been infected with the virus, the corollary of this being that no disease has been reported in parts of the country where there are no grey squirrels. These serological results, although circumstantial, provide good evidence that the grey squirrel is the natural reservoir of squirrelpox virus, transmitting it to the red squirrels with lethal consequences.
Until 2005 there was no evidence of squirrelpox virus in Scotland. In May that year grey squirrels potentially infected with the virus were detected for the first time in south west Scotland, near Newcastleton. Since then grey squirrels that have been exposed to the virus have been found expanding northwards.
In May 2007 pox disease was detected in Scottish red squirrels for the first time. This was in woodland just south of Lockerbie. Although no further outbreaks of pox disease were detected that year, in 2008 three outbreaks were detected; near to Langholm, north of Thornhill and to the west of Annan.

Research Undertaken

The Scottish Government, working through the auspices of Scottish Natural Heritage and Forestry Commission Scotland, published the Scottish Red Squirrel Action Plan 2006-2011. This set out a series of recommendations to help conserve the red squirrel in its current range in Scotland. As part of this plan, Moredun Research Institute is continuing to monitor blood samples from red and grey squirrels. Samples from all over Scotland are analysed to help predict the spread of the virus.

Evidence to date suggests that the resident population of Scottish grey squirrels have not previously been exposed to squirrelpox virus, but that grey squirrels carrying the virus from the south have emerged in the last few years. Research aimed at understanding more about the epidemiology of the virus and the feasibility of producing a vaccine to protect the red squirrels against the virus is also now underway.

Typical appearance of a red squirrel suffering from squirrelpox. Scabs are present on the nose and between the fingers. Conjunctivitis and scabs on the eyelids are particularly pathognomic. This was the first red squirrel found in Scotland suffering from squirrelpox.None of the pathological signs of disease in red squirrels are seen in infected grey squirrels. In fact no pathological changes in the grey squirrels have been detected so far. This makes understanding how the virus is transmitted between grey squirrels and from grey squirrels to red squirrels particularly difficult. Current work is thus focussed on trying to unravel what happens when a grey squirrel becomes infected with the virus and how the virus is then passed on to other squirrels.

Research, funded by the Wildlife Ark Trust, has also recently started on identifying possible vaccines and vaccination strategies for squirrelpox. Poxvirus vaccines have been successfully produced in the past for a variety of diseases, but one of the biggest challenges for squirrelpox may be in delivery of the vaccine to a wild population of animals.

Policy Implications

Efforts are currently being made to restrict the virus to south west Scotland. The government are about to launch their initiative on “Saving Scotland’s Red Squirrels.” Monitoring the spread of the virus will directly influence policies of the local conservation organisations in determining where best to place efforts to control the virus. Understanding how the virus is transmitted will also help to inform on the major risk factors influencing the appearance of disease in the red squirrels. 


Dr Colin J. McInnes, Moredun Research Institute


Ecosystems and biodiversity , Food, health and wellbeing

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