The research shows that squirrel poxvirus is threatening to wipe out red squirrels in some of the areas in which they remain in northern England within 10 years. In areas where the virus has been detected, the rate of decline in reds is 17-25 times higher than in places where there has been no outbreak.
Until now the reds' main survival challenge was believed to be competition with grey squirrels over resources. However, this research highlights the urgency for new conservation strategies for the red squirrel, a species that has been in Britain for the last 10,000 years and is protected under the UK's Wildlife and Countryside Act.
Researchers say that in the absence of a vaccine for the disease the only effective way of stopping the spread is to target grey squirrel control at the narrow entry points and corridors to England's 16 designated red squirrel refuges by killing the small numbers of greys that may come in.
Squirrel poxvirus is passed to reds by grey squirrels, and probably arrived to Britain with some of the greys that were introduced from North America over 100 years ago. The virus does not appear to harm grey squirrels, but is fatal to reds when they become infected.
Around 70 per cent of greys in England have been exposed to the virus. They are thought to transmit it to red squirrels, which develop sores and ulcers on their face, feet and thighs, and they usually die within two weeks.
The research team - Newcastle University, Queen Mary, University of London, and other partners* - makes its recommendations for grey squirrel management in two academic journals, Epidemiology and Infection and Biological Conservation. The Mammals Trust UK and Forestry Commission, England (Kielder Forest District) funded the work.
For the Epidemiology and Infection study, researchers analysed public records on red squirrel populations in Cumbria and Norfolk - both areas that suffered squirrelpox virus outbreaks. The team compared these figures with similar records from red squirrel strongholds in Scotland and Italy that have not been affected by the virus.
The team looked at how far red squirrels had retreated on a landscape scale, and found the annual distance was 76 square kilometres in Cumbria and 96 sq km in Norfolk. In contrast, the annual retreat in Italy was just 3.6 sq km and 4.4 sq km in Scotland. The red squirrel decline - measured in terms of the area they inhabit rather than population numbers - was 17-25 times higher in Cumbria and Norfolk where the virus was present than in Italy and Scotland where it was absent. (It is believed that red squirrels became extinct in Norfolk in the last ten years).
The team then simulated various management strategies using a computer model. Although results suggested that a continuous cull of more than 60 per cent of greys would be needed to save red squirrels on a landscape scale, researchers reasoned this approach would be costly, require complete coverage, long-term commitment and is unrealistic.
The team therefore recommend in the Biological Conservation study that the only practical and humane way forward is the strategy of red squirrel refuge areas where control of grey squirrels bringing the disease could be targeted locally along likely entry points.
In this second study, the team describes how it is applying computer technology to develop recommendations for England's biggest red squirrel refuge, Kielder Forest, Northumberland. The forest has played a leading role in developing conservation strategies for the red squirrel. The team identified four potential 'gateways' for grey squirrels and developed a strategy to manage the disease threat. So far there has been no reports of the virus or the disease in the forest. There are now plans to apply the approach to the other 15 squirrel refuges in Cumbria, Northumberland, North Yorkshire and Merseyside.
Dr Peter Lurz is a co-author of both studies and a research associate at the Institute for Research on Environment and Sustainability (IRES) at Newcastle University. He said: “It is vital we get this disease under control, especially as it is now threatening to spread across the border to Scotland with severe consequences for red squirrel conservation there.
He added: “We are not trying to wipe out the grey squirrel but as conservationists we have a duty to look after the red squirrel as it is a protected, native species. Small, targeted control at local entry points to refuge areas is an accepted management strategy and already works well in controlling populations of species like the deer and rabbit. It's a practical and affordable solution.”
Tony Sainsbury, Lecturer in Wild Animal Health at the Zoological Society of London (ZSL), commented: “The cases seen by ZSL's national red squirrel disease surveillance programme clearly demonstrate the severity and debilitating nature of the squirrel poxvirus disease on red squirrels.
“Given that the grey squirrel is an introduced species, and therefore the disease has occurred as a consequence of human action, the onus is very much on us to look for practical solutions to control the disease and protect red squirrels from this fatal disease. Our work concludes that targeted control is the most viable way forward.”
* Full list of research partners: Newcastle University; Queen Mary, University of London; Moredun Research Institute, Scotland; English Nature (Cumbria Team); Institute of Zoology, Zoological Society of London, Forestry Commission, England.
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