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Poxviral Disease in Red Squirrels Sciurus vulgaris in the UK: Spatial and Temporal Trends of an Emerging Threat

Abstract

The squirrel poxvirus (SQPV) is the probable mediator of apparent competition between the introduced invading gray squirrel (Sciurus carolinensis) and the red squirrel (Sciurus vulgaris) in the UK, and modeling studies have shown that this viral disease has had a significant impact on the decline of the red squirrel in the UK. However, given our limited understanding of the epidemiology of the disease, and more generally the effects of invasive species on parasite ecology, there is a need to investigate the transmission dynamics and the relative pathogenicity of the virus between species. We aimed to increase our knowledge of these processes through an empirical study in which we: (i) used pathological signs and transmission electron microscopy (TEM) to diagnose SQPV disease in red squirrels found dead during scanning surveillance between 1993 and 2005; (ii) detected antibody to SQPV using an enzyme-linked immunosorbent assay (ELISA) in the same animals; and (iii) mapped cases of the disease, and the gray squirrel distribution, using a geographical information system. We analyzed the distribution of cases of SQPV disease according to woodland type, a measure of squirrel density. SQPV disease occurred only in areas of England also inhabited by seropositive gray squirrels, and as the geographical range of gray squirrels expanded, SQPV disease occurred in these new gray squirrel habitats, supporting a role for the gray squirrel as a reservoir host of the virus. There was a delay between the establishment of invading gray squirrels and cases of the disease in red squirrels which implies gray squirrels must reach a threshold number or density before the virus is transmitted to red squirrels. The spatial and temporal trend in SQPV disease outbreaks suggested that SQPV disease will have a significant effect on Scottish populations of red squirrels within 25 years. The even spread of cases of disease across months suggested a direct rather than vector-borne transmission route is more likely. Eight juvenile and sub-adult free-living red squirrels apparently survived exposure to SQPV by mounting an immune response, the first evidence of immunity to SQPV in free-living red squirrels, which possibly suggests a changing host-parasite relationship and that the use of a vaccine may be an effective management tool to protect remnant red squirrel populations.

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Acknowledgments

The Institute of Zoology acknowledges the financial support of Natural England, Peoples Trust for Endangered Species, Joint Nature Conservation Committee, and the Zoological Society of London. The VLA acknowledges the financial support of DEFRA. The Moredun Research Institute acknowledges the financial support of SEERAD. The Tullie House Museum, Cumbria Wildlife Trust, Northumbria Natural History Society, and the Northumberland Wildlife Trust kindly allowed us to use sightings records, and we particularly thank Louise Bessant for help here. We also thank Forest Enterprise, Kielder Forest District, and in particular Bill Burlton and Neville Geddes for their support of the work. Bernadette Carroll assisted with the GIS analysis. Many veterinarians assisted with postmortem examinations, including Heather Ainsworth, Martin Cooke, Jane Cooper, David Couper, Andrew Cunningham, Javier Lopez, Hisako Ohira, Amalia Kountouri, Ann Pocknell, Taina Strike, and Maged Taema. Aviva Petrie assisted with the statistical tests. Maps of administrative boundaries in the UK were reproduced from Ordnance Survey map data by permission of the Ordnance Survey © Crown copyright 2001.

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Correspondence to Anthony W. Sainsbury.

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Sainsbury, A.W., Deaville, R., Lawson, B. et al. Poxviral Disease in Red Squirrels Sciurus vulgaris in the UK: Spatial and Temporal Trends of an Emerging Threat. EcoHealth 5, 305 (2008). https://doi.org/10.1007/s10393-008-0191-z

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Keywords

  • invasive species
  • apparent competition
  • Sciuridae
  • conservation