Poxviral Disease in Red Squirrels Sciurus vulgaris in the UK: Spatial and Temporal Trends of an Emerging Threat Anthony W. Sainsbury, 1 Robert Deaville, 1 Becki Lawson, 1 William A. Cooley, 2 Stephan S. J. Farelly, 2 Michael J. Stack, 2 Paul Duff, 3 Colin J. McInnes, 4 John Gurnell, 5 Peter H. Russell, 6 Stephen P. Rushton, 7 Dirk U. Pfeiffer, 8 Peter Nettleton, 4 and Peter W. W. Lurz 7 1 The Institute of Zoology, Zoological Society of London, Regent’s Park, London, NW1 4RY, UK 2 Veterinary Laboratories Agency, New Haw, Addlestone, Surrey KT15 3NB, UK 3 Penrith Veterinary Investigation Centre, Veterinary Laboratories Agency, Merrythought, Calthwaite, Penrith CA11 9RR, UK 4 Moredun Research Institute, Pentlands Science Park, Bush Loan, Penicuik, Edinburgh EH26 OPZ, UK 5 Biological Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK 6 The Royal Veterinary College, Royal College Street, London NW1 OTU, UK 7 IRES, Devonshire Building, School of Biology, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK 8 The Royal Veterinary College, Hawkshead Lane, North Mymms, Hatfield, Hertfordshire AL9 7TA, UK 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 Published online: October 16, 2008 Correspondence to: Anthony W. Sainsbury, e-mail: tony.sainsbury@ioz.ac.uk EcoHealth 5, 305–316, 2008 DOI: 10.1007/s10393-008-0191-z Original Contribution Ó 2008 International Association for Ecology and Health