1. Centre for Epidemiology and Risk Analysis, Veterinary Laboratories Agency, New Haw, Addlestone, Surrey KT15 3NB, UK
2. Department of Statistics and Modelling Science, University of Strathclyde, 26 Richmond St, Glasgow G1 lXH, UK
3. Population and Public Health Branch, Health Canada, 110 Stone Road West, Guelph, Ontario, N1G 3W4, Canada