1. Quantitative Veterinary Epidemiology Group; Wageningen Institute of Animal Sciences; PO Box 338 6700 AH Wageningen The Netherlands
2. Centre for Veterinary Epidemiology and Risk Analysis; School of Agriculture, Food Science and Veterinary Medicine; University College Dublin; Belfield Dublin 4 Ireland
3. Central Veterinary Research Laboratory; Department of Agriculture and Food; Backweston County Kildare Ireland
4. Department of Population Medicine; Ontario Veterinary College; University of Guelph; Guelph Ontario N1G 2W1 Canada