1. Veterinary Epidemiology Unit; Department of Agriculture and Rural Development for Northern Ireland; Dundonald House, Upper Newtownards Rd Belfast BT4 3SB Northern Ireland
2. Institute of Veterinary; Animal and Biomedical Sciences; Massey University; Private Bag 11222 Palmerston North 4442 New Zealand
3. AgResearch; Invermay Agricultural Centre; Puddle Alley, PO Box 50034 Mosgiel 9053 New Zealand