1. SAC Veterinary Science Division, Perth PH1 1HF,1
2. Moredun Research Institute, International Research Centre, Pentland Science Park, Penicuik, Midlothian EH26 OPZ,2
3. Veterinary Research Institute, 621 32 Brno, Czech Republic3; and
4. Veterinary Diagnostic and Investigational Laboratory, University of Georgia, Tifton, Georgia 317934
5. Biomathematics and Statistics Scotland, Edinburgh EH9 3JZ,5Scotland, United Kingdom;