1. Norwegian School of Veterinary Science, Department of Production Animal Clinical Sciences, P.O. Box 8146, 0033 Oslo, Norway
2. Department of Health Surveillance, Norwegian Veterinary Institute, P.O. Box 750, 0106 Oslo, Norway
3. Department of Laboratory Services, Norwegian Veterinary Institute, P.O. Box 750, 0106 Oslo, Norway
4. Norwegian Pig Health Service, Animalia, P.O. Box 396, 0513 Oslo, Norway