1. Swine Medicine; Department of Large Animal Sciences; Royal Veterinary and Agricultural University; Dyrlaegevej 88 DK-1870 Frederiksberg C Denmark
2. National Committee for Pig Production; Danish Bacon and Meat Council, Veterinary and Food Advisory Service; Axeltorv 3 DK-1609 Copenhagen Denmark
3. Department of Large Animal Sciences; Veterinary Epidemiology; Royal Veterinary and Agricultural University; Grønnegårdsvej 8 DK-1870 Frederiksberg C Denmark