^a Danish Bacon and Meat Council, Veterinary and Food Advisory Service, Axelborg, Axeltorv 3, DK-1609 Copenhagen V., Denmark

^b Royal Veterinary and Agricultural University, Department of Animal Science and Animal Health, Division of Ethology and Health, Bülowsvej 13, DK-1870 Frederiksberg C., Denmark

^c Royal Veterinary and Agricultural University, Department of Clinical Studies, Swine Medicine, Bülowsvej 13, DK-1870 Frederiksberg C., Denmark

^d Danish Veterinary Institute for Virus Research, Lindholm, DK-4771 Kalvehave, Denmark

A total of 999 pigs were included in the study. The pigs were blood sampled at weaning and subsequently every fourth week until slaughter. All pigs were examined for antibodies against M. hyopneumoniae (enzyme-linked immunosorbent assay), P. multocida toxin (enzyme-linked immunosorbent assay) and A. pleuropneumoniae serotypes 2, 5–7, 12 (complement-fixation tests). The most-common pattern (28%) of seroconversion was that of pigs first seroconverting to A. pleuropneumoniae serotype 2, followed by seroconversion to M. hyopneumoniae. Each herd had a dominant serotype of A. pleuropneumoniae to which most pigs seroconverted. Seroconversion to the respiratory pathogens occurred mainly in the growing-to-finishing units (8–24 weeks). The risk of seroconversion to the P. multocida toxin was very low (<20%) and occurred late.

None, four and seven herds tested seropositive to PRRS and to swine influenza virus subtypes H3N2 and H1N1, respectively, when testing 10 pigs per herd (selected randomly among the study pigs) at the age of 20 weeks.