Swine stocks are endemically infected with the major porcine pathogen ( ) . The factors governing the transition from colonizing residing in the tonsils and the exacerbation of disease have not yet been elucidated. We analyzed the sudden death of fattening pigs kept under extensive husbandry conditions in a zoo. The animals died suddenly of septic shock and showed disseminated intravascular coagulopathy. Genotypic and phenotypic characterizations of the isolated strains, a tonsillar isolate and an invasive type 2 strain, were conducted. Isolated from dead pigs belonged to type 2 strain ST28, whereas one tonsillar isolate harvested from a healthy animal belonged to ST1173. Neither growth, induction of neutrophil extracellular traps, nor survival in blood could explain the sudden deaths. Reconstituted blood assays with serum samples from pigs of different age groups from the zoo stock suggested varying protection of individuals against pathogenic type 2 strains especially in younger pigs. These findings highlight the benefit of further characterization of the causative strains in each case by sequence typing before autologous vaccine candidate selection.