Dendrogram grouping based on the SDS-PAGE WCP lysate profiles. Reference strains are designated A-O (Table 1), field isolates are designated 1–31 (Table 2), and outgroups are Pasteurella multocida (PM), Mannheimia haemolytica (MH), Pasteurella trehalosi (PT) and Actinobacillus pleuropneumoniae (AP). Reference strains were obtained between 1978 and 1990. Field strains 1–24, 25–29, 30–31 were obtained in 2004, 1999, and 1984, respectively. Three clade and three subclade designations are shown. Numbers at the nodes indicate percentages of bootstrap values after 1000 replicates.