One hundred and fifty-eight porcine strains of Pasteurella multocida, recovered primarily from cases of pneumonic pasteurellosis or progressive atrophic rhinitis (PAR) in England and Wales, were characterized by determination of their capsular types, presence or absence of the toxA gene and molecular mass heterogeneity of the heat-modifiable (OmpA) and porin (OmpH) proteins. Eighteen groups (clones) of strains were identified on the basis of specific combinations of capsular type, toxA status and outer-membrane protein (OMP)-type. The data provided evidence that different subpopulations of P. multocida are responsible for pneumonia and PAR in pigs. The majority (88 %) of cases of pneumonia were associated exclusively with non-toxigenic capsular type A strains of OMP-types 1.1, 2.1, 3.1 and 5.1 and capsular type D isolates of OMP-type 6.1. These strains were recovered from widespread geographical locations within England and Wales over a 12-year period and represented mostly single sporadic cases. The association of a small number of P. multocida variants with the majority of cases of porcine pneumonia suggests that these strains are not opportunistic pathogens of low virulence but represent primary pathogens with a relatively high degree of virulence. In contrast, the majority (76 %) of cases of PAR were associated with toxAcontaining capsular type D strains of OMP-type 4.1 and capsular type A and D strains of OMP-type 6.1. Toxigenic capsular type A strains associated with PAR and non-toxigenic capsular type A strains associated with pneumonia represent distinct subpopulations of P. multocida that can be differentiated by their OMP-types. The association of capsular types A and D with strains of the same OMP-types, and the absence and presence of the toxA gene in strains of the same OMP-types, suggest that horizontal transfer of capsular biosynthesis and toxA genes has occurred between strains representing certain subpopulations of P. multocida. INTRODUCTIONPasteurella multocida is the aetiological agent of progressive atrophic rhinitis (PAR) of swine and is of considerable economic importance to the pig-rearing industry throughout the world (Chanter & Rutter, 1989). PAR is characterized by atrophy of the nasal turbinate bones which, in severe cases, can lead to facial distortion. P. multocida strains associated with PAR usually produce a 145-kDa dermonecrotic toxin, which is encoded by the toxA gene (Lax et al., 1990;Buys et al., 1990). This toxin induces osteolysis in the turbinate bones and plays an important role in the pathogenesis of PAR (Kamp & Kimman, 1988). Toxigenic strains associated with PAR are most frequently of capsular type D (Eamens et al., 1988;Foged et al., 1988;Gardner et al., 1994;Lariviere et al., 1992), although toxigenic isolates of capsular type A can also be involved in the disease (Fussing et al., 1999;Sakano et al., 1992). Non-toxigenic P. multocida strains are not usually associated with PAR and confirmation of toxin production is important for the diagnosis and control of the disea...

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One hundred avian Pasteurella multocida isolates recovered from cases of fowl cholera and related infections in England and Wales over a 13-year period were characterised by capsular PCR typing and analysis of outer membrane protein (OMP) profiles. Sixty-eight percent of the strains were of capsular type A, 14% were type F, 5% were type D, 4% were type B and 9% were untypable. Nineteen distinct OMP profiles (OMP-types) were identified based mainly on molecular mass heterogeneity of the heat-modifiable (OmpA) and porin (OmpH) proteins. Fifty-six percent of the isolates were represented by 15 OMP-types, whereas 44% of the isolates were associated with four OMP-types. The extensive molecular mass heterogeneity of the OmpA and OmpH proteins supports previous findings that avian P. multocida strains are very diverse. Furthermore, the isolates studied were associated with different clinical symptoms and were recovered from a wide range of lesions and tissues. The high degree of strain diversity together with the wide variety of clinical symptoms suggest that certain avian strains of P. multocida are opportunistic pathogens of relatively low virulence. Strains of capsular types B, D and F, as well as the untypable isolates, were associated exclusively with specific OMP-types and represent distinct and widely disseminated clonal groups. These observations support the view that avian strains of P. multocida have a clonal population structure. Based on previous studies, the molecular mass heterogeneity of the OmpA and OmpH proteins might provide a selective advantage to P. multocida by generating antigenic variation.

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