Pasteurella multocida type A (PmA) has usually been considered to be a secondary agent of pneumonia in pigs. The general assumption was that unless some other primary pathogen such as PRRS, Influenza or Mycoplasma hyopneumoniae started the respiratory disease the PmA would not be a disease issue on it’s own. These Brazilian researchers challenged that traditional thinking and used some newer diagnostic technology to dig a bit deeper on the potential role of PmA as a primary pathogen.
Test pigs were challenged with eight “field strains” isolated from pneumonia and serositis in six Brazilian states. Eight groups of eight pigs each were intranasally inoculated with different strains of PmA (1.5 mL/nostril of 10e7 CFU/mL). The control group (n = 12) received sterile PBS. The pigs were euthanized by electrocution and necropsied by 5 days post infection. Visual lesions were recorded, and swabs and fragments of thoracic and abdominal organs were analyzed by bacteriological and pathological assays. The PmA strains were analyzed for four virulence genes by PCR and sequencing and submitted to multilocus sequence typing (MLST).
The eight PmA strains were classified as follows;
- five highly pathogenic (HP) for causing necrotic bronchopneumonia and diffuse fibrinous pleuritis and pericarditis;
- one low pathogenic for causing only focal bronchopneumonia;
- two non-pathogenic because they did not cause injury to any pig.
PCR for the gene pfhA that causes adhesions was positive for all five HP isolates.
The low and non-pathogenic strains did not contain the genes tpsB2 and pfhA2. MLST clustered the HP isolates in one
group and the low and non-pathogenic isolates in another. Only the non-pathogenic as a primary isolates matched sequence type 10; the other isolates did not match any type available in the MLST database.
Take Home Message
Not all strains of Pasteurella multocida type A are created equal in terms of their disease
causing abilities and this can help explain some of the variation in clinical outcomes when what
appears on the surface to be the same organism.
Submitted by Ed Metzger, DVM
Ref: Oliveira Filho JX, Morés MAZ, Rebellato R, Kich JD, Cantão ME, Klein CS, Guedes RMC, Coldebella A, Barcellos DESN,
Morés N. Pathogenic variability among Pasteurella multocida type A isolates from Brazilian pig farms. BMC Vet Res. 2018 Aug 22;14(1):244. doi: 10.1186/s12917-018-1565-2.