Abstract
BackgroundPasteurella multocida type A (PmA) is considered a secondary agent of pneumonia in pigs. The role of PmA as a primary pathogen was investigated by challenging pigs 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 dpi. Macroscopic 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 (toxA: toxin; pfhA: adhesion; tbpA and hgbB: iron acquisition) by PCR and sequencing and submitted to multilocus sequence typing (MLST).ResultsThe eight PmA strains were classified as follows: five as highly pathogenic (HP) for causing necrotic bronchopneumonia and diffuse fibrinous pleuritis and pericarditis; one as low pathogenic for causing only focal bronchopneumonia; and two as nonpathogenic because they did not cause injury to any pig. PCR for the gene pfhA was positive for all five HP isolates. Sequencing demonstrated that the pfhA region of the HP strains comprised four genes: tpsB1, pfhA1, tpsB2 and pfhA2. The low and nonpathogenic strains did not contain the genes tpsB2 and pfhA2. A deletion of four bases was observed in the pfhA gene in the low pathogenic strain, and an insertion of 37 kb of phage DNA was observed in the nonpathogenic strains. MLST clustered the HP isolates in one group and the low and nonpathogenic isolates in another. Only the nonpathogenic isolates matched sequence type 10; the other isolates did not match any type available in the MLST database.ConclusionsThe hypothesis that some PmA strains are primary pathogens and cause disease in pigs without any co-factor was confirmed. The pfhA region, comprising the genes tpsB1, tpsB2, pfhA1 and pfhA2, is related to the pathogenicity of PmA. The HP strains can cause necrotic bronchopneumonia, fibrinous pleuritis and pericarditis in pigs and can be identified by PCR amplification of the gene pfhA2.
Highlights
Pasteurella multocida type A (PmA) is considered a secondary agent of pneumonia in pigs
Our group has successfully developed a model to reproduce the disease in pigs inoculated with a field strain of P. multocida type A [7]. This model is useful for studying the pathogenicity of other P. multocida type A isolates in the specific pig host
All animals were negative for respiratory pathogens (Actinobacillus pleuropneumoniae, Haemophilus parasuis, Bordetella bronchiseptica, M. hyopneumoniae, porcine circovirus type 2 (PCV2) and influenza virus) when screened before and after inoculation
Summary
Pasteurella multocida type A (PmA) is considered a secondary agent of pneumonia in pigs. The role of PmA as a primary pathogen was investigated by challenging pigs with eight field strains isolated from pneumonia and serositis in six Brazilian states. According to Ross [6], the difficulty of reproducing the disease in the absence of infectious or noninfectious cofactors is a major limitation to demonstrating the primary role of P. multocida type A in pneumonic lesions in pigs. Our group has successfully developed a model to reproduce the disease in pigs inoculated with a field strain of P. multocida type A [7]. This model is useful for studying the pathogenicity of other P. multocida type A isolates in the specific pig host
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