Histopathological and immunohistochemical characterization of pneumonia caused by co-infection between Pasteurella multocida and Mycoplasma hyopneumoniae in swine.

Background: The infection with Mycoplasma (M.) hyopneumoniae is a major cause of pneumonia in pigs worldwide, but the interaction between M. hyopneumoniae and Pasteurella (P.) multocida has been described with low frequency and histopathological characteristics of this association have not been described. In Brazil this interaction is considered very important and is found in most cases of pigs with pneumonia and pleuritis at slaughter. The objective of this work was to examine 41 samples of swine lungs collected at slaughter, from which P. multocida had been isolated and to study the co-infection with M. hyopneumoniae by histopathology and immunohistochemistry (IHC). Materials, Methods & Results: Among lung tissue samples obtained from necropsy in swine with clinical signs of pneumonia, 41 that had shown positive results on bacteriological examination for P. multocida were selected. The histopathological fi ndings of bronchopneumonia were classifi ed in different degrees according to the extension of the lesion as follow: absent (0), mild (+), moderate (+ +) and severe (+ + +). Hyperplasia of bronchial and bronchiolar epithelium was classifi ed as absent (0) and present (+). The bronchus associated lymphoid tissue (BALT) hyperplasia lesions were classifi ed as absent (0), (+) when the peribronchial and peribronchiolar lymphocytic infi ltrate were discrete, (+ +) when the peribronchial and peribronchiolar lymphocytic infi ltrate was moderate and/or the presence of a few lymphoid nodules; (+ + +) highest amount of lymphoid nodules, and (+ + ++) when a large amount of lymphoid nodules compressing the bronchi and bronchioles were observed. All samples were submitted to IHC technique for M. hyopneumoniae p-36 protein. The main fi ndings in histological examination were BALT hyperplasia (97.6%), purulent bronchopneumonia (92.7%) and bronchial and bronchiolar epithelial hyperplasia (51.2%). BALT hyperplasia was mild (+) in 43.9%; moderate (++) in 31.7%; accentuated (+++) in 12.2%; accentuated with bronchium and bronchiole obstruction (++++) in 9.8% and absent (-) in 2.4%. The classifi cation of purulent bronchopneumonia was moderate (++) in 36.6%; accentuated (+++) in 34.1%; mild (+) in 22.0% and absentee (-) in 7.3%. Lesions compatible with a combined infection between P. multocida and M. hyopneumoniae were found in 97.6% of the samples by histopathology and in 61.0% by IHC. Discussion: There was a high frequency of lesions consistent with infection by M. hyopneumoniae in samples of lungs from which P. multocida was isolated, being detected at the same time by histopathology in 97.6% of samples, and 61% by IHC. However, 15 samples (39.0%) had microscopic lesions of BALT hyperplasia in different degrees and/or epithelial hyperplasia with no labeling in IHC M. hyopneumoniae. This could be due to the absence of the agent in chronic infections. On histopathological examination, the presence of BALT hyperplasia is considered very suggestive of infection with M. hyopneumoniae. In the study only one lung (2.3%) did not have this injury, and did not show labeling in IHC. Among samples positive for P. multocida in bacteriological examination, only three showed no lesion suggestive of the infection with the agent in histopathology, which could be due to a recent bacterial infection or its presence as a commensal in the lungs. Collectively, bacteriological examination, histopathology and IHC confi rmed the co-infection between M. hyopneumoniae and P. multocida in 97.2% of pneumonic lungs analyzed, suggesting the importance of the relationship between the agents in Brazil. Moreover, it was possible to determine the histopathological and immunohistochemical characteristics of the association between both bacteria.