Pathology, Tissue Metalloproteinase Transcription and Haptoglobin Responses in Mice after Experimental Challenge with Different Isolates of Pasteurella multocida Obtained from Cases of Porcine Pneumonia

Pasteurella multocida is a major cause of porcine pneumonia, but the pathogenesis of the disease is poorly defined. The aim of this study was to further understand the host response to infection by use of a mouse model of P. multocida pneumonia. Twenty female mice were divided into four groups (n=5). Three groups were infected with one of three isolates of P. multocida isolated from clinical cases of chronic porcine pneumonia with necrotizing, suppurative and non-suppurative lesions, respectively. The fourth group served as uninfected controls. Mice were killed 24h postinfection and samples were collected for bacteriology, histopathology and in-situ hybridization for detection of P. multocida. Measurements of expression of genes encoding matrix metalloproteinase 9 (MMP9) and tissue inhibitor of metalloproteinase 1 (TIMP1) in lung tissue and quantification of serum haptoglobin concentration were performed. P. multocida was found in the lung and spleen. Lung lesions were characterized by deposition of fibrin in alveoli and bronchioles, perivascular oedema, suppuration and necrosis. The cellular infiltration was mainly of neutrophils. Splenic neutrophilic infiltration was also evident. Minor differences in the severity and nature of lesions were seen according to the isolate of P. multocida used for infection. Intranasal infection of mice can therefore be used to evaluate the host response and lesions caused by P. multocida obtained from porcine pneumonic infections. The inflammatory response in this model is associated with increased tissue expression of genes encoding MMP9, TIMP1 and serum haptoglobin concentration.