Characterization and time course of pulmonary lesions in calves after intratracheal infection with Pasteurella multocida A:3.

Pasteurella multocida A:3 is a common cause of suppurative bronchopneumonia in calves and results in significant production losses and mortality. Here we describe the lesions in three calves at each of four time points (1 day and 4, 7 and 10 days) after experimental intratracheal infection with approximately 1x10(9) colony-forming units of P. multocida A:3 Moredun Research Institute (MRI isolate 671/90). Equivalent age- and time-matched sham-dosed negative control animals were also studied. Infected calves developed significantly elevated mean rectal temperatures (P<0.001) and respiratory rates (P<0.001) compared with negative control animals. Extensive consolidation of multiple lung lobes was present on each of the day/s post-infection (dpi). Histologically, large numbers of alveoli contained either or both polymorphonuclear neutrophils (PMNs) and oedema fluid (1 dpi). At 4 dpi a severe fibrinosuppurative bronchopneumonia had developed. At this time, PMNs and macrophages formed focal lesions containing central necrotic and mineralized debris, while the interlobular septa were severely distended by oedema. Early abscess formation was present in the lung parenchyma at 7 dpi and many of the interlobular septa were thrombosed. At 10 dpi abscesses within the lung parenchyma were mature and comprised of central necrosis with surrounding layers of PMN, macrophages and fibrous tissue. This study describes, for the first time, the commencement, nature and progression of lesions in bovine pneumonic pasteurellosis caused by P. multocida A:3 and provides the foundations for further investigation of the pathogenesis of this disease in cattle.