Interalveolar septal changes with Pasteurella haemolytica-induced pneumonia in exertion-stressed calves

This study compared the effects of exertion stress and exertion stress combined with P. haemolytica infection on cells in the pulmonary capillaries and interalveolar septa adjacent to pneumonic lung. Two calves stood on an Anamill treadmill to serve as nonexercised controls and four calves were run to exhaustion (exertion-stressed) on the treadmill. Two of these four calves were inoculated endobronchially with 5 x 10(9) CFU of virulent Pasteurella haemolytica immediately following treadmill exercise. All calves were euthanized 24 h later and lung tissue was collected from the right mid-caudal lung lobe, next to the area of the gross lesion in inoculated calves. Ultrastructural evidence of increased metabolic activity of pulmonary intravascular macrophages (PIMs) and slight alveolar edema were present after exertion stress, but no changes in numbers of PIMs and platelets were observed between exertion-stressed and control calves. Lungs of calves that were subjected to stress and experimental infection had increases in size, number, and metabolic activity of PIMs, and numerous platelets that were interspersed among the PIMs. These findings suggest that exercise has minimal effect on the cellular changes in interalveolar septa after 24 h. However, exercise with P. haemolytica infection can stimulate PIM activity at short distances (<2.5 mm) from the gross pneumonic lesion. It is possible that the alterations in PIMs are stimulated by cytotoxic products arising from a nearby lesion in which bacteria and infiltrated cells are present.