Submitted to: Veterinary Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2001
Publication Date: N/A
Interpretive Summary: Respiratory tract diseases are a leading cause of loss from disease in the cattle, sheep, and goat industries. Annual loss in the United States is estimated to exceed one billion dollars. Losses are from mortality, reduced feed efficiency, and slaughter condemnations, as well as prevention and treatment measures. Currently, scientists and veterinarians do not know all the factors leading to the development of pneumonia. As part of our ongoing studies to understand the disease process, we found that mast cells and histamine levels in early infected lung tissue were significantly lower than in surrounding normal lung tissue. Macrophages in the affected areas reacted strongly to substance P, suggesting an important extraneural contribution to the disease process. At 15 days after infection, mast cells were still scarce, histamine content was still decreased, and substance P was present in thickened unmylinated nerve fibers. At 45 days, ,the lungs had increased numbers of mast cells, but the histamine was still low. These findings suggest that mast cells and substance P both have an important role in the pathogenesis of lung lesions, an important factor that may be used to better control shipping fever of cattle. Corollary benefits include an increase in the profitability and international competitiveness of the U. S. cattle industry, a stronger rural economy, and a continued supply of inexpensive, wholesome beef and beef products for the American consumer.
Technical Abstract: To determine the density of mast cells (MCs) and extent of substance P (SP) during initiation and progression of Mannheimia ( Pasteurella) haemolytica pneumonia, 18 lambs were inoculated intrabronchially with M. haemolytica or saline, and lung tissue was collected at 1, 15, and 45 days post inoculation. At 1 day after bacterial inoculation, mast cells were absent in infected areas, and histamine content was reduced. Macrophages, infiltrating some of these affected areas, reacted strongly to SP, suggesting an important extraneural contribution to the inflammatory process. At 15 days, 2 lambs had pyogranulomatous abscesses where MCs were scarce and histamine content was decreased. SP immunoreactivity was present in some large macrophages bordering the pyogranulomatous areas and in thickened unmylinated nerve fibers in alveolar septa, terminal bronchioles, small and large bronchioles, small blood vessels, and occasionally in ganglion neurons. Enlarged bundles of myelinated nerves associated with bronchioles and bronchi also had extensive immunoreactivity. At 45 days, the fibrosing changes became more defined. Lungs had increased numbers of MCs; however, the histamine content did not correspond to this increased population. SP immunoreactivity appeared only in nerves with no extraneural reaction and was mild. This work demonstrates that MCs and SP both have an important role during the initiation and progression of pulmonary lesions in pneumonic pasteurellosis.