Pulmonary lesions induced by a Pasteurella haemolytica cytotoxin preparation in calves

Lesions in lambs experimentally infected with ovine adenovirus serotype 6 and Pasteurella haemolytica

Twenty-five colostrum-deprived lambs reared in isolation were inoculated with a US variant of ovine adenovirus serotype 6 (OAV-6) strain RTS-151, Pasteurella haemolytica, or a combination of the 2 agents. Although severe pulmonary lesions were caused by each agent, the lesions were more severe and lasted longer with the combined infection. Lesions induced by OAV-6 alone developed 6-9 days after inoculation and lasted for 15 days, the length of the experiment. The lesions were characterized by suppurative inflammation at the junction of the terminal bronchioles and alveoli. Air spaces were filled with neutrophils and sloughed epithelial cells, which often contained large intranuclear inclusions. Lesions induced by P. haemolytica alone developed within 1 day and persisted for no more than 10 days and were characterized by severe pulmonary edema with variable amounts of fibrin. Lesions induced by the combined infection had aspects of each infection alone and resulted in severe disease in 4 of 8 lambs that were permitted to live more than 1 day after inoculation with bacteria. Early pulmonary lesions included edema, limited fibrin deposition, and slight purulent bronchiolitis and alveolitis. Later lesions included necrosis and more fibrin. For lambs inoculated with both pathogens, resolution was incomplete 15 days after inoculation of virus (10 days after inoculation of P. haemolytica). The results presented here corroborate previous findings indicating that the RTS-151 variant of OAV-6 is common in lambs and acts in concert with P. haemolytica to cause severe and often fatal pneumonia.

Pasteurella haemolytica lipopolysaccharide-associated protein induces pulmonary inflammation after bronchoscopic deposition in calves and sheep

The lipopolysaccharide (LPS)-associated protein (LAP) was extracted from Pasteurella haemolytica serotype A1 strains L101 (bovine origin) and 82-25 (ovine origin). Extracts contained 0.017% total LPS and appeared as only two bands at 14 and 16.6 kDa after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. To determine the extent of pulmonary inflammation induced by LAP and its possible role in the pathogenesis of pneumonic pasteurellosis, LAP (500 micrograms in pyrogen-free saline [PFS]) was deposited by fiber-optic bronchoscopy into the dorsum of the caudal portion of the cranial lobe of the right lung of calves (strain L101 LAP) and sheep (strain 82-25 LAP). LPS (500 micrograms in PFS), 3-h P. haemolytica cultures (1.6 x 10(8) to 1.9 x 10(8) CFU in PFS), and PFS alone were deposited similarly as controls. At necropsy, 24 h after deposition, gross and histologic pulmonary lesions of calves and sheep given LAP, LPS, and P. haemolytica were similar and consisted of various degrees of acute bronchopneumonia (relative severities of lesions induced: LAP < LPS < live organisms). By subjective histologic interpretation and semiquantitative morphometry, animals given LAP had the highest percentage of macrophages per alveolar lumen and the lowest percentage of neutrophils. The lesions from animals given LPS were more severe than those given LAP, but the morphometric cell counts were similar. In contrast, animals inoculated with P. haemolytica had lesions typical of this agent, consisting of many neutrophils, proteinaceous exudate, and a few macrophages. Morphometrically, these lesions had the highest numbers of neutrophils and the lowest numbers of macrophages. These studies show that LAP can induce an inflammatory response in the alveolar lumens and may play a role in the pathogenesis of pneumonic pasteurellosis.

Pasteurella haemolytica leukotoxin enhances production of leukotriene B4 and 5-hydroxyeicosatetraenoic acid by bovine polymorphonuclear leukocytes

The influence of the leukotoxin of Pasteurella haemolytica on the generation of arachidonic acid metabolites by bovine polymorphonuclear leukocytes (PMNs) was investigated. PMNs released 5-, 12-, and 15-hydroxyeicosatetraenoic acids (5-, 12-, and 15-HETE) and leukotriene B4 (LTB4) upon stimulation with arachidonic acid. The leukotoxin preparations dose dependently enhanced the release of the 5-lipoxygenase products 5-HETE and LTB4 in arachidonic acid-stimulated PMNs, whereas the release of 12- and 15-HETE was not affected. The enhanced release of LTB4 and 5-HETE was not due to a decreased cellular retention of the 5-lipoxygenase products. In addition, leukotoxin preparations by themselves were also able to induce LTB4 and 5-HETE production in the absence of exogenous arachidonic acid. Generation of 5-lipoxygenase products by PMNs stimulated by leukotoxin may represent an important cellular event that occurs during infections with P. haemolytica.