Approximately 40% to 80% of all diseases of cattle involve the respiratory system (Lillie L E: "The bovine respiratory disease complex." Can Vet J 15:233-242, 1974). Bovine respiratory disease complex (BRDC) is a major problem in the U.S. cattle industry. BRDC consists of several clinical syndromes, the two most common being shipping fever of feedlot cattle and enzootic calf pneumonia usually seen in dairy calves. While it is now recognized that numerous viruses, stressful management practices, and environmental factors are important in the genesis of shipping fever, P. haemolytica biotype A, serotype 1 (Type A-1) is the main bacterial agent responsible for the clinical disease and pathophysiologic events leading to acute fibrinous lobar pleuropneumonia and subsequent death (Yates W D G: "A review of infectious bovine rhinotracheitis, shipping fever pneumonia and viral-bacterial synergism in respiratory disease of cattle." Can J Comp Med 46:225-263, 1982).
In a two year study conducted in Saskatchewan, Canada, P. haemolytica Type A-1 isolated from the lungs of 74% of cattle that died of shipping fever pneumonia (Schiefer B, Ward G E, Moffatt R E: "Correlation of microbiological and histological findings in bovine fibrinous pneumonia." Vet Pathol 15:313-321, 1978). Annual progress reports during the five year period 1987 to 1991 from the Department of Veterinary Science at South Dakota State University (South Dakota State University, Department of Veterinary Science, Animal Disease Research and Diagnostic Laboratory: Annual Progress Reports 1987-1991. Submitted to the NC107 Technical Committee on Bovine Respiratory Disease.) revealed that P. haemolytica Type A-1 was isolated from 48.7% of bovine pneumonic lungs. Thus, it appears to be the major bacterial agent causing pneumonia in cattle.
P. haemolytica serotype Type A-1 is the pathogen responsible for the fibrinous necrotizing lobar pleuropneumonia seen in shipping fever and purulent bronchopneumonia associated with enzootic calf pneumonia. Interestingly, other P. haemolytica serotypes (frequently ST2 and ST4 and occasionally ST7 and ST11) are innocuous inhabitants in many areas of the nasal cavity or upper respiratory tract (URT) of clinically normal feedlot cattle (Frank G H: "When Pasteurella haemolytica colonizes the nasal passages of cattle." Vet Med 83:1060-1064, 1988 and Wilkie B N, Shewen P E: "Defining the role that Pasteurella haemolytica plays in shipping fever." Vet Med 83:1053-1058, 1988). In clinically normal dairy calves, P. multocida may predominate in the URT flora, in which various serotypes of P. haemolytica can also be found. In contrast, P. haemolytica Type A-1 is barely detectable in the URT of feedlot and dairy calves (Frank G H: supra (1988) and Wilkie B N, Shewen P E: supra (1988)).
Exposure of calves to stress factors such as viral infection, marketing, shipping, processing at feedlots, and abrupt changes in climate leads to an explosive growth and colonization by P. haemolytica Type A-1 in all areas of the URT (Frank G H: supra (1988) and Wilkie B N, Shewen P E: supra (1988)). No other serotype of P. haemolytica is known to exhibit this type of increase. In shipping fever pneumonia, colonization of the URT with P. haemolytica Type A-1 is an important prerequisite to the development of the clinical disease and the ensuing fibrinous necrotizing lobar pleuropneumonia. Id.
In spite of its potential importance in the pathogenesis of the pneumonia, the mechanism of colonization that facilitates the explosive proliferation of P. haemolytica Type A-1 in the URT is poorly understood. It is nevertheless apparent that these organisms enter the lung via aspiration of droplet nuclei, colonized desquamated epithelial cells, or pharyngeal secretions. At the University of Minnesota (Whiteley L O, et al: "Pasteurella haemolytica and bovine respiratory disease: Current thoughts on its pathogenesis." Vet Int Med 6:1-12, 1992), large numbers of rapidly growing bacteria entering the alveolar spaces were found to interact with alveolar macrophages. The endotoxin released from the bacteria crosses the alveolar wall and activates the pulmonary intravascular macrophages, endothelium, neutrophils, platelets, complement, and Hageman factor leading to complex interactions of cells and inflammatory mediators. Progression of this inflammatory response with neutrophil influx is responsible for the acute lung injury that is associated with the disease. Leukotoxin, one of the major virulence factors of P. haemolytica, may enable the bacteria to survive by destroying phagocytic cells and impairing lung clearance mechanisms. Id.
Prevention of pneumonic pasteurellosis has been attempted in the past by the use of killed bacterins of P. haemolytica. However, it has been demonstrated that vaccination with bacterins may enhance the development of fibrinous pneumonia after challenge exposure. (Sanford, S. E., "Some Respiratory and Enteric Diseases of Cattle; An Update" Mod Vet Prac, 65(4): 265-268 (1984)). Immunization with live vaccines have been generally unsuccessful because of the low antigenicity of P. haemolytica and rapid inactivation by the healthy animal. (Henry, C. W., "Shipping fever pneumonia: a new look at an old enemy" Veterinary Medicine, 1200-1206 September (1984)).
More recently, attempts to develop a Pasteurella haemolytica vaccine have focused on the P. haemolytica leukotoxin. In a study to determine the interaction of P. haemolytica with bovine neutrophils, results demonstrated that optimal cytotoxin production occurred during the logarithmic phase of bacterial growth for P. haemolytica that was grown in a standard tissue culture medium (Baluyut, C. S., et al. "Interaction of Pasteurella haemolytica with Bovine Neutrophils: Identification and Partial Characterization of a Cytotoxin", Am J Vet Res, Vol. 42, No. 11, pages 1920-1926 (1982)). The authors concluded that ". . . s!ince this toxin affected the phagocytic cells, it was considered to be a virulence factor." Id. at page 1925.
U.S. Pat. No. 4,957,739 teaches a vaccine containing a purified P. haemolytica antigen, such as a leukotoxin component, where the antigen is purified from a cell-free supernatant or obtained by recombinant DNA technology. WO 91/15237 discloses a vaccine composition containing at least one immunogenic polypeptide from the group of P. haemolytica fimbrial protein, plasmin receptor protein, a 50K outer membrane protein and leukotoxin. U.S. Pat. No. 5,055,400 discloses DNA encoding P. haemolytica A-1 leukotoxin which is used to produce recombinant protein for the preparation of vaccines. U.S. Pat. No. 5,055,400 refers to the protective capability of cytotoxic supernate from P. haemolytica and cites U.S. Ser. No. 821,197 filed 27 Jan. 1986 now U.S. Pat. No. 5,165,924 as an example of such a vaccine.
Vaccination of calves with bacteria-free cytotoxic culture supernatant from P. haemolytica Type A-1 induced resistance to experimental challenge. (Shewen, P. E., et al. "Immunity to Pasteurella haemolytica Serotype 1." In Proceedings of the North American Symposium on Bovine Respiratory Disease (R. W. Loan, ed.). Texas A & M University Press, College Station, Tex. pp 480-481 (1984)). A cell-free vaccine containing leukotoxin and serotype specific surface antigens (PRESPONSE.RTM. Pasteurella haemolytica toxoid: American Cyanamid Co., Wayne, N.J.) was shown to be efficacious in preventing pneumonia in calves vaccinated twice followed by intratracheal challenge with live P. haemolytica. (Bechtol, D. T., et al., "Field Trial of a Pasteurella haemolytica Toxoid Administered at Spring Branding and in the Feedlot" Agri-Practice, Vol. 12, No. 2, pp.6-14 (March/April 1991)).
There remains a need in the art for improved Pasteurella haemolytica vaccines, such as a vaccine that confers active immunity in a single dose thereby eliminating the requirement of costly repeat administration and a vaccine that offers the convenience of being administered subcutaneously or intramuscularly.