Evaluation of an Experimental Vaccine for Control of Bovine Penumonia Induced by Pasteurella Haemolytica

Abstract

Among the members of the HAP group (Haemophilus, Actinobacillus, and Pasteurella) are bacteria of considerable importance in veterinary medicine. These, over the years, have caused considerable economic losses in cattle and swine herds. The pathogenic peculiarities in these bacteria require investigators to look beyond killed whole cell vaccines for the induction of the immune response. Compounding all of this is the fact that in many instances a strong immune response is not necessarily synonymous with protection.

P. haemolytica Type A-1, a conditional pathogen and normal inhabitant of the nasal cavity of clinically normal cattle, is the most frequent isolate from acute pneumonic pasteurellosis in cattle. Lesions formed by P. haemolytica are characterized by necrosis, vascular damage, fibrin exudation, numerous viable bacteria and variable leukocyte infiltrations. Virulence factors produced by P. haemolytica include; a) leukotoxin (LKT) , a potent exotoxin which affects ruminant leukocyte populations; b) capsule, which inhibits complement mediated serum killing; c) lipopolysaccharide, which alters neutrophil function and stimulates leukocyte adherence; and d) fimbriae, which enhances adherence of the bacteria to the upper respiratory mucosa. Increasing knowledge of the growth conditions conducive to expression of virulence factors should help in the development of efficacious vaccines.

A vaccine has been developed which contains many of the virulence factors described above. This vaccine has been tested in a single dose regimen in many vaccination/artificial challenge experiments and has proven to be efficacious and safe. Experiments detailed in this manuscript are representative of results obtained for similar tests during the development of this vaccine.