The protective effect of vaccination against experimental pneumonia in cattle with Haemophilus somnus outer membrane antigens and interference by lipopolysaccharide.

A semi-purified outer membrane anionic antigen (AA) fraction was isolated from Haemophilus somnus by a modified procedure of anion exchange chromatography to yield a protein fraction free of lipopolysaccharides (LPS). The AA fraction (1 mg) was administered with or without the homologous lipopolysaccharide (10 micrograms/kg body weight) as vaccines to groups of cattle twice, three weeks apart. A control group which did not receive any antigen was included in the trial. Six weeks after the first vaccination, the animals were challenged intratracheally with a virulent pneumonic strain of H. somnus (70986) and observed for clinical signs of respiratory disease. The cattle were euthanized six days later and the lungs were evaluated for the severity of lesions macroscopically as well as histopathologically. Vaccination with AA alone provided the best protection against pneumonia as indicated by significantly lower clinical scores, less extensive gross lung lesions and mild histopathological lesions with immune cell infiltration. However, when AA was combined with LPS in the vaccination, this protective effect was negated and the animals showed more detrimental histopathological lesions than the controls.