Pasteurella haemolytica : a study of pathogenicity and immunity in laboratory animals and sheep

Immunological work has hitherto been severely limited by the high resistance of laboratory animals and sheep to reasonably small doses of Past. haemolytica. This has also led to speculation as to the pathogenic significance of the organism in sheep disease.The present studies showed that true infections could be established in mice by two methods, namely (a) intracerebral inoculation, and (b) intraperitoneal inoculation of organisms suspended in mucin. Either method when used as a means of challenge was capable of demonstrating passive immunity. For this purpose however the second procedure was the more successful and it was also very satisfactory for demonstrating active immunity produced in mice by different vaccination procedures. Subcutaneous vaccination was less effective than immunisation by the intraperitoneal or intravenous routes. Immunity could be passively transferred by injecting serum from vaccinated mice into non- -vaccinated mice. Procedures leading, to non -specific changes in mouse -resistance were investigated.Very large doses failed to produce disease in cheep by intratracheal inoculation but were sometimes fatal when injected intravenously. The implications of using such doses are discussed in the light of experiments on toxicity of the organism. Invasiveness of Past. haemolytica in experimentally and naturally infected sheep was investigated.Each strain of the organism examined was classified as one of two Types, designated A and T. Although closely similar these two Types were distinguishable by certain in vitro characteristics. 133 Cases of enzootic pneumonia in sheep and of septicaemia in very young lambs were associated with Type A infections while septicaemias of older lambs yielded Type T organisms.Three- week -old lambs were highly susceptible to extremely small doses of an A strain which were completely resisted by young adult sheep. Intraperitoneal inoculation resulted in rapid death from peritonitis. Intravenous inoculation frequently produced prolonged infections with many of the lesions seen in field cases, including pneumonia. Pneumonia was also produced by intratracheal inoculation of larger doses.Low titre agglutinins to a T strain were found in the sera of all adult sheep tested but were frequently absent in lambs. By means of the passive mouse test it was found that normal adult sera tended to be more protective than normal lamb sera against challenge with a T strain.Using the active mouse protection test it was shown that the species Past. haemolytica is not immunogenically homogeneous. It also appeared that the majority of T strains examined shared an immunising antigen of minor importance. This antigen was not possessed by A strains.The use in sheep of a formol- saline vaccine prepared from a T strain stimulated the production of antibody which was capable of protecting mice from challenge with the homologous organism