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.
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
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.