From the three cases of pasteurellosis in sheep studied, five
different strains of pasteurellae were obtained, 182 from ram 1,
181 from ram 2, and 247, 247a, and 247c from ram 3.
Of these strains 182 and 247 were highly pathogenic for both
sheep and guinea-pigs, and almost non-pathogenic for rabbits and
pigeons. Both showed the same biochemical reactions and both
produced similar pathological changes in experimental animals
inoculated with live cultures. They resembled each other also in
morphology, staining and cultural characteristics. Pasteurellae 182
and 247 can therefore be regarded either as identical or so closely
related that they cannot be differentiated by the methods employed.
Both these organisms have a predilection for pulmonary tissue and serous membranes and both produced lesions in experimental animals that could not be differentiated from those found in natural cases studied.
These two organisms are considered to have been the cause of the
mortality among the Ryeland sheep at the experimental farm of the
University of Pretoria. An identical disease in experimental sheep
was produced by the injection of organ emulsions and cultures made
from the original cases from the University farm. Berkeveld filtrates
of organ emulsions from natural cases did not produce the disease.
So far no success has yet been attained with immunisation tests
in laboratory animals and no properly controlled immunisation
experiments have been carried out with sheep.
The pathogenesis of the disease under natural conditions is still
Pasteurellae 181 and 247a cannot be distinguished from each
other by the tests employed; both are very slightly pathogenic for
guinea-pigs and both have the same biochemical reactions, and they
agree in morphology, cultural and staining characteristics. Both
have originated from small colonies picked from primary cultures of
Pasteurella 247c does not resemble either of the two groups of
organisms mentioned above. It is entirely non-pathogenic for
These results indicate:
(1) that the small colonies picked from the primary growths
on media seeded with material from affected lungs yielded
cultures which were either non-pathogenic (247c) or only
very slightly pathogenic (247a, 181);
(2) that the large colonies obtained from similar growths gave
rise to highly pathogenic cultures (182 and 247);
(3) that when several colonies were picked from the same
primary growth, highly pathogenic, slightly pathogenic
and non-pathogenic cultures may be obtained e.g. cultures
247, 247a, and 247c);
(4) that if only one colony is picked from the primary growth
either a highly pathogenic culture (182) or one which
is barely pathogenic (181) may result.
It is possible that the non-pathogenic and slightly pathogenic
pasteurellae occur as saprophytes in the respiratory passages of sheep
in certain areas and that they invade the lungs only when the way
has been paved for them by the entrance of pathogenic pasteurellae
of the type 182 and 247. These latter enter the tissues and set up
disease under conditions which have not yet been determined.
In making a bacteriological study of a case of pasteurellosis,
therefore, it is recommended that several colonies of different sizes
be picked from the primary growth, and that the pathogenicity of
each one be studied separately. Only in this way may the presence
of pathogenic pasteurellae be determined.