I 
INFECTIOUS SWINE DISEASES IN THE UNITED STATES. 
827 
plague bacillus. The presence of this organism in the trachea 
of healthy pigs has been suggested as the cause of sporadic cases 
of swine plague and it may explain the frequent association of 
swine plague with hog cholera. What the conditions are by 
which these bacteria are enabled to produce disease in their 
host have not been clearly pointed out. The pathogenic organ¬ 
ism associated with the lesions in certain forms of broncho¬ 
pneumonia in cattle differs very slightly from this. In human 
pathology we find a striking resemblance in Micrococcus lajiceo- 
latus * to the swine-plague bacillus. While there are differ¬ 
ences between the behavior of this organism and the swine- 
plague bacillus, its manifold and varied pathogenic possibilities 
and its distribution in normal human saliva are worthy of notice 
in this connection. 
If we take the rabbit as the animal on which to test the 
pathogenesis of the bacteria belonging to the swine-plague group 
we find that those from different sources are very similar. In 
nature, the bacilli of swine plague, rabbit septicaemia, fowl 
cholera, and those located in the normal upper air passages of 
the various species of animals mentioned exist possessed of 
marked variation in virulence, that is, those which will kill a 
rabbit when inoculated subcutaneously with pure culture in 
from 16 to 24 hours to those which require from 3 to 10 days 
to destroy life. With the variations in the length of time we 
have corresponding differences in the lesions. Thus the viru¬ 
lent forms produce septicaemia while the attenuated varieties 
excite a severe puruleut infiltration about the place of inocula¬ 
tion and exudates on one or more of the serous membranes. 
Conversely, it has been shown that rabbits possessed of a cer¬ 
tain amount of natural or artificially produced resistance will, 
when inoculated with a virulent culture, die after the same 
period of time and with lesions similar to those produced by 
the attenuated virus in the susceptible rabbit, or, if the resist¬ 
ance approaches in degree to immunity the lesions may be re¬ 
stricted to single or multiple abscesses which develop slowly in 
* Welch Bulletin Johns Hopkins Hospital, HI., 1892, p, 125. 
