HOG CHOLERA. 
29 
a want of exact experimental data fixing this period. The ani¬ 
mals are first inoculated with a weak vaccine, and ten days after¬ 
wards with a stronger one, when they are considered bo be free 
from any danger to natural infection. But the all important 
question, when this period absolutely begins, has not been satis¬ 
factorily determined. 
With regard to the swine plague, my idea is, that as this dis¬ 
ease is not so acutely fatal, that this period will be found to be 
longer, say thirty days, and that it cannot be reduced without 
* using a virus so strong as to be dangerous. 
Another difficulty which we had to deal with this year was 
the uncertainty of the natural virus in its virulence. As is well 
known, the disease has been unusually mild in Nebraska during 
the summer and fall of 1886, and even in different localities it 
has varied much in its virulence. In some cases it took very 
much greater quantities of the same kind of a culture to produce 
death than others. Another fact of great practical importance in 
the endeavor to obtain a reliable vaccine is the rapidity with 
which the germ of swine plague loses in virulence under artifi¬ 
cially cultivated conditions, as well as the irregularity in this 
direction, between the germs of different cultures in different 
tubes, which were, however, all inoculated from the same original 
material and at the same time. 
These difficulties do not exist with anthrax to any such degree, 
though the cultivations do lose in virulence after the same original 
culture has been passed through many successive artificial genera¬ 
tions, as has been shown by Germany’s great pathogenic bacteri¬ 
ologist, Robert Koch. 
The really unpleasant and disturbing characteristics of the 
germ of swine plague surmount the production of a reliable vac¬ 
cine with difficulties which can be overcome, however. Pasteur 
claims for his anthrax vaccine, as well as that for rouget and even 
rabies, that when a cultivation of the germs has been subjected 
to a certain known line of treatment, and a certain degree of 
attenuation attained, that this degree of attenuation remains con¬ 
stant; that is, that a vaccine of the same and a constantly reliable 
degree of attenuation can always be manufactured from inoculat- 
