4 
The Colorado Experiment Station. 
generally, but all these things may be looked upon as accessory 
causes, inasmuch as they weaken the constitution and natural re¬ 
sistance to the disease. 
Hog cholera made its first appearance in the United States in 
the year 1833 and investigations have been conducted almost con¬ 
stantly since that time to determine the cause. Salmon and Smith, 
in 1885, isolated and described an organism which they called 
Bacillus cholerae suis and declared it to be the causative factor. In 
1903 de Schweinitz and Dorset demonstrated conclusively that the 
infecting agent existed in the body fluids and would pass through 
the finest porcelain filters. Dorset, Bolton and McBryde later dem¬ 
onstrated that the Bacillus cholerc e suis was not the specific germ 
of hog cholera, but that it was caused from an ultra microscopic 
organism that was non-filterable. 
Recently Walter E. King, of the Research Laboratories of 
Parke, Davis & Co., has reported the finding, and seemingly has 
demonstrated, a protozoan which is uniformly present in the blood 
of hogs affected with cholera. This organism has been named Spi¬ 
ro cliaet a suis and while the author assumes the probable relation¬ 
ship of this organism to hog cholera, as the causative factor, yet 
at this writing he admits that its significance in this connection is 
not definitely determined. 
susceptibility. 
The period of incubation is the time between exposure to the 
disease and the appearance of the symptoms and it varies widely 
in different diseases. In hog cholera it varies from four days to 
two weeks, depending upon the age of the hogs, their thrift, nat¬ 
ural resistance to the disease and the virulence of the infection. 
In most outbreaks of cholera there are a few individuals that 
have a high degree of resistance and do not contract the disease, 
and a few may have it and recover. All breeds of hogs, including 
the mule-foot, are about equally susceptible and there is no appre¬ 
ciable difference between the breeds as to the number that will ul¬ 
timately recover. 
SYMPTOMS. 
There are at this time no other serious diseases among hogs 
in Colorado, and any disease making its appearance among hogs 
should be looked upon with suspicion. The symptoms manifested 
are not uniform throughout, but the first indication is usually a 
refusal to come to the trough for food and to lie buried deep in the 
litter. There is fever, and the animal sways from side to side as 
