422 HOG CHOLERA 
also pointed out that if the immunizing serum was used in conjunction 
with the virus, or the simultaneous method, the pigs became immune 
for a much longer time. It is this serum, known as the Dorset-Niles 
serum, together with the use of the virus and serum or the simul- 
taneous method, that are now being employed as prophylactics against 
hog cholera. 
Geographical distribution. Hog cholera is widely disseminated 
throughout the central part of the United States. It exists, however, 
to a certain extent in practically every state in the Union. It is 
known in Great Britain, and it prevails to a greater or less extent on 
the continent of Europe. 
Etiology. The cause of hog cholera is a filterable virus. It exists 
in the blood, urine, lymph and all organs of affected swine. So far as 
known, the virus of this disease exists only in the tissues and excre- 
tions of infected hogs. No other species of animals are known to be 
affected with it, and its propagation outside of the body is unknown. 
In different outbreaks there seems to be a variation in the degree of 
virulence. The virus is quite resistant to heat but rather quickly 
attenuated or destroyed by decomposition processes in tissues. A 
5% solution of chloride of lime or a solution of from 3 to 6% cresol 
soap seem to be the most effective disinfectants but they require 
eonsiderable time when added to virulent blood. The virus seems to 
be destroyed rather promptly when exposed to light and drying, but 
in damp places especially “wallows’’ it may persist for months. 
There is much difference of opinion regarding the significance of 
B. sutpestifer in hog cholera. It is frequently found in the tissues of 
swine suffering from cholera but it is generally believed to be a second- 
ary invader. Uhlenhuth found it ina large percentage of healthy hogs. 
Dorset, Bolton and McBryde state that ‘“‘it must be admitted that 
a disease in hogs may exist which is due to B. suipestifer, and which 
has no connection with the filterable virus found by us in the out- 
breaks we have studied.” They believe, however, that such a disease 
“would be possessed of a low degree of contagiousness.” 
Glasser found this bacillus to be a normal inhabitant of the intes- 
tines of swine. 
The period of incubation varies from 7 to 14 days. Berry states that 
it varies from one to three weeks and perhaps longer. 
Symptoms. In uncomplicated cases of hog cholera the first 
symptom is a rise of temperature. This occurs before the animal 
