424 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



rabbits consecutively, the virulence is increased until it will 

 kill rabbits regularly in from six to seven days. This is then 

 termed a " fixed " virus. The vaccine is prepared by remov- 

 ing under aseptic precautions, the spinal cord from a rab- 

 bit dead as a result of an injection of fixed virus. This is 

 carefully dried in a desiccator at a constant temperature of 

 23 c. for a period of two weeks. The vaccine is an emul- 

 sion of this cord in physiological salt solution. The dry- 

 ing has so attenuated the virus that it may be injected with 

 impunity into man or animals. Later other injections are 

 made with emulsions from spinal cord dried for a shorter 

 period of time. Inasmuch as the disease has a relatively 

 long incubation period, the time elapsing between the bite 

 of a rabid dog and the normal appearance of symptoms may 

 be used for immunization. This method, first used by Pas- 

 teur, has proved highly successful. 



HOG CHOLERA 



This disease of swine is one of the most important affect- 

 ing the live stock industry. There was much difficulty in 

 the earlier study of the disease due to confusion with other 

 swine diseases, and particularly because from 1885, when 

 Salmon and Smith described the organism Bacterium 

 chblerce suis as the cause of the disease, until 1904 the 

 scientific world labored under a misunderstanding of the 

 cause. In the latter year De Schweinitz and Dorset demon- 

 strated that the typical hog cholera in the United States 

 is due to a filterable virus. This discovery led to the devel- 

 opment of satisfactory methods for immunization against 

 the disease. The organism which causes hog cholera is 

 classed as a filterable virus, inasmuch as it will pass readily 

 through porcelain filters. The organism in the blood serum 

 of hogs will remain virulent for many weeks. It may be 

 destroyed by disinfectants, but is relatively resistant. 



Hog cholera usually manifests itself as an acute disease, 



