DISEASES OF FARM ANIMALS 19! 



about to die from cholera. The general theory 

 upon which this double vaccine is used is that of 

 giving the animal an infectious disease and at the 

 same time a treatment which enables the animal to 

 resist the infection. When the hog is through with 

 it he is in exactly the same condition as though he 

 had gone through a natural exposure and recovered. 



General Method. We start this work with cer- 

 tain hogs that are immune usually because they 

 have passed through an outbreak. It has been 

 shown that when such immune hogs are treated 

 with large injections of virulent blood under the 

 skin or into a vein, that they do not usually become 

 sick, but their own blood develops a peculiar prop- 

 erty that gives protection to other hogs that are 

 naturally susceptible. 



When the blood or rather blood serum from this 

 specially treated immune hog is injected into the 

 bodies of healthy susceptible hogs, the latter be- 

 comes likewise immune, but the immunity so gained 

 lasts only a short time, possibly four to six weeks, 

 and is then gradually lost. If we give a small in- 

 jection of virulent blood at the same time, or soon 

 after the immunizing serum is given, then the 

 treated hog becomes immune for a long period, per- 

 haps for life. 



The Serum Hog. The specially immunized hog 

 which produces this immunizing serum is known as 

 a hyperimmune, and to save words will be hereafter 

 mentioned as such. The simply immune hog may 

 be prepared for producing serum in either one of 

 three ways, (i) By three rapidly increasing doses 

 of virulent blood serum injected under the skin at 

 intervals of seven to ten days; (2) by one enor- 

 mously large injection of virulent serum under the 



