HOG CHOLERA 335 



have been formed under the stimulus of the disease-pro- 

 ducing organism. The amount of protective bodies that 

 are thus produced as a result of an attack of the disease 

 is not sufficiently great so thai the blood can be drawn and 

 introduced into the body of another animal for the pur- 

 pose of imparting immunity. If, however, such an immune 

 animal is injected with large quantities of the blood of a 

 hoir that is ill with the <lis<-a>e. the stimulus imparted by 

 the introduction of the virus will cause the animal to form 

 additional protective bodies, so that it will be practicable 

 to draw the blood and use it in protecting other animals. 

 The hogs thus treated are said to be hyper-immunized. 

 The protective serum is secured by bleeding the immunized 

 hog. This is done by cutting off the tip of the tail. 

 About five or six cubic centimeters of blood to one pound 

 of body weight is drawn. This process is repeated three 

 times, at weekly intervals. The animal is then given an- 

 other injection of the virulent blood, is then bled twice 

 i'l-om the tail, and after the usual interval is bled from 

 the throat. The blood is beaten with a wire as soon as 

 drawn from the animal to remove the fibrin and prevent 

 clotting. One half of one per cent, of carbolic acid is added 

 as a preservative. 



Before the serum is used in the field it is necessary to 

 determine its protective power. This is done by injecting 

 varyhm- amounts into susceptible pigs that are inoculated 

 at the same time with some virulent blood. In this manner 

 it can be determined how much must be used in actual 

 work to protect an animal. It will be seen that the prepa- 

 ration of the serum is expensive because of the large num- 

 ber of hogs that must be used and the labor involved. 

 Many of the States have established laboratories for the 

 preparation of the serum. 



By the use of the serum alone a passive immunity is 



