248 AGRICULTURAL BACTERIOLOGY 



cholera is yet unknown, although it can be transferred 

 by the use of blood from a sick hog. The introduction of 

 a large quantity of the virus into the body of the immune 

 hog causes the formation of an increased amount of the 

 protective bodies. The immunizing process is thus repeated 

 until the blood contains such a quantity of protective sub- 

 stances that, when transferred in practicable amounts, it 

 imparts a considerable degree of immunity. 



The disease virus is obtained from a sick hog by bleeding 

 from the throat. This virulent blood is usually introduced 

 into the blood vessels of the animal to be hyper-immunized, 

 which, when its blood is sufficiently high in the protective 

 bodies, is bled for the anti-serum by cutting off a piece of 

 the tail. The animal can be bled several times in this way, 

 a fresh cut being made each time until this appendage is too 

 short for further use. The final bleeding is then made 

 from the throat. 



In the preparation of diphtheria antitoxin the horse is 

 used to produce the anti-bodies. The animal is not suscep- 

 tible to diphtheria; hence, the organisms themselves can 

 not be employed to stimulate the production of anti-bodies. 

 The horse is, however, susceptible to the toxin of the diph- 

 theria organism. The organism is grown in the laboratory 

 in beef broth, which is filtered through porcelain to remove 

 all the bacteria, and gradually increasing doses of this fil- 

 trate are then injected into the body of the horse. At 

 first only very small doses can be administered without 

 killing the animal ; but after recovery from the first injec- 

 tion, repeated doses of increasing amounts are applied, 

 the effect of which is to produce the protective anti-bodies 

 in the blood of the animal. The blood is then drawn 

 from the jugular vein ; it is allowed to coagulate in order 

 to remove the clot, and the blood serum is used. Not only 

 does this serum protect an individual from acquiring diph- 



