Prevention. Imnmnization. 51 



disease lias decreased considerably in the course of the last 

 30 to 50 years, owing to such improvements of the ground. 

 When this is not practicable, and the danger of infection re- 

 mains, the appearance of the disease can be prevented by means 

 of protective vaccination; otherwise, the immediate change of 

 pasture or stable often results in the disappearance of the 

 disease. 



Immunization. Arloing, Cornevin & Thomas have stated 

 that cattle raised in blackleg districts mostly contract the disease 

 at a young age, while animals brought there from disease-free 

 regions are liable to be infected at an older age. This is ex- 

 plained by the supposition that the young animals, during their 

 stay in blackleg districts, become immunized early by repeated 

 absorption of smaller amounts of virus. When these authors 

 produced a mild attack of the disease by means of injection 

 of small amounts of virus, they also found that the inoculated 

 animals later resisted otherwise fatal infections. Based on this 

 experience they have devised a very valuable and j^racticable 

 method of protective vaccination, which has since been modified 

 in many ways by other authors. 



I. Two inoculations with attenuated virus. (Method of 

 Arloing, Cornevin & Thomas.) A vaccine is prepared from 

 diseased, powdered muscles by attenuating tlie virus through 

 heating to a high degree of temperature. For this purpose, 

 the diseased part of the muscle is ground in a mortar with, some 

 water, and dried in a thin layer at 37° C; then the dry mass 

 is mixed with 2 parts of water, and the one half heated at 

 100-104°C., the other at 90-94°C. for 7 hours. The former 

 makes the first weaker vaccine, the latter the second stronger 

 one. 



The high temperature attenuates the virulence of the 

 ^drus, and the longer and stronger it has been heated the more 

 attenuated it becomes. The spores, however, in spite of the 

 influence of the heat, remain capable of germinating, but the 

 toxin having been weakened through heat, the spores are not 

 capable of exerting their pathogenic influence. Being to a cer- 

 tain degree protected against the phagocytes by the coagulated 

 albumin, the spores begin to germinate and to multiply slowly, 

 and thus little by little, the newly produced toxin immunizes 

 the tissues. The immunity reaches the necessary degree in 8 

 to 10 days after the second vaccination. Until then, however, 

 the animals have a lower resistance aganst an artificial or 

 natural infection (negative phase). 



The practical value of the protective vaccination is already 

 definitely established. The first experiments proved that ani- 

 mals, vaccinated at the end of the tail, subsequently withstand 

 the intramuscular inoculation of a very virulent culture without 

 harm, and that the immunity obtained lasts at least 17 or 18 



