DISEASES OF CATTLE 307, 



effective methods for freeing an infected pasture from blackleg is to 

 allow the grass to grow up high, and, when sufficiently dry, to burn 

 it off. One burning off, however, is not sufficient to redeem an in- 

 fected pasture, but the process should be repeated several years in 

 succession. This method, however, is in many instances imprac- 

 ticable, as few cattle owners can afford to practice it, and the onlyj 

 means left for the protection of the animals is vaccination. 



Immunization by Vaccination. Three French Veterinarians, 

 Arloing, Cornevin, and Thomas, were the first to discover that cattle 

 may be protected against blackleg by inoculation with virulent ma- 

 terial obtained from animals which have died of this disease. Later 

 they devised a method of inoculation with the attenuated or weak- 

 ened blackleg spores which produced immunity from natural or ar- 

 tificial inoculation of virulent blackleg germs. Their method has 

 undergone various modifications both in regard to the manufacture 

 of the vaccine and in the mode of its application. Kitt, a German 

 scientist, modified the method so that but one inoculation of the 

 vaccine was required instead of two, as was the case with that made 

 by the French investigators. The vaccine prepared and distributed 

 by the Bureau of Animal Industry combines the principle of Arlo- 

 ing, Cornevin, and Thomas and the modification of Kitt. 



By vaccination we understand the injection into the system of 

 a minute amount of attenuated that is, artificially weakened-^ 

 blackleg virus. This virus is obtained from animals which have 

 died from blackleg, by securing the affected muscles, cutting them 

 into strips, and drying them in the air. When they are perfectly 

 dry they are pulverized and mixed with water to form a paste, 

 smeared in a thin layer on flat dishes, placed in an oven, and heated 

 for six hours at a temperature close to that of boiling water. The 

 paste is then transformed into a hard crust, which is pulverized and 

 sifted and distributed in packages containing either 10 or 25 doses. 

 This constitutes the vaccine, the strength of which is thoroughly 

 tested on experiment animals before it is distributed among the 

 cattle owners. This vaccine, which is in the form of a brownish 

 dry powder, is mixed with definite quantities of sterile water, filtered, 

 and the filtrate injected by means of a hypodermic syringe under 

 the skin in front of the shoulder of the animal to be vaccinated. The 

 inoculation is usually followed by insignificant symptoms. In a few 

 cases there is a slight rise of temperature, and by close observation a 

 minute swelling may be noted at the point of inoculation. The im- 

 munity conferred in this way may last for eighteen months, but 

 animals vaccinated before they are 6 months old and those in badly 

 infected districts should be revaccinated before the following black- 

 leg season. 



The effect of the vaccine prepared by this Bureau in preventing 

 outbreaks of the disease and in immediately abating outbreaks al- 

 ready in progress has been highly satisfactory, and it is not to be 

 doubted that thousands of young cattle have been saved to the stock 

 owners during the six and a half years in which the vaccine has been 

 distributed. More than 7,700,000 doses have been sent out during 



