178 MANUAL OF BACTERIOLOGY. 



but it has been shown that small-pox protects against cow- 

 pox. 



Immunity Produced by Inoculation with Bacteria of 

 Diminished Virulence. Pasteur conceived the idea of 

 attenuating the virulence of the bacilli of fowl-cholera by 

 prolonged exposure to the air. He made use of the atten- 

 uated virus as a vaccine against the disease. 



A nearly similar principle was shortly afterward applied 

 by him to the preparation of a vaccine against anthrax. 

 When anthrax bacilli were cultivated at a temperature of 

 43 C., Pasteur obtained bacilli of very slight virulence. 

 Such bacilli did not produce death when inoculated into 

 animals that were ordinarily susceptible. Yet animals that 

 were vaccinated with this virus were able afterward to 

 resist inoculation with fully virulent anthrax bacilli. (See 

 Bacillus anthracis, Part IV.) 



In the case of erysipelas of swine (French rouget; Ger- 

 man, Schweinerothlauf) Pasteur secured bacilli of dimin- 

 ished virulence by injecting virulent bacilli into relatively 

 insusceptible animals. The animal used was the rabbit. 

 The bacilli were passed through several rabbits in succes- 

 sion. Cultures taken from the last of the series produced 

 a milder form of the disease and an amount of immunity, 

 the value of which is in dispute. 



In still another disease, black leg of cattle or symptomatic 

 anthrax ( French, charbon sytnptomatique ; German, Rausch- 

 brand), an attenuated virus is secured by the use of heat. 

 The pulp from the infected muscle of a diseased animal, 

 containing the bacilli, is squeezed from it and heated to a 

 temperature of 95 to 99 C. for six hours. The dried 

 material mixed with water constitutes the vaccine. The 

 Department of Agriculture of the United States now fur- 

 nishes this vaccine free to farmers. The results of this 

 method are said to be very gratifying. 1 



1 See Recent Annual Reports, Bureau of Animal Industry, U. S. 

 Department of Agriculture. 



