726 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



or lesions, while at the same time sufficient specific, infectious qualities 

 must be present to produce mild reactions. The successful vaccine 

 is attenuated or modified to a degree which insures both safety and 

 activity. The following are the more important methods used to 

 modify viruses: 



Attenuation by growth at a temperature above the optimum. This is 

 illustrated by Pasteur's method of preparing anthrax vaccine. 



Attenuation by passage of the virus through some species other than 

 the animal for which the virus is specific. Smallpox vaccine may be 

 regarded as a virus modified by passage through a heifer or other 

 animal. 



Attenuation of the virus by drying at constant temperature. The 

 Pasteur method of prophylactic treatment for rabies is based upon this 

 method. 



Attenuation by chemicals. The growth of certain pathogenic 

 bacteria in the presence of weak antiseptics reduces their disease- 

 producing activities. 



Other methods of modifying viruses for the purpose of active im- 

 munization: 



The simultaneous method or hypodermic application of the virus 

 together with protective serum, as in hog cholera vaccination. 



The association or combination of the specific pathogenic bacteria 

 with those of other species as illustrated by the apparent restraining 

 action of yeasts upon pyogenic bacteria and the antagonism which 

 Ps. pyocyanea exerts toward Bact. anthracis. 



The filtration of liquid cultures of pathogenic organisms, such as 

 Bact. diphtheria or B. tetani, and the consequent separation of the 

 organisms from the toxin. The toxin is used to immunize animals in 

 the production of antitoxin. 



The destruction of young living cultures of specific bacteria by 

 moist heat at a temperature slightly above their thermal death-point. 

 Heated cultures of B. typhosus and Bact. pestis are used as prophylactics 

 against typhoid fever and bubonic plague. 



There are many vaccines in practical and experimental use at the 

 present time. Among those which are of recognized value as shown by 

 extensive practical use and reliable clinical statistics, the following 

 are the most important: smallpox vaccine, blackleg vaccine, rabies 

 vaccine,^ typhoid vaccine and perhaps Pasteur's anthrax vaccine. 



