658 ACTIVE IMMUNIZATION 



For obvious reasons this method is being extensively tried out on the 

 lower animals, especially the chimpanzee, before it is applied to human 

 medicine. 



2. By inoculation with a modified virus or with microorganisms at- 

 tenuated or modified according to various methods. 



(a) By passing the virus through a lower animal, as the passage of 

 smallpox through the heifer when the virus is incapable of producing 

 smallpox, although vaccinia confers a specific immunity against small- 

 pox. A vaccine for swine erysipelas is prepared in the same manner 

 (Pasteur) by passing the bacillus through the rabbit several times, 

 which increases its virulence for the rabbit but decreases it for swine. 



(6) By exposing suspensions of microorganisms to heat. They are 

 usually grown on a suitable solid medium, suspended in salt solution, 

 and exposed to a temperature at or just above their thermal death-point 

 for just sufficient time to kill or attenuate them in so far that they can- 

 not multiply. The same result may be secured by longer exposure to a 

 lower temperature. To secure a potent vaccine the principle of minimum 

 exposure at the minimum temperature should be observed, the question of 

 viability being controlled by culturing the vaccine. Most bacterial 

 vaccines are prepared in this manner. Of course, the fact that a vaccine 

 is sterile could be confirmed by exposing it to a very high temperature, 

 but in this case the product may be of no more value than so much salt 

 solution. Usually an exposure of 53 to 60 C. for from one-half to one 

 hour is sufficient, and only exceptionally are these limits exceeded. 



(c) By exposing the microorganism to air and light. The first 

 bacterial vaccine (chicken cholera) was accidentally prepared by Pasteur 

 in this manner. 



(d) By desiccating or drying the virus. This is the method of 

 vaccination in rabies, as the virus contained in the spinal cord of rabbits 

 is dried for varying lengths of time, emulsified, and injected. The longer 

 the period of drying, the greater the attenuation, and in this manner the 

 strength of the vaccine and the progress of immunization are under 

 control. 



(e) By exposing the microorganism to a high temperature for vary- 

 ing lengths of time. Anthrax vaccine, for the immunization of lower 

 animals, is prepared in several strengths by exposing suspensions of the 

 bacilli to 42 C. for varying periods of time. 



(/) By exposing the microorganisms or their products to certain 

 ch&nical germicides, as in the preparation of anthrax vaccine by Roux, 



