34 INFECTION 



more of the natural defenses it can anchor and remove from the 

 field. 



Their virulence can be lessened by cultivation at a higher 

 temperature than the body, 42.5-47C.; by drying; the exposure 

 to light; the action of chemicals; compressed oxygen; and by 

 passing the organism through the bodies of non-susceptible 

 animals. The attenuation or weakening of the pathogenic powers 

 of bacteria is useful for the production of various vaccines which 

 are valuable in preventive medicine. 



By growing the anthrax bacillus atahigh temperature, 42.5C., 

 it becomes so avirulent that it is incapable of destroying sheep or 

 rabbits. It is then used as a vaccine to prevent infection with 

 virulent bacilli. By exposing the spinal cords of animals dead 

 from hydrophobia to the action of drying for various periods, 

 Pasteur was able to attenuate the virus, so that it would not 

 produce hydrophobia, but on the contrary, it, by repeated 

 inoculation, caused immunity. The inoculation of monkeys 

 (which are non-susceptible) with hydrophobia virus attenuates 

 it. The growth of the small-pox organism in the cow, causing 

 cow-pox, so reduces the virulence of the germ that it is incapable 

 of producing small-pox in man, but only vaccinia; infection with 

 this gives immunity against small-pox. The flesh of animals 

 that have died from quarter-evil is so changed by heat and desic - 

 cation that if it is injected into susceptible animals, they do not 

 succumb but are vaccinated against infection with the virulent 

 organism. 



When we speak of attenuation of virulence we usually refer to 

 the effects on experimental animals and specify what attenuation 

 is meant when they are to be used as vaccine. A very interesting 

 pathogenic, yet attenuated, form of streptococcus is to be met 

 in subacute endocarditis. These organisms produce serious or 

 even fatal valvulitis, and yet have no effect upon lower animals. 

 They are extremely hard to remove from the body. They 

 have accustomed themselves to residence in the body, have estab- 



