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l8 DIMINUTION IN VIRULENCE 



gated") virulence are frequently employed as vaccines in the pro- 

 duction of artificial immunity. The following are some of the 

 chief methods employed : 



1. By prolonged culture in artificial media, as described above. 

 This method was introduced by Pasteur in the case of fowl 

 cholera. The loss of virulence is a progressive one, and cultures 

 ten months old are devoid of virulence. 



2. By cultivating the organism at a temperature above the 

 optimum for saprophytic growth. This was also introduced by 

 Pasteur, and is used in preparing the vaccines to anthrax. The 

 organism is cultivated at a temperature of 42*5 C., and all 

 virulence is destroyed in about six weeks, though the cultures 

 retain their power of growth unaltered. The first vaccine is prepared 

 by allowing growth to continue at this temperature for twenty- 

 four days. In appearance the bacilli are unaltered, but they have 

 lost the power of killing rabbits and guinea-pigs, though they 

 are still fatal to mice. The second vaccine is cultivated at a high 

 temperature for a fortnight only ; it is virulent to mice and 

 guinea-pigs, but not to rabbits. 



The process may also be carried out by a short exposure to a 

 higher temperature. Chauveau's vaccine consists of blood con- 

 taining anthrax bacilli, heated to a temperature of 50 to 55 C. for 

 ten or fifteen minutes. The bacilli remain alive, but are mitigated 

 in virulence. 



3. In some cases the addition of various chemical antiseptics in 

 minute amounts to the culture medium has a similar effect. This 

 is the case with anthrax also. Addition of chemical substances is 

 also made with the idea of destroying toxins, but this is a different 

 phenomenon. 



4. The virulence may be destroyed by drying. This method 

 was introduced by Pasteur for the preparation of a vaccine against 

 rabies. We have already described the method by which he 

 obtained the fixed virus and its action on rabbits. He found that 

 by suspending the spinal cords of rabbits dead of this fixed virus 

 over caustic potash at a temperature of 23 C. the virulence was 

 entirely removed in fifteen days. Drying for a shorter time 

 diminished the virulence, but did not remove it entirely. 1 



1 The more modern idea is that the process of drying kills off a large 

 number of the pathogenic organisms, and that the use of the dried cord is 

 merely another method of giving very minute doses of virus of normal 

 strength. 



