BIOLOGICAL THERAPEUTICS 529 



■ Vaccination is especially indicated when we desire to confer an active 

 and therefore lasting immunity to the animals. Of course^ there are 

 types of infectious diseases in which it is not the organisms which produce 

 the disease, but their toxins. The ultimate result, however, is the same, 

 since in such cases instead of the germicidal action of the protective sub- 

 stances created in the body, antitoxic substances are produced which take 

 their place. 



Taking all of these factors into consideration, it appears natural 

 that the greatest and the surest achievement may be expected from immu- 

 nization in those diseases in which the specific cause of the disease has. 

 been definitely established, and as a matter of fact, the attained suc- 

 cesses have proved the most reliable where these conditions have been ful- 

 filled. 



Preparations Used in the Production of Immunity and as Diagnostic 



Agents : 



In the production of immunity three principal methods are used. 



A vaccine is either a living, attenuated or weakened virus of the dis- 

 ease. The reduction of the virulence is accomplished by subjecting the 

 virus to a degree of heat or desiccation or other unfavorable influences, 

 not sufficient to destroy it, but only to weaken it. Good examples 

 of vaccines are anthrax vaccine and blackleg vaccine. In some 

 instances the fully virulent organism is used for immunizing purposes, 

 as, for instance, in abortion ; or, again, an unattenuated virus may be used 

 when passed through certain species of animals which increases the viru- 

 lence for the particular species through which it is passed, but reduces its 

 virulence for other species of animals, as, for instance, suspensions of a 

 brain emulsion of fixed rabies virus, passed through rabbits is used for the 

 immunization against rabies in other species of animals. 



Vaccines produce a mild attack of the disease, thereby rendering the 

 animal resistant against subsequent exposure to natural infections. 



Bacterins (bacterial vaccines) are standardized suspensions in physio- 

 logical salt solution (or in oil) of the killed pathogenic bacteria. Vac- 

 cination with bacterins against certain diseases such as hemorrhagic 

 septicemia, white scours and other infections is now a procedure of estab- 

 lished success. Bacterins are standardized to a definite number of dead 

 organisms per mil either by actual counting of stained organisms in a 

 blood counting apparatus or by mixing a definite amount of blood and the 

 suspension of the bacterin to be counted and then establishing the rela- 

 tive proportion of organisms to the number of red blood cells in the 

 preparation, from which by simple calculation it is possible to establish 

 the number of organisms contained per mil. 



Bacterins contain the dead organisms and their products through 

 which immunity is established by a stimulation of the body to the forma- 

 tion of protective substances against the subsequent attacks of the viru- 

 lent germ. 



Sera contain the protective substances and when injected into the 

 body either exert a bactericidal action against the virulent virus or act as 

 antitoxins possessing a neutralizing power over the poisonous products 

 (toxins) of the disease producing organisms. 



For the preparati&tuf Hani'pnj»/is«MSiSUif®lly horses are used, prin- 



