226 PRINCIPLES OF VETERINARY SCIENCE 



In the preparation of these biologic therapeutic agents, it is 

 essential that the organism that causes the disease be used. 

 When prepared with skill and applied to cases where they are 

 certainly indicated, they will yield good results because of their 

 specificity. The three classes of biologies that are used the most 

 are: (1) vaccines, (2) bacterins, and (3) sera. 



Vaccines are agents containing living virulent bacteria, or 

 bacteria that have had their vitality and virulency reduced by 

 heat or chemicals. They are used purposely to produce a mild 

 attack of disease, thereby rendering the animal resistant when 

 subsequently exposed to a natural infection. Examples are 

 blackleg and anthrax vaccines, which are extensively and success- 

 fully used throughout sections where these diseases are prevalent. 



Bacterins are similar to vaccines except that the bacteria in 

 them have been killed and cannot produce disease even in very 

 large doses. When introduced into the body they stimulate 

 the production of protective substances which act against sub- 

 sequent attacks of the particular virulent germ. Examples of 

 bacterins in common use are those for influenza and suppuration 

 in horses, and distemper in dogs. Theoretically, bacterins should 

 be useful in the treatment of every infectious disease of which 

 the causative organism is known. Practically, they do not 

 always give desired results, as it is impossible to immunize 

 against many chronic infectious diseases. 



Sera for therapeutic use are obtained from the blood of animals 

 that have received one large or several smaller doses of infectious 

 organisms. As a result of this treatment, specific antibodies 

 are produced in large quantities in their blood. These anti- 

 bodies exert a bactericidal action on virulent organisms, or 

 contain antitoxins that will neutralize bacterial toxins. When 

 used alone they can only tide the animal over the period of 

 exposure. When used in conjunction with living germs the 

 production of antibodies is stimulated to such an extent that 

 the animal is rendered permanently immune. Examples of 

 specific sera that have proved successful are those for hog cholera, 

 tetanus, rinderpest, and calf scours. 



The process of conferring protection by treatment with these 

 biologic products is spoken of as immunization. Immunization 

 is recommended either when other methods of prevention are 

 impracticable, or when it is desirable to strengthen the body 



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