PROTECTIVE AGENCIES 55 



Lessening of Virulence. The virulency of pathogenic bacteria 

 can be lessened by a number of procedures, such as : 



1. Inoculating animals which are only slightly susceptible and 

 obtaining the bacteria from them. 



2. Growing the bacteria artificially at temperatures slightly 

 higher than the body temperature of susceptible animals; that is, 

 generally higher than the optimum temperature of such bacteria. 



3. Growing bacteria in the presence of small amounts of anti- 

 septics. 



4. Growing for a few generations, on artificial culture media. 

 Certain pathogenic bacteria will lose a good deal of their virulency 

 when cultivated on such a medium for some time. The pneumo- 

 coccus (the cause of pneumonia) is an example. 



Attenuation. Whenever virulent bacteria, through natural or 

 artificial means lose some of their virulency they are said to have 

 become attenuated and the process is called attenuation. 



Avirulence. When a virulent bacterium loses all its virulency and 

 becomes non-pathogenic, it is said to have become avirulent. Some 

 such avirulent bacteria cannot be made virulent again, but there are 

 others which can be made so by being introduced successively into 

 very susceptible animals. 



Mixed fcifection. Sometimes more than one species of bacteria 

 infects an animal; this is known as a mixed infection. A symbiosis, 

 previously defined, would come under this head, and such symbiotic 

 associations may be very detrimental to the infected animal and lead 

 to a very virulent form of the disease. 



The tetanus bacillus, as already stated, is an anaerobic bacterium. 

 If a wound becomes infected with it and an aerobic bacterium which 

 will absorb the oxygen, the tetanus bacillus has a better chance to 

 multiply and form more toxins, and a most virulent form of the 

 disease will result. Streptococci and diphtheria bacilli; staphylo- 

 cocci and colon bacilli often unite in symbiotic association in pro- 

 ducing virulent mixed infections. 



Mortality. All of the primary signs and symptoms and pathologic 

 changes of infectious diseases, such as the loss of appetite, weakness, 

 inability to work, prostration, elevation of body temperatures, degen- 

 erative changes in the organs, disturbances of circulation, etc., are 

 due to the elaborated and absorbed toxins. 



Infectious diseases may have a high or low mortality, but there is 

 practically no infectious disease which in every case leads to death. 

 On the contrary, the majority of all such cases end, fortunately, in 

 recovery. 



Protective Agencies. There are a number of factors which protect 

 the body against the invasion and multiplication of pathogenic organ- 

 isms, and there are other factors which limit both the multiplication 

 of bacteria and the amount of toxins formed in cases where the 

 bacteria have gained entrance to the body of the sick animal. If 



