Transmissible Diseases. 95 



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decomposition of the carcass, the bacteria are set free 

 unless, as frequently happens, the process of putrefaction 

 destroys them. 



Spread of pathogenic bacteria. The bacteria are 

 spread from place to place by insects which have bitten 

 the living animal or have fed on the carcass ; by birds, and 

 by carnivorous animals, as dogs. The bacteria may be 

 carried long distances in water, or by wind as dust in the 

 air ; on the clothes and shoes of attendants ; on farm im- 

 plements as hoes, shovels, etc. ; and by the shipment of 

 infected objects such as hides. 



The most important manner in which the disease-pro- 

 ducing bacteria are spread is through the transfer of 

 diseased animals from place to place. All of our import- 

 ant transmissible diseases have been imported to this 

 country in the bodies of animals. Constant watch has 

 to be maintained to prevent the introduction into this 

 country of some diseases that are widespread in Europe, 

 such as foot and mouth disease and contagious pleuro- 

 pneumonia of cattle. 



Susceptibility to transmissible diseases. In order 

 that transmissible diseases may develop, it is necessary to 

 have the causal organism enter the body of a susceptible 

 animal host. Not all animals are susceptible to any dis- 

 ease. When an animal does not afford a suitable medium 

 for the growth of the parasite, it is said to be immune. 

 Such an immunity is ' ' natural. ' ' Again immunity may 

 be "artificial," i. e., induced by a natural attack of the 

 disease, or by the introduction of some immunizing sub- 

 stance into the body, as in the employment of vaccines 

 in black leg and anthrax. Some diseases affect but a 

 single kind of animal, as typhoid fever in man, or hog 

 cholera in hogs. Again a large number of kinds may be 



