CHAPTER XIX 



THE RELATION OF MICROORGANISMS TO 

 DISEASES OF ANIMALS 



Communicable diseases. The diseases of animals may be 

 divided into two classes: the organic or constitutional, 

 which are due to the faulty operation of some organ; and 

 the communicable, which are caused by the invasion of the 

 body by some organism and the growth thereof with the 

 formation of substances that have a harmful action on the 

 body. The latter are termed communicable diseases, be- 

 cause the passage of the causal organism from the diseased 

 to the healthy animal is sufficient to spread the trouble. 

 The organic diseases can not be thus transmitted, for they 

 are not due to the presence of a living organism in the 

 body. The communicable diseases are also termed infec- 

 tious, contagious, and preventable, since the prevention of 

 their spread can be accomplished by stopping the trans- 

 mission of the organism. In some instances the knowledge 

 of the manner of transmission is so complete that if stock- 

 men could be induced to put that knowledge into practice 

 the diseases would soon disappear, while in other cases the 

 information is not yet sufficiently complete to enable their 

 spread to be prevented. 



Such transmissible diseases as tuberculosis, contagious 

 abortion, hog cholera, Texas fever, and glanders entail 

 an enormous tax on the livestock industry of the world, as 

 will appear in the discussion of the specific diseases. Since 

 the prevention of disease is a problem that must always 

 rest in the hands of the farmer himself, rather than in any 



230 



