INFECTIOUS DISEASES OF CATTLE. 



By Drs. D. E. SALMON and THEOBALD SMITH. 



GENERAL INTRODUCTION. 



The importance to the fanner and stock raiser of a general knowl 

 edge of the nature of infectious diseases need not be insisted on, as it 

 must be evident to all who have charge of farm animals. The growing 

 facilities for intercourse between one section of a country and another 

 and between different countries cause a wide distribution of the infec- 

 tious diseases once restricted to a definite locality. Not only the ani- 

 mals themselves, but the cars, vessels, or other conveyances in which 

 they are carried may become agents for the dissemination of disease. 

 The growing tendency of specialization in agriculture which leads to 

 the maintenance of large herds of cattle, sheep, and swine makes infec- 

 tious diseases both more common and more dangerous. Fresh animals 

 are l>eing continually introduced which may be the carriers of disease 

 from other herds, and when this is once introduce*! into a large herd the 

 losses become very high, because it is difficult, if not impossible, to 

 check a disease after it has once obtained a foothold. 



These considerations make it plain that only by the most careful super- 

 vision by intelligent men who understand the nature of infectious dis- 

 eases and their causes in a general way can these be kept away. v We 

 must likewise consider how incomplete our knowledge concerning many 

 diseases is, and probably will be for some time to come. The sugges- 

 tions and recommendations offered by investigators may, therefore, not 

 always bo correct, and may require frequent modification as our in form a 

 tion grows more comprehensive and exact. 



An infections disease may be defined as any malady caused by the 

 introduction into the Inxly of minute organisms of a vegetable or animal 

 nature which have the power of indefinite multiplication and of setting 

 free certain peculiar poisons which are chiefly responsible for the nior 

 bid changes. 



This definition might include diseases due to certain animal para- 

 sites, such as trichina', for example, which multiply in the digestive 

 tract, but whose progeny is limited to a single generation. Py mininnn 

 consent the term infectious is restricted to those diseases caused by 



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