INFECTIOUS DISEASES OF CATTLE. 



By Drs. D. E. Salmon and Theobald Smith. 



[Revised in 1908 by John E. Mohler, A. M., V. M. D.] 

 GENERAL INTRODUCTION. 



The importance to the farmer 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 grow- 

 ing facilities for intercourse between one section of a country and 

 another, and between different countries, cause a wide distribution of 

 the infectious diseases once restricted to a definite locality. Not only 

 the animals 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 hogs, 

 makes infectious diseases more common and more dangerous. Fresh 

 animals are being continually introduced which may be the carriers 

 of disease from other herds, and when disease is once brought into a 

 large herd the losses become very high, because it is difficult, if not 

 impossible, to check it after it has once obtained a foothold. 



These considerations make it plain that only by the most .careful 

 supervision by intelligent men who understand the nature of infectious 

 diseases and their causes in a general way can these be kept away. 

 We must likewise consider how incomplete our knowledge concerning 

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

 suggestions and recommendations offered by investigators may, there- 

 fore, not always be correct, and may require frequent modification as 

 our information grows more comprehensive and exact. 



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

 introduction into the body 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 morbid changes. 



This definition might include diseases due to certain animal para- 

 sites, such as trichinse, for example, which multiply in the digestive 

 tract, but whose progeny is limited to a single generation. By com- 

 mon consent the term "infectious" is restricted to those diseases 

 caused by the invasion and multiplication of certain very minute uni- 

 cellular organisms included under the general classes of bacteria and 



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