INFECTIOUS DISEASES OF CATTLE. 



Revised by John R. Mohler, A. :M., V. M. D., 

 Assistant Chief, Bureau of Animal Industry. 



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 

 growing facilities for intercourse between one section of a country 

 and another, and betAveen different countries, cause a wide distribu- 

 tion of the infectious diseases once restricted to a definite locality. 

 Not only the animals themselves, but the cars, vessels, or other con- 

 veyances in which they are carried may become agents for the dis- 

 semination 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 infec- 

 tious 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 investi- 

 gators, therefore, may not always be correct, and may require fre- 

 quent 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 caused by certain animal 

 parasites, such as trichinae, for example, which multiply in the diges- 

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

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

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