INFECTIOUS DISEASES OF CATTLE. 



Revised Ij.v John R. Mohleh, A. M., Y. JI. D., 



Assistant Chief, Bureau of Auimul IndiisU-y. 



GENERAL INTRODUCTION. 



The importsmee, to the farmei and stoclc 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 coimtry 

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

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

 Xot onlj' the animals themselves, but the cars, vessels, or other con- 

 veyances in which they are carried may liecome 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 Ijrought 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 footliold. 



These considerations make it plain that only liy 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- 

 qiTent modification as our information grows more comprehensive 

 and exact. 



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

 introduction, into the bod_Y, 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 miglit include diseases caused ]>y certain animal 

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

 ti\'e tract, but whose progeny is limited to a single generation. By 

 common consent the terni "infectious'' is restricted to those diseases 



