224 DISEASES OF CATTLE, SHEEP, GOATS, ETC. 



cattle, sheep, and hogs, makes infectious diseases more common 

 and more dangerous. Fresh animals are being continually intro- 

 duced 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 may, therefore, 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 re- 

 sponsible for the morbid changes. 



This definition might include diseases due to certain animal 

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

 gestive tract, but whose progeny is limited to a single generation, 

 By common consent the term infectious is restricted to those dis- 

 eases caused by the invasion and multiplication of certain very 

 minute unicellular organisms included under the general classes 

 of bacteria and protozoa. Nearly all the diseases of cattle for which 

 a definite cause has been traced are due to bacteria. Among these 

 are tuberculosis, anthrax, blackleg, and tetanus (or lockjaw). Some 

 diseases, such as Texas fever and nagana, are traceable to protozoa, 

 while others, like actinomycosis and aspergillosis, are caused by 

 fungi. Those diseases of which the cause is unknown or imper- 

 fectly worked out are pleuro-pneumonia, rinderpest, foot-and-mouth 

 disease, rabies, cowpox, malignant catarrh, and dysentery. 



Bacteria may be defined as very minute, unicellular organisms 

 of a plant-like character. They multiply in two ways. The bac- 

 terium elongates and then divides in the middle to form 2 daughter 

 cells. These go through the same process at once, and thus 4 cells 

 are produced. The division of these leads to 8, the division of 8 to 

 16, and so on indefinitely. The rapidity with which this multiplica- 

 tion takes place depends upon the nature of the bacterium. The 

 bacillus of tuberculosis multiplies very slowly, while that of anthrax 

 multiplies with great rapidity, provided both are in the most favor- 

 able condition. Another mode of reproduction, limited to certain 

 classes of bacteria, consists in the formation of a spore within the 

 body of the bacterium. Spore formation usually takes place when 

 the conditions pertaining to the growth of the bacteria become un- 

 favorable. The spores are much more resistant to destructive agents 

 than the bacteria which produced them. The anthrax spore may 

 live several years in a dried state, but the anthrax bacillus perishes 



