DISEASES OF CATTLE 339 



the chances may be small, human life is too valuable 

 to be risked unnecessarily. 



The old opinion was that tuberculosis in man or ani- 

 mals was inherited. We now know that it is an infectious 

 disease that is rarely inherited. It is also sometimes 

 attributed to dark, dirty stables, but these are not the 

 cause. A filthy stable can no more produce tuberculosis 

 if the germs are not present, than can a fertile field pro- 

 duce a corn crop if no corn is planted. The disease will 

 spread more rapidly in dark, unsanitary barns, just as 

 corn will yield more on good land. Ten per cent of the 

 cattle that come from the ranges of Nevada, and that 

 never were in barns, are tuberculous. We must distinguish 

 between the germ which is the cause, and the surround- 

 ings which favor its growth. 



One of the chief sources of the spread of the disease 

 is the creamery. Milk from many herds, some of which 

 are diseased, is mixed at the creamery, and the skimmed 

 milk is returned to the farm for feeding calves or hogs. 

 And these animals are infected. To prevent this loss, the 

 milk must be pasteurized, as is now done in the cream- 

 eries of Denmark. 



The other important source of infection is the purchase 

 of diseased animals. To guard against this, one who has 

 a sound herd should not add animals that are not tuber- 

 culin-tested. Even tested animals are not safe if they 

 come from badly diseased herds. One should always 

 buy from a sound herd. 



Animals usually do not show signs of the disease until 

 they are in the last stages. Fortunately a method has 

 been found by means of which diseased animals may be 



