152 



Bulletin 225. 



Practically the only way tuberculosis gets into a herd of healthy 

 cattle is by the introduction of a tuberculous animal. It has often 

 happened that farmers who have perfectly healthy cows, buy a nice 

 looking cow that is tuberculous, although the disease was not at all 

 in evidence, and sooner or later this animal infects a very large 

 number of animals in the herd. The buying of infected animals is 

 largely responsible for the spread of tuberculosis in cattle. 



The history of tuberculosis in cattle shows that when it is once 

 introduced into a previously uninfected district its tendency is to 

 spread from farm to farm with a rapidity which depends largely 



Fig. 34. — Tuberculous ulcers in the intestines of a tuberculous cow. These are 



not common in cattle. 



upon the activity of the cattle traffic. If the interchange of animals 

 between herds is frequent the disease usually spreads rapidly. If, 

 on the contrary, there is but little interchange of animals, tubercu- 

 losis spreads slowly in the newly infected community. This observa- 

 tion relates to the spread among herds; other conditions govern the 

 spread of tuberculosis after infected animals are added. The latter 

 factor is controlled by the degree of contact between the diseased 

 animals and their associates, and the sanitary and other conditions 



