112 



Bulletin 250. 



traffic. If the interchange of animals between herds is frequent the dis- 

 ease usually spreads rapidly. If, on the contrary, there is but little inter- 

 change of animals, tuberculosis spreads slowly in a newly infected 

 community. This observation relates to the spread among herds ; other 

 conditions govern the spread of tuberculosis in the herd after infected 

 animals are added. The latter factor is controlled by the degree of con- 

 tact between the diseased animals and their associates, and the sanitary 

 and other conditions to which the herd is subjected. The increase in the 



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Fig. 131. — Tuberculous ulcers in the intestines of a cow. 

 These are not common in cattle. There are many worm 

 nodules on the intestines that are frequently mistaken 

 for tuberculous nodules. 



cattle traffic is one reason why there is more danger of spreading tuber- 

 culosis now than there was a generation ago. 



If a tuberculous cow is placed in confinement with other cattle, she will 

 convey the disease to them more certainly and more quickly than when 

 the animals are at liberty. As bearing upon this point, it has been noted 

 repeatedly that tuberculosis spreads more rapidly in herds when they are 

 confined in winter than when they are at pasture in summer, and there is 

 reason to believe that this difference is due, not to the season, but to the 

 intimacy of contact. Moreover, tuberculosis once introduced spreads with 

 increasing rapidity as the centers of infection are multiplied. So long 

 as there is but one infected herd from which it can spread in a district, 



