VII TUBERCULOSIS OF THE COW 127 



this happened, the heating apparatus of the dairy was found 

 to have been out of order just at the time when milk was 

 supplied to the set of animals that had reacted. 



From the practical point of view in relation to human 

 infection, the essential point to consider is the extent to which 

 these tuberculous cows yield tubercle bacilli in the milk. Up 

 to a few years ago it was generally accepted that tubercle 

 bacilli were only found in milk when tuberculosis of the udder 

 was present. This is now known not to be the case, and 

 tubercle bacilli may gain access to the milk from cows 

 which neither clinically nor on post-mortem examination 

 present any signs of udder disease. We owe our knowledge 

 upon this matter largely to Mohler and Schroeder in America, 

 and more recently to the last Eoyal Commission on Tuberculosis 

 in this country. Some of Mohler's ^ conclusions were as follows : 



The tubercle bacillus may be demonstrated in milk from 

 tuberculous cows when the udders show no perceptible evidence 

 of the disease, either macroscopically or microscopically. 



The bacilli of tuberculosis may be excreted from such an udder 

 in suflScient numbers to produce infection in experimental animals 

 both by ingestion and inoculation. 



The presence of the tubercle bacillus in the milk of tuberculous 

 cows is not constant, but varies from day to day. 



Cows secreting virulent milk may be affected with tuberculosis 

 to a degree that can be detected only by the tuberculin test. 



The physical examination or general appearance of the cow 

 cannot foretell the infectiveness of the milk. 



The milk of all cows which have reacted to the tuberculin 

 test should be considered as suspicious, and should be subjected 

 to sterilisation before using. 



Schroeder ^ came to very similar conclusions. He found 



. . . among tuberculous dairy cows that retain the appearance 

 of health and are not known to be affected until they are tested 

 with tuberculin, 40 per cent or more actively expel tubercle bacilli 

 from their bodies in a way dangerous to the health of other animals 

 and persons. 



The general evidence justifies the conclusion that tuberculous 

 cows do not expel tubercle bacilli until some time after they contract 

 the affection. 



Dairy cows that have been affected with tuberculosis three 



' Bulletin No. 4I, Bureau Animal Industry U.S. Dept. Agriculture, 1903. 

 2 Circular 118, Bureau Animal Industry U.S. Dept. Agriculture, 1907. 



